Other groups of insects can infest dry and seasoned wood. These include the termites (order Isoptera), powderpost beetles (Lyctidae), false powderpost beetles (Bostrichidae), deathwatch beetles (Anobiidae), carpenter ants (Formicidae: Formicinae), and the oldhouse borer just mentioned. These insects may be very damaging because they can reinfest timbers until the wood is completely hollowed out, leaving only a deceptive outer shell. When wood is infested by the beetles in this group, this outer shell is perforated with the exit holes of the successive generations of insects.
There are other insect species that do superficial or only occasionally serious damage to wood, but which are included in this book under more appropriate headings. For example, the spider beetles (Ptinidae) and hide beetles (Dermestidae) can bore into wood when seeking a place to pupate, and they sometimes severely damage structures used for storage of their principal food materials - grain, hides, and offal (the inedible parts of butchered animals).
Fungi probably cause as much damage as termites to wood structures. They require about the same environmental conditions for their growth and development as the subterranean termites (Reticulitermes) and dampwood termites (Zootermopsis) do for theirs. The elimination of these conditions results in control for both termites and fungi.
Fungi also are important in supplying important nutrients or even the entire food supply of certain wood-destroying insects.
Becker (1968) found that some strains of Aspergillus flavus, Penicillium funiculosum, and Trichoderma viride were highly toxic to the larvae of Hylotrupes bajulus in small, sapwood samples of Pinus sylvestris L. On the other hand, of the soft-rot fungi tested, Fusarium aquaeductuum and Phialophora aurantiaca accelerated the larval growth rate 10- to 30-fold compared with the control insects.
Young larvae of the deathwatch beetle, Xestobium rufovillosum (De Geer), an anobiid, are absolutely dependent on fungi that develop in their tunnels in dry wood, becoming less dependent as they mature (Campbell and Bryant, 1940). On the other hand, larvae of the cerambycid Ergates faber (L.) become increasingly dependent on fungi with each molt (Becker, 1942, 1943). Some anobiids are so dependent on fungus-infected wood that they discontinue feeding at the point where fungal infection stops (Becker, 1950, 1951). The ambrosia beetles of the Scolytidae and Platypodidae feed on fungi grown in their brood galleries; these galleries are the extent of their damage to the wood. The ambrosia beetle Xyleborus ferrugineus (F.) can go through 1 generation on a diet lacking either fungus or ergosterol, the major sterol of fungi. Apparently, there is an adequate carry-over of sterol from the preceding generation that fed on diets with fungus. However, the second generation fails to pupate if it is not supplied with either fungi or ergosterol in its diet (Norris et al., 1969).
Dominik (1966) found that, among the species of beetles attacking building timbers, some attacked wood heavily infected with fungi, some attacked sound, uninfected wood, and others attacked either infected or uninfected wood. He observed that a single genus (Anobium) contained species that required and species that shunned fungus-infected wood. Anobium punctatum (De Geer) occurs outdoors in Great Britain where wood has resisted fungal attack, possibly because of sheltered location or good drainage. Old branch scars on trunks of trees or areas of exposed sapwood are almost always infested, and the freedom from fungal decay in such situations led Hickin (1963a) to suspect that some fungicidal material had been secreted into the wood by the developing larvae.
Digestion of cellulose by coleopterous larvae was once believed to be dependent on symbiosis with cellulose-secreting microorganisms. This has since been shown to be untrue, although such microorganisms may be important in connection with the vitamin requirements of the insects. There is no longer any doubt that cellulase is present in the digestive enzymes of many, but not all, woodboring beetle larvae. The nutrition of woodboring Coleoptera appears to depend upon more factors than the enzyme complex in the larval gut, and much further research is still needed (Parkin, 1940).
A remarkably close relationship between an insect and a fungus is seen in the woodwasp Sirex and the Basidiomycete Oidia. The spores of the fungus are carried by the adult siricid in 2 small, invaginated, intersegmental sacs (figure 140) at the base of the ovipositor, described by Buchner (1928, 1930). During oviposition, the spores are deposited in the egg tunnel and develop to produce a mycelium that penetrates the surrounding wood. The woodwasp larva is believed to be benefited by the action of the fungal enzyme, which continues to digest wood particles in the alimentary tract of the insect (Francke-Grosmann, 1939, 1957; Parkin, 1942; Middlekauff, 1960; Stillwell, 1966; Morgan, 1968).
The ambrosia beetle Platypus wilsoni (Swaine) (figure 133, right) possesses punctures concentrated on the caudal half of the pronotum that were shown to be mycangia (Farris and Funk, 1965). The male of the scolytid beetle Gnathotrichus sulcatus (LeConte) (figure 133, left) carries 3 species of symbiotic fungi in special enlargements (mycangia) of the forecoxal cavities. This is the only ambrosia beetle that has been recorded as carrying more than 2 symbiotic fungi (Funk, 1970). Graham (1967) discussed the results of an investigation by W. Schedl, who reported 8 different types of mycangia in 26 species of ambrosia beetles examined:
In the head, there may be mandibular, pharyngeal, or external ventral pockets; in the thorax, there may be tubular "glands," dorsal pits, pockets between the pro- and mesonotum, coxal cavity spaces, or pockets in the elytra. Most occur only in females of a species, but some occur only in males. At present, the process by which pores are transferred into and out of the mycangia is not understood.
The relationship of termites and fungi has been the subject of many investigations. Fungi in decaying wood may furnish nitrogen, vitamins, and other substances beneficial to termites, and may also destroy toxic volatile material or extractives in the wood. Fungi are also known to produce attractants or feeding stimulants. On the other hand, they can remove certain nutrients or produce toxic metabolites. Whether fungi are beneficial or not depends on the species of termite and fungus and other variables (Becker and Kerner-Gang, 1964; Smythe et al., 1971).
Representatives of all the classes of fungi are associated with termites. Slight deterioration of wood by certain Basidiomycetes of the brown-rot type increases its nutritional value to termites (Light and Weesner, 1947; Becker, 1948, 1965), and the same may be said of certain Ascomycetes and Fungi Imperfecti, the soft-rot fungi (Becker, 1948). Other species or even other strains of the same species may have no effect, or may even be toxic to termites (Lund and Engelhardt, 1962; Becker and Kerner-Gang, 1964; Kovoor, 1964; Becker, 1965; Smythe and Coppel, 1966a). For rearing termites in the laboratory, Becker (1969b) suggested the use of wood blocks with about 3 to 10%, weight loss owing to attacks by brown-rot Basidiomycetes, such as Coniophora puteana, Lenzites spp., Polyporus spp., or Merulius lacrymans. The mycelium of certain nontoxic fungi can be added with additional benefit.
Workers of the eastern subterranean termite, Reticulitermes flavipes when exposed to both decayed and sound wood of 6 wood species, ate 1.4 to 2 times more decayed than nondecayed wood. During an 8-week test, survival of termites was highest on wood decayed by Daedalea quercina and Poria oleracea. Termite survival was better on ovendried decayed wood than on less dry sound wood for all wood species except a walnut (Juglans nigra), on which there was no survival (Smythe et al., 1971).
The question is sometimes asked whether subterranean termites will feed on sound wood that is not infected by fungi. To obtain information on this subject, Pence (1957) collected Reticulitermes hesperus from sound, white Douglas-fir lumber and placed the insects on sterilized and ovendried strips of the same wood, dyed with India ink. Within a few hours, their intestinal tracts were filled with the dyed wood. When given a choice, they fed on black wood exclusively in the presence of light, but fed on either black or white (undyed) wood indiscriminately when placed in darkness. There is no doubt that termites attack and destroy perfectly sound timber, but the galleries they form in the wood are soon infected with fungi, which probably are a useful amendment to their diet.
The eastern subterranean termite was attracted by aqueous extracts of 6 species of wood decayed by the fungus Lenzites trabea, to 3 species decayed by Poria cocos, and to 1 each decayed by Lentinus lepideus and Daedalea quercina (Smythe et al., 1971). Attractancy of wood decayed by L. trabea should not be construed to imply usefulness of the fungus to termites under natural conditions; in fact, Smythe et al., (1971) found it to be associated with generally decreased termite survival. However, there has been much recent investigation on the subject of attractant fungi because of the possibility that their extracts, combined with suitable insecticides, may provide specific control, or that fungal parasites and pathogens may be used in biological control (Esenther and Gray, 1968; Sands, 1969).
Unique genera and species of oxymonad, trichomonad, and hypermastigote flagellates (protozoa) that occur in the intestinal tracts of termites are found only in the 4 most primitive termite families: Mastotermitidae, Hodotermitidae, Kalotermitidae, and Rhinotermitidae. The great majority of these flagellates ingest wood particles, and appear to be indispensable for the survival of the termites. Although there appears to be no doubt that the flagellates are responsible for cellulase activity in the hindgut of the lower termites, it seems that they do not contribute substantially to the nitrogen (for protein) requirements of the insects. Possibly, the bulk of the nitrogen is supplied by fungi present in the wood (Honigberg, 1970). The higher termites (Termitidae) possess protozoa such as found in other insects and in other invertebrates, and even in the large intestines of some vertebrates, but they do not depend on them for the digestion of wood. In fact, as a rule they do not feed solely on wood or cellulosic material, and what wood they do eat is usually much more decayed than the wood eaten by the lower termites.
The function of the fungus in the fungus combs of the Termitidae "appears to be mainly the breakdown of lignin, but it probably also supplies nitrogenous materials and possibly other factors, such as vitamins" (Sands, 1969).
Termites feed on wood, and throughout a large area of the world they are the most destructive insects to wood structures. This is a measure of the importance of their natural role in nature - breaking down and returning to the soil and atmosphere the enormous tonnage of dead and fallen trees and other cellulosic material that is continuouslv accumulating on the earth's surface. They are important pests of agricultural crops, forest nursery seedlings, and range grasses, and also damage an enormous amount of stored food and household furniture and commodities, including even most plastics (Snyder, 1935, 1955b; Harris, 1961; Ebeling, 1968). Damage from termites plus the cost of controlling them probably amounts to approximately a half billion dollars per year in the United States alone (Ebeling, 1968). Our knowledge of the history of South America would probably have been much more complete if it had not been for termites, for they are said to have eaten most of the books more than a century old (Howse, 1970).
Termites are found in tropical, subtropical, and in most temperate climatic zones. They are increasing their range and density northward, being favored by the accelerated urbanization incident to the "population explosion," particularly in areas where central heating of buildings affords them a particularly favorable environment for the establishment of colonies.
Much has been written about termites, yet until recently the largest and most comprehensive work, particularly on termite biology, was a book titled Termites and Termite Control. It was edited by C. A. Kofoid, and published by the University of California Press in 1934 (partially revised in 1965), and includes the contributions of 35 experts on termite biology, taxonomy, and control. In 1969 and 1970, a treatise of 2 volumes, titled Biology of Termites and edited by Kumar Krishna and Frances M. Weesner, was published. Twenty-five scientists contributed to this exhaustive treatise, which covered many aspects of termite biology, systematics, distribution, behavior, social organization, and control. Termites- Their Recognition and Control, by Harris, published in 1961; and Termites-A World Problem, by Hickin, published in 1971, are books on the distribution, classification, economic importance, and control of termites, as well as wood-preservation procedures, written from a world standpoint. An excellent popular book on termites was written by Howse (1970).
The lower termites (e.g., drywood and damp-wood termites) can be easily collected and then cultured and observed under glass. The higher termites are more difficult to culture, but successful techniques for culturing have been developed for certain species in all families, and have been described by Becker (1969a). An observable termite colony should be a fascinating subject for classroom demonstration.
In Wisconsin, the approximate northern limit for the eastern subterranean termite, Reticulitermes flavipes, coincides with an annual minimum isotherm of -30 °C (-22 °F). Upward movements of termites from the soil cease near zero C, and they overwinter primarily at a soil depth of from 3 to over 4 ft (1 to 1.5 m). In this way, the insects can escape adverse weather conditions, such as dryness or low temperature. It appears that the most adverse effect of winter is to confine them to a zone below an adequate source of food, which is usually near the ground surface. The survival of termites in cold climates depends on their ability to repopulate during the warm season (Esenther, 1969).
The situation in Wisconsin is probably typical of all areas in which subterranean termites are extending their range into colder regions. Esenther (1969) points out that only "man-oriented" colonies have been found in Wisconsin. He believes that they were introduced originally on infested lumber and spread mainly through new colony formation facilitated by their subterranean tunneling. Unlike the situation in the warmer southern parts of the range of the species, dissemination by flight must be relatively insignificant in the north. The rate of development of an incipient colony in the north is too slow, and therefore the termites are not likely to survive the rigors of the first winter to form alates. Thus, reproduction is by neotenic individuals, those which attain sexual maturity without attaining the alate (winged) form.
Becker (1970a, b) has observed differences between Reticulitermes flavipes in Wisconsin and Hamburg, Germany, when compared with what he considers to be bioecological races of that species in South Carolina and Hallein, Austria. The northern forms were the most active gallery builders, and demonstrated daily rhythms of activity that were lacking in the southern forms.
A society can be developed only if its members are long-lived. This in turn depends on an adequate and continuous food supply. Termites solved this problem by acquiring the ability to use wood-cellulose as food. The principal termite pests in the United States are in the families Rhinotermitidae and Kalotermitidae, the members of which depend on protozoa (mastigophoran flagellates) in their hindguts to break down cellulose. Termites do not possess these protozoa when they are born; they must obtain them by proctodeal feeding, that is, feeding upon liquid intestinal content taken from the anal aperture of an older termite. Every time the termite molts, the lining of the hindgut is shed, along with the entire body cuticle, and the protozoa are lost. Refaunation takes place by proctodeal feeding (Andrew, 1930; Honigberg, 1970).
In some areas of the world, termites belong primarily to the family Termitidae. The termitids are responsible for most of the earthen termite mounds, some as much as 10 meters in height, which form a characteristic feature of many landscapes of the African and Asian tropics. These termites have no intestinal fauna of the types that can aid in digestion. They may consume grass, leaves, humus, the manure of herbivorous animals, and decaying wood. The Macrotermitinae have spongelike fungus combs in their nests. They are constructed of chewed wood and feces, and are built up to fit the chambers of the nest. These termites feed on fungus (Termitomyces) in the combs. The function of the fungus in digestion appears to be mainly the breakdown of lignin, but probably other factors are supplied, such as nitrogenous materials and vitamins (Sands, 1969). Thus, termites have an abundant, continuous food supply, and this, coupled with their longevity, the potential immortality of the colony, and their ability to care for their eggs and young and protect the colony against natural enemies and the elements, allows for the development of enormous numbers of individuals. For example, in a large colony of the moundbuilding termite Nasutitermes exitiosus (Hill) in Australia, 11.05 kg of termites were removed, representing 2.5 million insects, of which about 87% were of the wood-destroying worker caste (Gay and Wetherly, 1970). Such enormous numbers of termites result in a great capacity for destruction of wood structures, with no seasonal letup in tropical areas. Most of the foregoing factors also favor another social insect, the ant, as a successful and persistent pest of man. Ants may not have as constant a food supply, at least not in nature, but they can store foods in their well-protected nests.
Termites live what is known as a "cryptobiotic" mode of life. They live in enclosed passageways, either entirely in the wood in which they feed or partly within the wood and partly within soil. At certain times of the year, depending on the species, a certain percentage of the colony develops wings and changes from the whitish color of the nymphs to the distinctive dark or black color of the winged reproductives (alates). The latter fly off to form new colonies.
The alates are the members of the colony most likely to be seen by the homeowner. They are the potential kings and queens. The homeowner often confuses alates with "winged ants." The abdomen of the termite is broadly joined to the thorax, while the thorax and abdomen of the ant are joined by a narrow petiole or "waist" (figure 66). The termite has straight, beadlike antennae, while those of the ant are elbowed. Unlike the castes they left behind, the termite alates are heavily pigmented. The fore- and hindwings of the alate termite are approximately equal in length, and usually extend from 25 to 33% of their length beyond the end of the abdomen when folded. The hindwings of the alate ant are much shorter than the forewings, and the folded wings rarely extend beyond the end of the abdomen (figure 67).
After the flight of the alates, their wings break off near the base. Males and females pair off and begin a small excavation for a new nest. Subterranean termites (Rhinotermitidae), for example, may excavate their nest in wood found after digging into the ground, or between a piece of wood and damp ground, or in a crevice in wood on damp ground. (Galleries eventually extend deeply into the ground.) The pair then mate, and the first eggs are laid. The egg-laying capacity of the queen increases as she grows older. Queens of some moundbuilding tropical termites can lay as many as a thousand eggs per day for as long as 25 years.
Several years may pass before all castes are present in a new colony. The complete colony consists of the primary pair of reproductives (royal pair) and three castes: (1) the workers, which feed on wood or fungi and, by regurgitation and excretion, provide food for the young and the other castes; (2) the soldiers, which in the United States are usually large-headed individuals with massive jaws that guard the nest entrances and the royal pair; and (3) usually 2 kinds of supplementary or substitute reproductives known as neotenics. These may be either lightly pigmented and with short wing pads (brachypterous) or very lightly pigmented and without wing pads (Krishna, 1969).
Chemicals that are secreted to the outside of the bodies of insects for caste regulation, attraction, communication, trail-marking, etc., are called pheromones. A colony of social insects (termites, ants, wasps, or bees) maintains its social cohesiveness primarily through the utilization of such chemicals. They are produced in specialized tissues known as exocrine glands. In response to specific stimuli, these glands evacuate their contents into the environment (Blum, 1970).
Termites continually groom one another by means of their mouthparts to obtain desired secretions or exudates containing pheromones. Among the pheromones they obtain in this manner are some that are believed to inhibit the formation of additional members of the sex or caste from which the hormones are obtained, thus serving as a regulatory mechanism to prevent a disproportionate ratio of males, females, and soldiers in a colony (Luscher, 1956a, b, 1961; Weesner, 1956). Luscher (1961) stated that the queen can inhibit sexual development of other potential reproductives, even if her abdomen is covered with varnish, thus covering all integumental glands and the genital opening, but that inhibition is no longer possible if the anus is blocked. He concluded that the inhibitor substance must be given off with the excrement.
There is considerable evidence that the "royal pheromone" that prevents Kalotermes flavicollis (F.) from undergoing the final molt is produced by the mandibular glands of the sexual forms. In 20 trials, when one of these glands from a primary sexual female was implanted in the abdominal cavity of a nymph shortly before the final molt, adult differentiation was blocked or inhibited, depending on how close to the final molt the nymph was at the time of implant (LeBrun, 1972).
Reproductives can also stimulate the development of a caste. For example, when a group of nymphs of Kalotermes flavicollis is separated from soldiers, some will differentiate into soldiers. The number of soldiers produced is much greater when reproductives are present. The effect of reproductives on soldier production was graded as follows: pair of reproductives (king and queen) > 2 queens = 1 queen = 2 kings > 1 king (Springhetti, 1970). Miller (1969) pointed out that, among the lower termites (all families but Termitidae), there is no evidence that the various castes are genetically different; their caste destinies are the expression of social and environmental factors. Even "workers" can become sexuals, and in laboratory colonies have been able to reconstitute all castes of the colony when they were sufficiently numerous.
The role of pheromones in the structure and formation of termite colonies, particularly when the order Isoptera as a whole is considered, is extremely varied and complex. The amazing extent to which an understanding of the role of pheromones has already developed from the pooled results of world-wide investigations is concisely discussed by Howse (1970). No doubt an even broader understanding will result from current investigations.
Supplementary reproductives (neotenics) are required for rapid increase in numbers of termites in a colony. When groups of workers and nymphs of the western subterranean termite, Reticulitermes hesperus, were separated from the mother colony, they formed a new colony in 6 to 8 weeks, utilizing supplementary queens developed from some of the short-winged nymphs found in every large colony (in addtion to the nymphs that develop into the alates that leave the colony). A supplementary queen can produce more eggs (60 to 80) in a day at the height of egg-laying than the primary queen in the first 2 years of the colony's development (Pickens, 1934a).
Extracts of the trail-marking pheromone from either Reticulitermes flavipes or R. virginicus were attractive to both these species and to R. hesperus, but not to the dampwood termite, Zootermopsis angusticollis (Smythe and Coppel, 1966b). Several synthetic analogs of the trail-marking pheromone of R. viginicus have been prepared, and the molecular structures to which these compounds owe their pheromone-mimicking characteristic have been identified (Tai et al., 1971).
Insect pheromones are generally rather species-specific. Therefore, it is of special interest that 6 subterranean termite species (Reticulitermes flavipes, R. virginicus, R. hesperus, R. tibialis, Coptotermes formosanus, and Leucotermes speratus [the latter from Japan]) all responded to 4 trail- marking pheromone analogs (Matsumura et al., 1972). The nonspecificity of these compounds would be advantageous in any attempt to use them as lures for trapping purposes in a control program. Two of the pheromone analogs have been found to be easily synthesized (Tai et al., 1971).
There are many other nonpheromone substances, some found in nature and some artificially produced, that have effects similar to those caused by pheromones. For example, wood rotted by the fungus Lenzites trabea produces an attractant for Reticulitermes flavipes as well as other species of Reticulitermes and Coptotermes. The fungus induces trail-following by termites similar to that induced by the trail-marking pheromone secreted by the sternal glands of these insects (Esenther et at., 1961; Esenther and Coppel, 1964; Allen et al., 1964b; Smythe et al., 1965, 1967a, 1967b; Esenther, 1969). Column chromatography of the unsaponifiable lipids from pine wood on which L. trabea was cultured yielded 2 well-separated fractions that were highly active in choice tests and in a trail-following test with Reticulitermes lucifugus (Rossi). The unsaponifiable lipids of the workers yielded only a single active fraction, but it corresponded to one of the fractions obtained from the wood (Ritter and Coenen-Saraber, 1969).
The attractance of 8 compounds formed in wood by wood-rotting fungi (Basidiomycetes) was tested, using 5 termite species as test insects. Five compounds were generally attractive, and 3 deterred termites or had no attraction. Acids were usually attractive, but aldehydes were attractive in only a few cases (Becker, 1964). Many organic compounds have been found to be attractive to R. flavipes (Watanabe and Casida, 1963).
A trail-marking scent for Nasutitermes exitiosus (Hill) was isolated and identified by B. P. Moore in Australia (Anonymous, 1967d), and was found to be an unsaturated diterpenoid hydrocarbon; only 10
At the United States Forest Service's Wood Products Insect Laboratory in Gulfport, Mississippi, it was observed that Reticulitermes flavipes followed marks made by a certain ballpoint pen with blue ink, but other available ballpoint pens did not have this effect. This phenomenon was thoroughly investigated by Becker and Mannesmann (1968). In their investigation, they used 55 termite species from 21 genera of 4 families. They found 3 ballpoint inks that served as trail-markers for many termite species, and 3 others that attracted fewer species. Figure 69 shows workers of R. lucifugus following a spiral line made with a ballpoint pen with ink containing a glycol compound. The termites tend to follow a tangent to the curve, and must make repeated corrections in their direction of movement. The inks were less effective for Mastotermes and species of Kalotermitidae than for species of Rhinotermitidae and Termitidae. Nine glycol compounds, including some used in ink for ballpoint pens, proved to be trail-marking substances of varying degrees of efficacy. Diethylene glycol monoethylether and diethylene glycol monobutylether were very effective for almost all termite species, including Kalotermitidae. Also, some of the decomposition products (aldehydes and acids) produced when wood was attacked by Basidiomycetes were found to be attractants and trail-markers. Reticulitermes lucifugus is unable to react to scent trails if its antennae are partially amputated unless at least 8 segments are retained.
The possibility of practical utilization of termite attractants in wood baits is discussed under "Termite Baits" latter in this chapter.
The term "workers" is retained because of common usage, although there is reason to believe that termites in the family Rhinotermitidae, in which Reticulitermes is one of the genera, have no definitive workers. Weesner (1965) stated that in Reticulitermes those forms that had generally been considered to be workers might instead be "fairly size-stable individuals functioning as workers, but still capable of molting and differentiating into slightly larger individuals, soldiers, or alates." True workers exist in the family Termitidae.
The growth of a colony from the primary pair of reproductives is slow. Only a few eggs are laid the first year, and they require an average of over 50 days to hatch. The first 2 instars require only 14 to 18 days each. The second and third instars require 1 and 2 months, respectively, and the fifth instar may be the dominant one, and last for as long as 2 years. In sufficiently large colonies that have a large amount of fraternal feeding (trophallaxis), large, well-matured workers and reproductive nymphs of the sixth instar may develop. There may be a seventh instar of still larger workers, and in this instar the perfect reproductive stage is attained in the reproductive caste. Even under the most highly favorable conditions, flights of alates cannot be expected before the third or fourth year. Later, the supplementary reproductives can be expected to greatly accentuate the growth of the colony, and tens of thousands of individuals can develop. (Pickens, 1934a).
The question is often asked as to how many years are required for a newly constructed building to show the first signs of subterranean termite infestation. If the infestation started from a primary pair of reproductives, at least 3 or 4 years would pass before even a few swarmers could be seen, even if the building became infested at the time of construction. Evidence of structural damage should not appear for an even longer period. On the other hand, if the building were built over or close to a strong existing colony, perhaps located in a buried stump or roots, substantial damage from termites could be noticeable within a year.
When condtions of food, moisture, and temperature are satisfactory, subterranean termites can live without any contact with the ground, although this is rare. In most areas, the ground serves as a protection against extremes of surface temperatures, as well as being a moisture reservoir. Subterranean termites are known to change the depth of their nests in the soil to accommodate to changing temperature or moisture requirements.
Subterranean termites in the United States generally span the distance between the ground and the first wood members of the substructure of a building by working their way up through intervening wood or by building "shelter tubes" over concrete foundation walls. Intermittent sections of shelter tubes can also be seen on structural wood members. Periodically, the workers must return via the shelter tubes to their galleries in moist soil to regain moisture through their cuticles to replenish what was lost in the relativeldry wood where they have been feeding.
The tubes do not conduct moist air to the areas of termite activity, as is often recorded in the literature. A humidity sensor, inserted into termite galleries in joists only 18 in. (45 cm) above-ground and directly connected to the ground by shelter tubes, showed that the humidity was identical with that of the atmosphere surrounding the joists (Ebeling, 1968). However, the protection that the shelter tubes provide against air movement probably helps to preserve the termites' moisture to some extent. Likewise, shelter tubes are not built to keep out light, for termites have no aversion to light if they are protected, as in termitaria under glass. The principal function of shelter tubes appears to be protection against natural enemies, especially ants.
The worker termites build shelter tubes from particles of sand or earth, or minute particles of wood, or both, coating them with a gluelike substance that they secrete. Fecal material is also used as a cement. (For Zootermopsis, excrement is likely to be the principal construction material.) The particles may be tightly packed, or they may adhere so loosely as to form a coarse filigree. The western subterranean termite uses at least 4 types of shelter tubes. Usually, all types can be seen in the crawl space under an infested house on a "raised foundation."
Utility or Working Tubes. These form runways through which termites feeding on the wood above the foundations can return periodically to the moist atmosphere of their subterranean galleries in order to replenish lost body moisture. These tubes are wide, flattened, and usually extend from the soil to the wooden construction above it (figure 72, A).
If the tubes are broken as rapidly as they are repaired by termites, and if the insects have no other sheltered passageways through which to return to their nests, and if there is no source of moisture other than the damp soil below, those termites above the broken portions of the tubes will perish.
Exploratory or Migratory Tubes. These (figure 72, B) are similar to utility tubes, but are not so strong, usually do not reach the wood above, and have small exit holes. Figure 73 shows one of these tubes being constructed by a worker termite.
Suspended or Drop Tubes. These are utility tubes, built downward from a wood member to the ground. They are lighter colored than the other types of tubes because they contain more wood fiber (figure 72, C). At the top of the figure, note the 2 egg sacs of the western black widow spider attached beneath the subflooring.
Three types of shelter tubes are shown in figure 74: A, a utility tube against the concrete foundation; B, a utility tube reaching from soil to subfloor, but not attached to the foundation; and C, a suspended or drop tube. Note the much lighter color of the latter. Also, note the block of wood, 1), which was left lying on the ground and became infested with subterranean termites.
Swarming Tubes. These are constructed at swarming time to provide for the exit of alates. They usually extend aboveground about 4 to 8 in. (10 to 20 cm), but under particularly favorable circumstances may extend considerably farther, and may reach wood members of the substructure. Swarming tubes are often found around or beneath floor furnaces or in other warm places.
Formation and Defense of Galleries in Wood
Subterranean termites primarily attack the soft spring growth of the infested wood, in contrast to drywood termites and other nonsubterranean species which burrow indiscriminately across and with the grain of the wood, excavating broad pockets or chambers connected by tunnels (figure 75). Termites tend to feed in a structural wood member until only the harder wood portions and a fragile outer shell remain (figure 76).
The workers of subterranean termites have strong mandibles (figure 77), very large in relation to the size of the head. They are able to tear up and consume wood, but do not eat all the wood chewed out of a gallery. They transport some of it to the rear, and may pile it into a natural cavity in wood or soil, or into a large, unused gallery; much of it is mixed with fecal plaster and packed along the walls of the gallery. Workers also sustain the other castes by oral or anal feeding, care for the eggs, feed the very young nymphs (larvae), queens, kings, and soldiers, and in fact, they perform all the duties of the colony except for reproduction and defense. The solders (plate I, 3; figure 70, E) are responsible for defense. Thrusting their large heads and powerful jaws into breaks in the gallery system, they can defend it against the ant, the termite's worst enemy. Only in case of major destruction of the gallery system is the termite in danger from insect predators. After swarming, the alates are, of course, easy prey for predatory insects and birds.
The First Signs of Infestation
In California, the rainy season is in late fall, winter, and early spring. During this period, flights of subterranean termites take place on warm, sunny days after a rain. (Flights during this period can also be initiated by heavy irrigation or sprinkling in the vicinity of a house). The alates may be the first indication of infestation observed by the homeowner, and it is important that he be able to distinguish them from winged ants (figure 66 and 67) and from the alates of the drywood termite, Incisitermes minor. The total length of the alates of the drywood termite is 11 to 12 mm (about a fourth longer than that of the subterranean termites), the body is dark brown, and the head and thorax are reddish.
Another common indication of the presence of subterranean termites is the occurrence of dark areas or blisters in flooring. One can easily crush these areas with a knife or screwdriver. Shelter tubes may be seen in the crawl space. Sometimes, the distinctive sound of the soldiers may be heard. If they are disturbed, as by a knock on an infested wall, floor, or subfloor wood member, the soldiers jerk their bodes and heads violently up and down and produce an audible tapping as the head capsule strikes the roof or floor of their gallery.
Possible Extent of Damage
Subterranean termites may infest the "mud-sill," usually the first structural wood member encountered on their way up from their subterranean galleries. If the sill is of termite-resistant wood or has been chemically treated, the termites generally build their shelter tubes over this wood member, and attack support structures that rest on the mudsill (girders, joists, and cripples), as well as adjacent wood structures, such as flooring and studding (figure 78, left). These wood members may be hollowed out by the termite feeding. Shelter tubes, constructed of earth, bits of wood, and excrement, extend up into the infested timbers (figure 78, right), and are among the definite indications of subterranean termite infestation. The infested timber becomes hollowed out to the extent that it is vulnerable to ants and other predators, and the termites must construct shelter tubes to protect themselves from their attacks.
Other Indigenous Species of Subterranean Termites
Another species of Reticulitermes, R. tibialis Banks, may be found in some inland areas, such as the San Joaquin and Sacramento valleys and certain high deserts in California, and in general the inland areas of the Pacific Coast eastward to the Rocky Mountain region and Texas. The alates can be distinguished from those of R. hesperus by their pale, almost whitish, wings, with brownish veins in the forearea,as compared with the dark wings of R. hesperus (Weesner, 1965). The head of the soldier of R. tibialis is short, broad, and dark, compared with the long, narrow, and pale head of the solider of R. hesperus (Pickens, 1934b). Where these species overlap in distribution, R. hesperus prefers cool, shady, moist places, while R. tibialis requires open, sunny, drier locations. The 2 species are equally able to damage wood structures.
In the deserts of Arizona, southeastern California, and Mexico another subterranean termite, Heterotermes aureus (Snyder), occurs which can be very destructive, although much of its damage is probably attributed to species of Reticulitermes. The alates, which are nocturnal fliers, can be distinguished from those of Reticulitermes by their pale color. The soldiers of the 2 species are difficult to distinguish.
In the East, Reticulitermes flavipes (Kollar) is the most important termite pest. It extends along the Atlantic Coast north to Maine, and occurs to some extent in Canada, generally up to the line where the average annual minimum temperature does not fall below -20 °F. It extends southward throughout the eastern and midwestern states to the Gulf and into Mexico and Guatemala. It is the most common and widely distributed termite in North America, but does not occur in the western United States. It is similar in appearance to R. hesperus, but is somewhat smaller, and it has similar habits (Smith and Johnston, 1962; Weesner, 1965).
At its western limits, R. flavipes overlaps the range of R. tibialis; in the Great Lakes region, its range meets that of R. arenicola Goellner; and to the south it overlaps the range of R. virginicus (Banks) and R. hageni Banks. It can be distinguished from these species on the basis of size or color of body and wings, color of tibia, location of ocelli, or a combination of these features (Weesner, 1965).
All infestations noted have been in harbor cities, initially in boats, ships, dredges, piers, and floating drydocks, indicating that this species came to the United States aboard ship. It will probably eventually have the same range of distribution in latitude in this country as it has in other areas of the world.
The Formosan termite is a more vigorous and aggressive species than the indigenous North American subterranean termites, indicated by more extensive tube and tunnel building, the rapidity with which a new food source is located and attacked, and the greater variety of materials attacked. Coptotermes formosanus was compared with 2 indigenous termite species of the eastern United States. It ate more and survived better than Reticulitermes flavipes, which in turn ate more and survived better than R. virginicus (Smythe and Carter, 1970). Coptotermes formosanus was found to be much more aggressive in tunnel building than R. flavipes and R. virginicus. This characteristic, coupled with its greater tolerance to the currently used soil insecticides (aldrin, chlordane, dieldrin, and heptachlor) results in its being more difficult to control. Coptotermes formosanus could penetrate 5 cm of soil containing 500 ppm of insecticide, whereas Reticulitermes could not. This is near the recommended concentration for chlordane, and double the concentration recommended for aldrin, dieldrin, and heptachlor. (See table 1, chapter 2.) Pending the completion of field investigations recently in progress, the dosages currently prescribed for soil treatment directed against our native termite species should be increased when treating for C. formosanus (Beal and Smith, 1971).
The most obvious characteristics that distingtush the Formosan termite from native species of subterranean termites are its larger size and pale yellow body color, the oval shape of the head of the soldier (figure 79, 80) compared with the rectangular head of the Reticulitermes soldier (plate I, 3; figure 70, E), and the hairy wings compared with the absence of hairs on the wings of most of the native species.
The habits of the Formosan termite are similar to those of native subterranean termites. They make nests in wood in or on the ground, in hollows they have excavated from tree stumps or posts, or in the hollow spaces in the walls, floors, or attics of buildings. Earthen shelter tubes are built over objects the termites cannot penetrate, such as concrete or pressure-treated wood, although they can penetrate through cracks in the surface of the treated wood to reach interior portions that have not been treated chemically.
The enormous area covered by some Formosan termite gallery systems was revealed by an excavation of such a system in Louisiana in an earth-fill known to be only 10 years old (King and Spink, 1969). The galleries of a single colony totaled about 1,900 ft (580 m) in length, and covered about 1.4 acre (0.57 ha). The galleries ranged from 5 to 117 cm in depth, but in some areas they are known to extend down to a depth of 3 m. The primary nest (figure 81) of the gallery system was found 48 cm below the soil surface. The nest was 53 cm wide and 48 cm high, and was built of carton, a mixture of soil and masticated wood that had been cemented together by the saliva and excrement of the termites. The same kind of material lined the galleries.
The nest was encased in a 7-mm layer of cemented sand, and rested on a layer of cemented carton and sand. Small cavities separated by thin walls of the carton were found throughout the nest. In the center of the nest were 6 physogastric supplementary queens, about 14 mm long, and in near-by cavities were several batches of eggs. In the outer layers of the nest, soldiers were the dominant form, but workers became more numerous toward the center.
The primary king and queen, along with workers and soldiers, were found in a small piece of cypress wood. The gravid queen was 17.5 mm long and 4.5 mm wide at the broadest part of the abdomen. From the nest, galleries extended to adjacent small pieces of wood.
As with other subterranean termites, if only a portion of a Formosan termite colony is destroyed, even if that portion includes the queen (figure 79), supplementary reproductives will develop from individuals in the isolated group.
Warm, sultry evenings, especially following rain, are favorable for extensive flights. The flights usually begin before sundown and end before midnight. Alates of Coptotermes formosanus are strongly attracted to lights. Indigenous subterranean termites (Reticulitermes spp.) fly only during the day.
In California, the western drywood termite is found under natural conditions as far north as Mendocino County and the Sacramento Valley. It is abundant in coastal regions, and extends eastward into Arizona in peripheral desert regions, in mountain canyons, and along streambeds. Colonies of this species have been transported to, and at least temporarily established in, various parts of the United States. Drywood termites are very amenable to accidental distribution because they may infest commonly transported articles, such as boxes, crates, and furniture, can tolerate low moisture conditions for long periods, and the colony is often small, infesting only a small volume of wood, and can therefore be readily transported for long distances.
Unlike the powderpost termite, Cryptotermes brevis, the western drywood termite is seldom seen in furniture and other small wooden products. However, an interesting observation on such an infestation was made by R. E. Wagner in October, 1973 (personal communication). In a bookcase having no contact with wooden members of the house structure, he found many predrilled 3-mm holes, 5 of which had been plugged with the typical drywood termite sealoff. A recently established royal pair of drywood termites was found about 10 mm deep in each hole, evidently individuals from the annual flight of alates. The house had been fumigated about 10 years earlier, but had become reinfested.
In newly developed residential tracts in California, some of the orchard trees are retained in the dooryards. The dead limbs on such trees often harbor drywood termites, particularly if the trees are walnut, and these may serve as sources of infestation for the newer houses. In southern California, homes in new residential tracts tend to become infested by drywood termites sooner and in greater numbers than by subterranean termites (Ebeling, 1968). Most often, drywood termites infest these homes as alates originating in older buildings in near-by areas. However, infested telephone poles, posts, and piles of lumber or firewood can also be sources of further infestation.
The lumberman, builder, and homeowner and, in lawsuits, the lawyer, often inquire as to where, in the sequence of events from the planing mill to the finished home, infestations of such insects as drywood termites, woodboring beetles, and woodwasps originate. It is important to know the life histories and habits of such insects.
Description of Drywood Termite Alate. The alate (plate I, 4; figure 82) is dark brown, and has smoky-black wings with black veins. It may be distinguished from the alate of the western subterranean termite by its larger size (about 11 to 12 mm long) and by the fact that it has a reddish-brown head and thorax, while the subterranean termite is black throughout.
Habits. The first evidences of drywood termite infestation are usually piles of brownish fecal pellets (figure 83) below "kickout" holes or chinks and cracks in the infested wood, particularly where outer walls of the wood member have become excessively thin from prolonged infestation. The pellets are elongate, averaging about 0.85 mm in length, with rounded ends, and with 6 flattened or roundly depressed surfaces. Longitudinal ridges occur at the angles between the 6 surfaces. The shape of the fecal pellet is the result of pressure exerted by 6 plates of rectal epithelium and 6 rectal grooves (Child, 1934). Another indication of the insects' presence may be the flight of alates during warm, sunny days in the fall months.
Besides infesting the dead branches of common native trees and shade and orchard trees, dry-wood termites also infest utility poles, posts, and piles of lumber (particularly sapwood of redwood) in lumberyards. They generally enter houses through attic vents or shingled roofs but, particularly in hot, dry localities, they are often found in the substructure, where they may have entered via the foundation vents. They attack rafters, ridgepoles, and sheathing in the attic, windowframes and sills, door and window jambs, doorsills and, in the substructure, mainly the floor joists and adjoining structural timbers. They may also infest wooden furniture or other wooden materials within the home.
Pence (1956b) found that under extremely dry conditions, individuals of Incisitermes minor in a wood cavity sealed themselves in thoroughly with carton and huddled together to conserve moisture. One individual in such a group survived in kiln-dried wood placed in a silica gel desiccator for 245 days. Its abdomen was then completely flat from loss of water. When given access to water, the termite drank until it became turgid and then continued its life normally. Because most kalotermitids can live in dry wood and do not require shelter tubes leading to the ground, they can damage wooden furniture, even if it is moved about frequently. This, plus the ability of a colony to exist in a very small piece of wood, results in kalotermitids and another nonsubterranean termite species being easily dispersed from one area to another. They often appear in areas far removed from regions in which they are indigenous, and it is then urgent that such localized infestations be eradicated.
The Royal Pair
After a flight of winged reproductives and subsequent breaking off of their wings, a mated pair of drywood termites will select a place to enter wood. Pairs work together to make a hole (figure 84), and then seal themselves in (Harvey, 1934). The hole serves as the entrance to the "royal cells," and is about 10 cm deep. The "royal pair" enlarge the hole, and the queen lays her first eggs. Generally, 2 to 5 nymphs hatch from the first eggs and begin an enlargement of the burrow. The nymphs (plate I, 5; figure 82) perform the duties of the worker caste of the higher termites. The queen then lays more eggs in the advanced part of the main passage of the burrow.
Toward the end of the second year, after the colonizing pair has entered the wood, the colony generally consists of the primary king and queen, one soldier, and a dozen or more nymphs (Harvey, 1934). By this time, the abdomen of the queen has become broader and longer, and she can lay a greater number of eggs (figure 85). From late spring to late fall, the primary queen lays from 1 to 12 eggs each day for 7 to 10 days, ceases egg-laying for a month or more, and then resumes at the same rate as before. Harvey (1934) found that at a temperature of 80 °F (27 °C) and a relative humidity of 83% the eggs in 1- and 2-year- old colonies were hatched in an average of 77 days. He believed that maximum egg-laying capacity is reached when the queen is 10 to 12 years old, and then declines rapidly. A secondary queen then takes over her functions.
The time required for Incisitermes minor to develop from egg to alate or soldier is believed to be more than 1 year. It has been estimated that a colony 15 years old might contain a primary king and queen, 1 or more supplementary reproductives, 120 soldiers, and 2,600 nymphs. Thus, the colonies are small compared with those of the subterranean termites, which may contain millions of individuals.
The relatively small colonies result in correspondingly less rapid and less severe damage to buildings than that which is caused by most subterranean termites, but proliferation of colonies in a building can result in extensive infestation.
During a 4-year period, quarantine officials intercepted Cryptotermes brevis 47 times in cargo entering the continental United States from the Bahamas, Cuba, Hawaii, Honduras, Philippines, and South Africa, and they finally intercepted an extensively infested ship. Piles of fecal pellets were found in wooden bunks and along paneled corridors. To avoid the damage to electronic equipment that would result from the use of methyl bromide, sulfuryl fluoride was used as a fumigant. A total of 282 lb (128 kg) of gas was used to fumigate 70,480 cu ft (1,997 cu m) of space (Mahaney, 1972).
Powderpost termites have been said to have smaller fecal pellets than the drywood termites just discussed (Light, 1934b). However, we were unable to distinguish pellets taken from a live colony of powderpost termites in 5-ply Philippine mahogany veneer, imported from Puerto Rico 5 years earlier, from the pellets of Incisitermes minor; both appeared to be identical in size and shape.
The alates of Cryptotermes brevis are 10 to 11 mm in total length, and their wing membranes are colorless, with brownish veins. The caste that most conspicuously distinguishes this species is the soldier. The head is mostly black, almost as broad as long, and high in front. The strongly truncated head is distinctly concave, and is rough in front (figure 86).
Description. Zootermopsis angusticollis occurs in coastal forests from British Columbia to Baja California. The alates (plate I, 7) may be as long as 25 mm, including wings - about twice the length of the drywood termite. Their color ranges from yellowish brown to cinnamon-brown to chestnut. The soldiers (figure 87) may be 15 to 20 mm long. The head is somewhat narrowed in front, unlike that of a related species, Z. nevadensis, or of Incisitermes minor. figure 88 shows a soldier severing the head of a carpenter ant with its powerful mandibles. The nymphs are white or somewhat cream-colored, but the abdomens of the older nymphs are colored by the intestinal contents, which can be seen through the integument (plate I, 8; figure 87).
As with drywood termites, fecal pellets, colored according to the kind of food eaten by the termite, may be found throughout the colony. The pellets are hard, usually elongate, rounded at both ends, but are only slightly hexagonal and lack the longitudinal ridges that give drywood termite pellets their strongly sculptured appearance. The pellets of Z. angusticollis are about 1 mm long, compared with an average of 0.85 mm for drywood termites. If the wood is extremely damp, the pellets are often spherical or irregular, and usually stick to the sides of the galleries. In drier wood, the pellets collect in the bottom of various chambers, or the termites may expel them the way drywood termites do.
Swarming. Most swarming takes place during August, September, and October, on warm, sultry evenings just before sunset, but alates may occasionally be found flying in every month of the year. They often appear after the early rains, and are attracted to lights.
Damage. Damage from dampwood termites can be greater than from subterranean termites, for they have more of a tendency to work their way upward from the foundation to the roof rafters. In one instance in San Francisco, they were found on the fifth floor of a concrete fire-proof building (Mallis, 1969). Infestations of dampwood termites may be found wherever there are wood-earth contacts. Poles, piling, bridge timbers, and other structures built over or near water are subject to attack. Down timber or, in the city, old, infested buildings serve as reservoirs for swarmers and invite termite attack.
Although Zootermopsis species have been called "rottenwood termites," Weesner (1965) pointed out that colonizing pairs are usually situated in sound wood, including felled logs, standing dead trees, and dead portions of live trees, and that only mature colonies invade rotten wood.
In the coastal areas, Z. nevadensis swarms mostly in summer and early fall, but in inland areas at higher elevations, the peak of the swarming appears to occur in the spring. This species seems to have about the same capacity for destruction of wood as Z. angusticollis, but it does not occur in the more densely populated areas and therefore is not so often recognized as a pest.
Gnathamitermes perplexus (Banks) is a large, brown species, reaching a length of 16 mm, including wings. It is the common desert termite in California and Arizona, and extends into almost all the southwestern desert regions (Weesner, 1965). It is primarily a soil-dweller, but builds extensive earthen workings aboveground to cover vegetation, stumps, posts, and wooden structures. This behavior, plus the superficial damage the termite can do to wood, results in it being an occasional pest. A similar species, G. tubiformans (Buckley), occurs in Texas, New Mexico, and Mexico.
Amitermes spp. are somewhat smaller than Gnathamitermes, rarely exceeding 12 to 13 mm in length, including wings. The alates are dark, and even black. The soldiers have mandibles usually less than 75% as long as the width of the head, and the outer margins are recurved from the base to the tip. Unlike Gnathamitermes, Amitermes spp. do not build extensive earthen structures aboveground. They attack wood directly, entering at the point where it contacts the ground, and are greater economic hazards than other desert-dwelling members of the family Termitidae (Weesner, 1965). Amitermes spp. have been known to girdle and kill young citrus trees.
Subterranean and drywood termites require entirely different control measures. Subterranean termites have their nests in the ground, and control consists of structurally or chemically isolating the wood structures of a building from these nests.
Such measures have no value against drywood termites. Drywood termites remain in infested wood, and must be controlled there. Fumigation of a building for drywood termites would kill the subterranean termites that happened to be in the wood structure at the time, but would not destroy the entire colony, and in fact would not kill the reproductive or potentially reproductive individuals. Therefore, the control of subterranean and drywood termites will be discussed in separate sections.
Successful treatment generally depends on a thorough and accurate diagnosis. Although swarming termites (flights of winged reproductives called alates) are often a sign that wood members of a building are infested, this is not true in all cases. For example, termites may inhabit an earth-filled porch, feeding in buried wood that was carelessly shoveled in when the original fill was made (figure 89). If the porch was properly sealed away from the foundation, the termites may not have had access to wood members of the building, and the alates may have emerged from cracks in the porch cap or other openings reaching the earth fill.
Important signs of subterranean termite infestation are shelter tubes (figure 72, this chapter). Shelter tubes are proof of termite infestation, but their absence does not necessarily mean that a structure is free of termites. The insects may reach sills and other wood members through cracks or voids in the foundation wall, under the outside stucco, or from earth-filled porches, steps, terraces, or patios. Among California homes with no wood contact with the ground that are infested with subterranean termites, more than half of the infestations originate in earth-filled extensions of the main foundation.
Moisture caused by condensation or leakage invites termite attack or dry rot. This is particularly true in subframing adjacent to the firstfloor showers, bathrooms, toilets, sinks, laundry facilities, and other plumbed areas. All moist areas of a building should be examined with particular care and the source of moisture eliminated. Blisters or darkened areas on the flooring sometimes indicate the presence of termites. Such areas are easily crushed, sometimes revealing the light-colored termite workers and a few soldiers.
A job for Professionals. Locating termite colonies in a building and determining the best way to combat the insects is very difficult and strenuous work. The homeowner can seldom inspect so thoroughly as the trained and experienced inspector that a professional termite operator would send out to do the work. The inspector must crawl over dust, mud, and debris, often in a space barely high enough to admit his body. He must crawl the entire distance around the inside of the foundation, inspecting and probing the wood structure, and must investigate areas under showers, sinks, and in the vicinity of floor furnaces.
Locating the source of a termite infestation is particularly difficult when the building has been constructed on a concrete slab, for the inspector cannot crawl beneath the floor as he can when inspecting joist-type construction. Generally, the termite operator must guess where to drill through the slab to inject his soil insecticide.
A reliable operator will provide the homeowner with a diagram of the inspected premises, indicating the points of infestation and hazardous conditions, and an estimate of control and repair cost. In areas where drywood termites are a potential problem, inspection for these insects should be included, and usually requires access to the attic area. On the diagram, the inspector will indicate with the letter "S" the approximate locations where subterranean termites or their damage are found. The letter "K" (for the old genus Kalotermes) is indicated where drywood termites are found or suspected.
It takes a long time for termites to cause appreciable damage in addition to that which is revealed by the inspection. There is rarely cause for an operator to impress the homeowner with the urgency of immediate action. If a great deal of work is recommended, it may be wise to investigate the reliability of the operator and to compare his diagnosis and competitive bid for the job with those of other termite operators.
Along with his diagram of the locations of termites and termite-conducive conditions, the inspector will make an estimate of the cost of control and if necessary, for repair or replacement of damaged wood. Some termite operators offer free inspections, others charge an inspection fee if no further work is done on the inspected premises, and some operators include an inspection fee in their estimate of the work to be done. The homeowner may arrange for a periodic inspection service done by experienced personnel, usually once a year.
The Standard Report Form. In California, all termite control companies, in accordance with state law, must file a standard report form with the Structural Pest Control Board for every piece of property their agents inspect. This is mandatory, regardless of whether an infestation is found or an inspection fee is charged. Copies of all reports filed within the preceding 3 years are available from the board for a nominal fee. The condition of a property with regard to structural pests is thus fully documented and disclosed.
The termite reports are usually requested by the escrow company during real estate transactions. They are also generally required by any lending institutions. However, a termite inspection is not legally required when a house changes ownership. The buyer or his lending agent must ask for it. It is a good idea to have a termite inspection before buying a house, even if the buyer has to pay for it.
The termite reports made by inspectors also serve as a basis on which state investigators may examine the competency of termite operators against whom complaints of substandard work or fraudulent practices have been lodged. Should comparison of the reports of several pest control firms indicate that the complaints against one of these firms are valid, further direct investigation is conducted, and can ultimately result in revocation of licenses held by the firm or its representative or both.
Faulty Home Construction and Maintenance
No method of construction or treatment has yet been devised that is an absolute guarantee against infestation by subterranean termites. The builder and homeowner can take certain measures to decrease the possibility of infestation. Figure 90 is a sketch of a residence, showing where such preventive measures can be taken.
Wood-to-Ground Contacts. Wood members should never be allowed to rest on the ground, but should rest on concrete foundations or piers. These are ordinarily not susceptible to termite penetration, although the insects may gain access to wood through cracks that develop in the concrete (figure 90 ). Termites can build shelter tubes over the surface of concrete, but the tubes are then exposed and may be destroyed. The ground below the tubes may then be chemically treated.
Specifications of the Uniform Building Code should be followed, particularly those pertaining to the minimum clearance between wood members of the substructure and the ground, the size, location, and screening of vents, and the installation of termite shields where required. The rules for foundation clearance and ventilation also pertain to wood-frame porches and steps. Nowhere should wood be allowed to come in contact with the ground (figure 90 , , , , and ). Attention to building code specifications will be of little avail if earth is piled against the foundation or pier posts after construction (figure 90 ). Planter boxes built on the ground should be separated from the main structure by at least 4 in. (10 cm); if possible, they should be kept below the level of the first wood in contact with the foundation wall.
To eliminate breeding places for termites in or on the soil, buried stumps, logs, and roots should be removed from the building site. Scrap lumber should be removed from the ground under the building after its completion (figure 90 ).
Care should be taken not to include wood scraps with the earth used to fill extensions of the foundation, such as porches, terraces, patios, and steps. This is very important, for termite colonies in these fills appear to be the sources of over half the subterranean termite infestations of structures. Even if these fills are properly sealed away from the foundation at the time of construction, earthquakes, sonic booms, or subsidence of the earth may break the seal, allowing termites within them to reach the mudsill and other wooden members.
Foundation forms and stakes should be completely removed (figure 90 ). If parts of forms and broken stakes have been left in the ground and cannot be removed, they may be soaked with an oilborne toxicant.
Avoidance of Excess Moisture. Because subterranean termites are favored by moist soil, every practicable site-grading and construction measure should be employed to prevent accumulation of moisture around and under the house. In addition, vents (figure 90 ) should be sufficiently large and numerous to remove moist air from the area beneath the house, and should be placed so as to provide cross-ventilation wherever practicable. Shrubbery should not be allowed to obstruct the vents (figure 90 ). Sprinkling against the sides of stucco houses should be avoided or minimized because the resulting dampness favors termite infestation. Termites have access to the mudsill from outside the foundation if stucco has become separated from it. Stucco will adhere to the foundation more firmly if the latter is "dashed" with a rich cement mixture before the stucco is applied.
All leaking faucets or other plumbing leaks should be corrected, and downspouts should carry water away from the building (figure 90 , ). There is often no practicable repair measure for shower-stall leaks. The shower-stall pan may be replaced with copper, lead, fiberglass, or masonry pans. A watertight wall should overlap the pan interior. The wood used in framing the pan should be pressure-treated with appropriate chemical preservatives to avoid termite infestation or fungal infection.
If drainage and ventilation do not reduce soil moisture sufficiently, it is helpful to cover exposed soil with heavy roofing paper. In the case of slab-on-ground construction, a "vapor barrier," usually consisting of a sheet of some synthetic material impervious to water, may be placed on the ground before the concrete for the slab is poured.
Besides reducing the possibility of termite infestation, measures that decrease under-area moisture also decrease the likelihood of decay from fungal infection. They may also prevent warping of floors and doors, result in greater comfort in summer by reducing humidity in the living space of the home, and save fuel in winter by preventing dampness.
Chemical Treatment of Lumber
Lumber "pressure-treated" with an approved wood preservative such as pentachlorophenol (C6Cl5OH) or Wolman salts resists attack by termites and fungi. It is considered too expensive to treat all lumber, even in the substructure, so treated lumber is ordinarily used only where it is most subject to attack by termites or decay. Since most of this lumber is structurally vital and difficult to replace, the cost of treatment is well justified.
Lumber thus treated offers the additional advantage of forcing the termites to tube around it and thus expose their activities to view. In an experiment in which pressure-treated wood, with untreated wood on top of it, was in contact with the ground for 4 years, termites tubed over the treated wood in 5 out of 20 samples, and damaged the untreated wood on top. Where more than 0.0625% of aldrin, chlordane, dieldrin, or heptachlor was added to the wood preservative, termites were not able to tube over the treated wood during the same period (Beal, 1969).
When wood is subjected to leaching, it is best protected by oils or oilborne preservatives. When the treated wood will not be in contact with the ground or water, or where it may require painting, waterborne preservatives are generally used. Chemicals and their uses are given in (1) Federal Use Specification TT-W-571d (current revision), (2) Standards of the American Wood Preservers' Association, and (3) Standards of the National Woodwork Manufacturers' Association. A brush coat of a termite- and fungus-resistant solution is helpful for treating sills and other wood members subject to termite attack if they have not been pressure-treated, but it is not so effective in all cases as pressure treatment.
Sills also have been constructed of naturally durable or naturally resistant wood, having extractives making them resistant and/or repellent to termites.
As used in the United States, grade-marked "foundation-grade" California redwood or 100% heartwood of southern tidewater red cypress, or very pitchy southern pine (lightwood), or heartwood of eastern red cedar (less resistant than the above) are suggested when durable wood is desired (St. George et al., 1963). It is generally recognized, however, that pressure-treated wood is more reliable.
A structural building board composed of wood elements coated with Portland cement, frequently referred to as "century board," was found to be resistant to 2 termite species, Coptotermes formosanus and Zootermopsis angusticollis. The resistant property of wood coated with cement was found to be the result of the creation of a physical barrier into which the termites were unable to penetrate (Allen and Dolan, 1970).
The addition of long-lasting toxicants to concrete foundations can deter subterranean termites, and is often useful even where soil treatment is made. In Australia, Gay and Wetherly (1959) substituted a 0.5% emulsion of dieldrin for the normal mixing water to prepare a termite-proof concrete. They found that workers of Nasutitermes exitiosus and Coptotermes lacteus, confined in cylinders of treated concrete, were affected after 2 hours of contact, and that complete mortality occurred in 24 hours; exposure of 1 hour was sufficient for complete mortality. They also found that while the termites could make their way through a "no-fines" concrete containing only coarse aggregates, this type of concrete, when treated with dieldrin, was not penetrated. F. J. Gay (correspondence) stated that laboratory tests had shown aldrin to be as effective as dieldrin when used in the same way and at the same concentration. The strength of the concrete was not adversely affected by the treatment, and accelerated weathering procedures did not destroy its insecticidal effectiveness.
In the United States, a 75% dieldrin wettable powder was added to water used in concrete, so that concentrations of the toxicant in cement blocks were approximately 0.1 and 1.6% by weight. A week later, the insecticide in these blocks resulted in a 100% mortality for Reticulitermes flavipes workers having contact with them for 1 minute, and for Nasutitermes columbicus, nasutes exposed 10 minutes (Allen et al., 1961). After 16 months, the newly cracked surfaces of laboratory-aged mixtures were equivalent in toxicity to the original surfaces of new mixtures (Allen et al., 1964a).
Information has recently become available on tests with insecticide-treated blocks placed on soil in the termite-infested pine forest in Gulfport, Mississippi, where so many tests had been made by the USDA Southern Forest Experiment Station with insecticide-treated boards and stakes (see "Insecticides and Concentrations," following). concrete blocks of the standard mix for slab on-ground construction, containing chlordane and dieldrin wettable powders (WP) and emulsifiable concentrate (EC) at concentrations from 0.04 to 1.33% were placed on ground where subterranean termites (Reticulitermes spp.) were known to be abundant. Wooden bait boards 1 x 6 x 6 in. (2.5 x 15 x 15 cm) were placed under the concrete blocks to encourage termite activity, and on top of the blocks to better determine the presence of termites. Measures were employed to provide humid, shaded conditions for 1 year, and then wooden covers were placed on the units to simulate conditions under a house. After 5 years of exposure under field conditions, termites had not tubed over any of the dieldrin-treated blocks, and on only 1 of 10 blocks treated with 0.125 and 0.25% chlordane WP. Five of the 10 untreated blocks were completely tubed over in 2 years (Beal, 1971).
Under actual foundation construction practice, treatments would probably be even more effective than the field tests indicated, for about a third of the water in concrete runs off the top of the slab during construction, and would be present in the soil for long periods, as soil treatments have indicated (Johnston et al., 1971). However, at the time of publication of the report, the use of insecticides for termiteproofing of concrete had not yet been registered by appropriate state and federal agencies (Beal, 1971). It appears that dieldrin is banned for any type of termite control other than soil treatment, so it is unlikely that anyone will seek registration of the insecticide for use in concrete.
Metal shields have usually been of little or no practical value against termites, principally because they are seldom installed properly. Even when shields are of proper design and materials and are properly installed, termites may build tubes over their edges. Some termite operators have found the sharp edges of the shields to be hazardous when they are inspecting or doing repair work under a building. Shields appear to be favored nesting places for Argentine ants. It is not uncommon for disintegration of the metal to take place within a few months when such nests are present, presumably because of the formic acid given off by the ants (Scott, 1960). The undersides of shields are often filled with carton by termites, thus presenting no barriers to them (Isherwood, 1957).
Correction of Structural Deficiencies
After a building is constructed, termite control may involve structural alterations or repairs that correct certain adverse environmental conditions.
Grade. Older buildings sometimes have only "crawling ditches" to allow inspection and chemical treatment. If a building does not have at least 18 in. (45 cm) of clearance between the soil and the lowest horizontal wooden members of the substructure, the inside grade should be lowered by excavation. In areas with high water tables, lowering the inside grade and digging ditches are undesirable if excessive dampness and/or standing water develop.
Where frame or stucco construction is used, additional foundation material is sometimes added to bring the sill at least 6 in. (15 cm) above grade. As a rule, a few linear feet of the mudsill and cripples are removed at a time, the new foundation is built up to the desired height, and a new substructure is built upon it. In certain cases, particularly where low foundations and resultant inadequate clearance beneath the building exist, the entire structure may be elevated and the whole foundation is raised to meet it.
Gutters. When wood sills on foundation walls are at or below grade, a gutter may be required to lower the soil or grade level to below the sill. The bottom of the gutter should be at least 4 in. (10 cm) below the sill. Gutters are usually made of preformed concrete slabs or poured concrete. Preformed slabs are used as retaining walls and as the bottoms of gutters. Joints should be properly filled with hot pitch, and the gutter should be at least 8 in. (20 cm) wide to allow removal of debris.
Flash Wall. The flash wall (figure 91) is a solid, poured-concrete facing, firmly bonded to the foundation, and extending at least 4 in. (10 cm) below the top of the foundation and a minimum of 3 in. (8 cm) above the natural grade level. It should be a minimum of 3 in. thick, and the top must be sloped outward to provide adequate drainage away from the wall of the house.
The flash wall is used as a relatively inexpensive alternative to raising the foundation when the latter procedure would normally be indicated - as when the sill is below the top of an adjacent concrete slab or below the earth grade. It is most justified when raising the foundation would be excessively expensive, as when the floor joists rest on the sill. Flash walls have often proved unsatisfactory because water may enter from above, and the flash wall is likely to separate from the foundation, thus providing a hidden access for termites.
The earth fill that is contained in concrete porches, terraces, patios, and steps has already been discussed as a potential source of subterranean termites. Although the earth is originally sealed away from the mudsill, this seal may be broken by earthquakes, sonic booms, gradual subsidence of the earth, or by temperature extremes. Termites then have access to the wood. To remedy this, termite operators usually cut out a strip of the slab of the porch, terrace, etc., adiacent to the wall of the building, and excavate the earth fill until at least 4 in. (10 cm) of the foundation are exposed. The exposed soil is always saturated with a suitable liquid insecticide, and reinfestation in the treated area is unlikely. The exposed portion of the foundation and the cut edges of the slab caps are then cleaned, and cracks (if any) in the foundation are flushed with toxicant and sealed. Concrete is then poured into the excavated area (figure 92). This procedure is particularly common in California and is called the "cut-and-seal" or "sealoff."
An ornamental wall adjacent to a porch slab (figure 92, lower left) was once commonly contstructed in California. The sill in this wall is vulnerable to termite infestation. Mechanical alteration consists of removing a section of slab along the wall and sealing the earth away from the foundation with concrete, as just described. The soil is first treated with liquid insecticide and cracks, if any, in the foundation wall are flushed with toxicant and sealed.
Treatment of Wood Substructure with Pentachlorophenol Emulsion
Pentachlorophenol, C6Cl5OH, is one of the principal toxicants used for pretreatment of timber destined for residential construction, telephone poles, railway ties, fenceposts, etc. "Mayonnaise-type" oil emulsions containing 10% pentachlorophenol are known as "Woodtreat-TC," and have long been used for applying a maximum quantity of termiticide and fungicide on subarea wood members not pretreated before construction. Currently Woodtreat-TC also contains 0.5%, technical heptachlor (0.36% actual heptachlor).
Using a paintbrush or a specially designed "caulking gun," mayonnaise-type emulsions can be applied in a thick layer containing about 20 times more toxicant than can be applied with a single brush coat of an unemulsified oil solution. For brush application, the emulsion should be stirred before use until it has a creamy consistency, for it is then easier to apply.
As the emulsion breaks, the toxic solution is gradually released to penetrate into the wood. Termites have a tendency to burrow as closely as possible to the wood surface without causing the infested wood member to collapse. Therefore, penetration of toxicant into the galleries is quite likely. Woodtreat-TC can be applied to such wood members as mudsills or floor plates, either for prevention of infestation by termites or to get rid of an existing infestation. It is particularly effective when applied generously where 2 wood members come together, as at the intersection of a girder, joist, or cripple on a mudsill, or where a stud meets a floor plate.
Aside from preventing conditions favorable to subterranean termites, such as a damp subarea or a dangerously high grade level, either outside or inside the foundation, the application of Woodtreat-TC to termite-infested or termite-susceptible wood members is probably the most feasible thing the homeowner can do for himself. The author controlled 2 subterranean termite infestations in his own home, one of them originating from an earth-filled patio, with Woodtreat-TC which was applied liberally on the sill and at the intersection of the sill and cripples in the entire infested area beneath the house. In his garage, he applied the material at the intersections of all studs and the mudsill. The infestation of subterranean termites had been so severe that the tapping noise of termite soldiers could be heard easily every time the garage door was opened. The tapping noise is made by the termite soldier by banging its massive head up and down against the ceiling and floor of the gallery. This is considered to be an alarm signal (Stuart, 1969). The tapping noises are detected through the subgenual organs in the termites' legs (Howse, 1970). The treatment appears to have been completely successful. Although a portion of the sill is below grade level and becomes wet during every heavy rain, the infestation has not reappeared in the 12-year period since the treatment was applied.
When applying Woodtreat-TC, inhalation of vapor should be avoided as much as possible, and if skin is accidentally contaminated, the material should be removed with soap and water. If much work with it is contemplated, chemical-resistant gloves and a respirator designed to remove organic vapors should be worn.
Pretreatment of Soil with Insecticides
One of the most important means of isolating a building from subterranean termites is the insecticide-treated soil barrier. Soil treatment is most effectively done before and during construction of the foundation, and this is called "pretreatment." It is of particular importance when concrete slab-on-ground construction is employed, for treatment of the soil is difficult after the concrete has been poured. Concrete slab was once stated to be a termite barrier, but experience has proved that this is not the case.
There are 3 types of slab-on-ground construction: monolithic, suspended slab, and floating slab. Figure 93, A, represents a cross section of the monolithic type, in which the floor (cap) and footing are poured in one continuous operation so that there are no "cold joints" that might eventually form cracks and allow access for subterranean termites. At B is shown a "suspended slab" extending completely across the top of the foundation. The foundation and the floor are constructed as independent units. Even though a vertical crack may develop in the foundation, this type of construction prevents enlargement of the crack. With both these types of construction, chances for entry of termites around the foundation are minimal, but other avenues of termite entry (such as cracks in the cap slab, or apertures around utility pipes) are as common as with other types of slab-on-ground construction.
In figure 93, C and D show "floating slab" construction, in which the slab or floor either rests on a ledge of the foundation or is independent of it. Entry for termites is made possible 'by expansion joints. The difference in the coefficient of expansion between the concrete and the filler in the expansion joints results in a pulling away of these materials from each other as soon as there is sufficient temperature change. This is also true of the supposedly termiteproof fillers or caulking substances placed around utility pipes extending through a slab.
Termites also penetrate through cracks that can form anywhere in the slab. The incidence of such cracks is greatly minimized by wire-mesh reinforcement of the concrete cap.
The principal difficulty with slab-on-ground, as far as termites are concerned, is that this type of construction makes treatment more difficult and expensive than where raised-foundation (joist-type) construction is employed.
Insecticides and Concentrations for Soil Treatment. The recommended insecticides for either pretreatment or for treatment of known infestations at any period after construction are the same: aldrin at 0.5% chlordane at 1%, dieldrin at 0.5%, and heptachlor at 0.5%. These insecticides may be used either as water emulsions or in solution in an oil not heavier than No. 2 fuel oil. Oil solutions should not be used under concrete slabs, for they may penetrate to the surface of the slab, causing unsightly stains or weakening the adhesives used for parquet flooring or tiles.
The insecticides are purchased in the form of emulsifiable concentrates that differ in the percentage of toxicant each contains. The concentrate must be diluted with the correct volume of water to obtain a solution for application that contains the recommended amount of toxicant for termite control. Emulsifiable concentrates blend readily with water. The globules of insecticide are largely sieved out by the first 0.75 in. (2 cm) of soil through which they pass (Ebeling and Pence, 1958; Beal and Carter, 1968). The insecticide penetrates to a greater depth in moist soil than it does in dry soil (Beal and Carter, 1968). Despite the shallow insecticide barrier, good protection is obtained against termites. The remarkable efficiency of this barrier is indicated by experiments of the United States Forest Service's Southern Forest Experiment Station at Gulfport, Mississippi (,Johnston, 1960). Boards were laid on treated soil, or wood stakes were driven into it, in forested land heavily infested with the eastern subterranean termite, Reticulitermes flavipes (figure 94). Aldrin, chlordane, dieldrin, and heptachlor have given complete protection for periods of 23 to 27 years to this date (1975), depending on when the material was applied (Johnston et al., 1971; Smith et al., 1972). Test boards and stakes have always been destroyed by termites within a year in untreated plots in these tests. Tests with granular formulations were begun much later, but the granules have provided a 100% control for a 12-year period where 0.031% dieldrin or 0.062% aldrin, chlordane, or heptachlor were used. Granules with a lower percentage of insecticide have already failed (Smith et al., 1972). [Labels for aldrin and dieldrin granules to be used for termite soil treatment have been voluntarily withdrawn by the sole registrant (Ruckelshaus, 1972b).]
In Hawaii, experiments were begun in 1958 to investigate the relative persistence of 7 insecticides which were mixed with 3 different types of soil and poured into prepared holes in randomized plots. Dosages of insecticide were as recommended by the manufacturer, and at 25% of those dosages. In 6 months, and at yearly intervals for 9 years, samples were used for laboratory bioassay tests, with the subterranean termite Coptotermes formosanus used as the test insect. The relative persistence of the insecticides, based on the percentage of the quantity applied that remained after 7 years, was in the order DDT > dieldrin > chlordane > aldrin and converted dieldrin > heptachlor and heptachlor epoxide > lindane > sodium arsenite. The relative toxicity of the treated soil samples to termites was in the order dieldrin > aldrin > heptachlor > chlordane > DDT > lindane > sodium arsenite (Bess et al., 1966). Later, bioassays showed that also after 9 years, "dieldrin and aldrin were clearly superior to the other termiticides in the test" (Bess and Hylin, 1970).
When the emulsifiable concentrate is diluted with a fuel-oil carrier, the insecticide stays in solution regardless of how deeply the solution penetrates. Therefore, fuel-oil solutions are useful when it is desirable for the insecticide to penetrate to a greater depth in a limited area, for instance, around a stake that cannot be removed. A limited degree of wood penetration, particularly in the direction of the grain, is also possible with fuel-oil solutions.
In the United States Forest Service tests in Mississippi, benzene hexachloride (BHC), at a concentration of 0.8% of the gamma isomer (lindane), was 100% effective for only 8 years, but heavy rainfall (about 2 m annually) in the test area may have adversely affected lindane, which is soluble in water to the extent of 10 ppm. The initial toxicity of lindane to termites is extremely high (Ebeling and Pence, 1958), and this insecticide is often used in southern California in attempts to exterminate termites under slabs. Lindane is relatively volatile, but when the objective is to create an effective barrier throughout the area beneath a concrete slab, volatility could be beneficial, even though the more volatile insecticides are generally the least persistent under other conditions (Bess et al, 1966).
Under the slab-on-ground foundations of 32 houses, Fahey and Osmun (1971) recovered 49% of the lindane applied 10 years earlier. This was a greater percentage than was recovered from the other organochlorine insecticides, although the latter are ordinarily more persistent. The percentages of aldrin, heptachlor, chlordane, and dieldrin recovered were 35.1, 37.8, 41.2, and 46.2, respectively.
Application Procedure and Dosage. Termite operators generally apply insecticide emulsions with power sprayers (figure 95), but high pressure is not required, and it is possible to apply the emulsions with a sprinkling can. To treat the ground where a concrete slab is to be poured, including attached porches, apply 1 gal (about 4 l) of the diluted insecticide emulsion to 10 sq ft (about 1 sq m) as an over-all treatment. Over-all treatments applied to washed and ungraded gravel fills, or fills of an absorbent material (e.g., cinders), should be increased by 0.5 gal per 10 sq ft (2 l per 1 sq m). Apply 2 gal per 5 linear ft (7.5 l per 1.5 m) to critical areas, such as along the inside of foundation walls and around utility pipes, entrances, and interior partition foundation walls; also, use 2 gal per 5 linear ft along the outside of the foundation.
Ideally, a narrow trench should be dug around the foundation to within about 1 ft (30 cm) above the foundation footing. The soil should be treated as it is replaced at the rate of 1 gal (4 l) per 5 linear ft for each foot of depth from grade level to footing, but allowing 2 gal per 5 linear ft for the first foot in depth from ground level. The treated backfill from the trench should then be covered with a layer of soil. According to the Environmental Protection Agency's instructions, the trench should be no wider than 6 in. (15 cm) (Ruckelshaus, 1972b).
Although the dosage for the over-all treatment under a building should average 1 gal per 10 sq ft (about 4 l to 1 sq m), in actual practice, termite operators tend to apply more insecticide emulsion where the chances for termite penetration are greatest, as around utility pipes, and less, or even no emulsion, in other areas. According to EPA instructions, if concrete foundation slabs are to be poured on treated soil, a polyethylene sheeting or other waterproof material shall be placed over the treated soil, unless the concrete is to be poured on the day of the treatment. This is to avoid washing away of insecticide by rain (Ruckelshaus, 1972b).
Unless treated ground or fill is to be promptly covered with a vapor barrier or by the slab, precautions must be taken to prevent disturbance of the treated soil that might break the continuity of the insecticide barrier. To avoid surface flow of the spray liquid away from the application site, treatments should not be made when the soil or fill is very wet, or after heavy rains.
In the treatment of joist-type houses at the time of construction, generally not all the crawl space is sprayed. Two gal (7.5 l) of the dilate emulsion is applied per 5 linear ft (1.5 m) to the inside of foundation walls and around piers and utility entrances.
Along the outside of foundation walls, including the part opposite entrance platforms, porches, etc., apply 2 gal per 5 linear ft where the foundation is deep, and apply 1 gal per 10 sq ft (4 l per 1 sq m) of soil surface as an overall treatment only when the attached porches, entrance platforms, utility entrances, etc., have covering slabs on fill or ground. Treat all voids of masonry block walls and piers, using 1 gal per 5 linear ft applied from grade to footing.
Is Soil Treatment for Subterranean Termites an Environmental Hazard? To be effective, soil treatment for subterranean termites should result in a very long-lasting barrier. As stated earlier, tests made by the United States Forest Service have already demonstrated that certain chlorinated hydrocarbons have the ability to create such a barrier for at least 21 to 25 years, and no one knows how much longer. Environmentalists may inquire as to the possibility of pollution from such long-lasting residues. Experiments have shown that there is neither downward nor lateral migration of insecticide that is of any practical consequence.
In tests with emulsions of 0.5% dieldrin and heptachlor and 1% chlordane made with 7 soils from various parts of the United States and at a dosage of 1 pt per sq ft (about 0.5 l per 930 sq cm) of soil surface, in most tests more than half of the insecticide was found in the upper 0.75 in. (2 cm) of soil. The emulsion penetrated least in arid soils from Arizona and eastern Oregon and most in soil from Missouri. An increase in soil moisture at the time of application increased penetration, but in any case only extremely small percentages of insecticide reached to a depth of 5 in. (13 cm) within 24 hours or within 4 years (Beal and Carter, 1968; Carter et al., 1970). Considering the inconsequential migration of the insecticides from the application site and their insolubility in water, soil treatment for termite control does not appear to offer any significant threat of environmental pollution.
Soil residues can be physically removed, as when soil is eroded by water in floods, etc., but this is rarely a problem in treated soil under buildings.
Rate of Infestation of Newly Constructed Residences
The decision of the prospective buyer of a new home as to whether the cost of a pretreatment is a justifiable insurance for his investment depends on how soon it is likely to become infested with termites. Extensive field experiments were made in large areas in southern California to determine the rate of infestation by termites in newly constructed residences built in tracts that had just been devoted to citrus or walnut growing. Soil was treated in lots on which houses with adjoining garages were to be built, using at least 8 insecticides in each of 12 tracts where slab-on-ground foundations were constructed, and in each of 10 tracts where joist-type (raised) foundations were used. In each tract, untreated lots equal in number to treated ones were left as checks (Ebeling and Wagner, 1962).
In cooperation with the California Structural Pest Control Board, a study was made of the incidence of termite infestation for 11 years after the lots were treated. On 90 treated and 86 untreated lots inspected, on which slab foundations were used, no infestation of subterranean termites was found. Of the 77 inspected treated lots in which the joist-type foundations were used for the houses, 1 infestation of structural timber was found in a garage (slab foundation) where the ground had been treated with 20% toxaphene. The infestation was obviously the extension of a vigorous colony over which the building was constructed.
No termites were discovered in any of the houses. However, they were found in wood debris on the ground under 5 of the houses. Of the 72 joist-type houses on untreated soil, infested wood debris was found under 7, and termites were also found in a form board that had not been removed after the concrete was poured. Thus, in 11 years, subterranean termites were found in the wood structure of only 1 building (a garage) out of a total of 167 treated and 158 untreated properties (Ebeling, 1968). Subsequent inspection reports of termite operators, obtained from the Structural Pest Control Board, showed the dates of transfer of ownership of the above properties and indicated that they had changed ownership averaging about once every 3.5 years. The above data show that a very long period is required for subterrancan termites to become established in southern California in new tracts when construction is in accord with modern building codes. On the other hand, in many areas where houses are 35 to 50 years old, nearly every house in the block has had one or more infestations of both subterranean and drywood termites. It must be remembered that most of these older structures would fall far short of current building regulations, particularly relating to earth-wood contacts, grade levels, and clearance. These factors, along with the greater period of exposure to termite attacks, are probably responsible for the record of heavy infestation by subterranean termites in the older buildings.
In some areas, as in the forested regions of the Gulf states, much of the ground is continuously infested with subterranean termites. The termite colony may be very large when it first attacks a new building, having developed to that size on subterranean and surface food that was available before the land was cleared for construction. In such areas, a large percentage of new houses can be expected to be infested in less than 2 years, and in some cases damage from termites is evident before construction is complete (Redd, 1957).
Drywood termites infest a greater percentage of residential properties in southern California, and reach them sooner than do subterranean termites. In 11 years following construction, dry-wood termites were found in 35 out of 325, or 10.8%, of inspected properties (Ebeling, 1968). They tended to spread in the direction of the prevailing winds from infested buildings, poles, dead tree branches, etc. In garages that usually had their doors left open during the day, the incidence of infestation was much greater than in those in which the doors were usually kept closed.
Treatment After Construction
Treatment after construction is not greatly different from pretreatment for buildings on raised foundations, except for the earth-filled extensions of the foundation, such as porches, patios, etc. For these, it is necessary to drill through the concrete cap to apply insecticide. Sometimes termites eventually build shelter tubes from the ground to the nearest timber, or there may be many swarming tubes, particularly around a floor furnace. Sprays can then be applied to the entire subfloor area, as well as to the inside of foundation walls. The entire subfloor area is also generally sprayed if termites have been found in scraps of wood on the ground, even though the wood is removed.
A Special Problem with Slab Foundations. Postconstruction treatment of buildings with slab-on-ground construction is much more difficult than pretreatment. Holes must be drilled either downward through the concrete or horizontally through the foundation in order to inject an insecticide. For vertical injection, the proximity of the holes to one another should depend on the nature of the earth or fill below the slab, If the slab is underlain by heavy soil, 3 ft (1 m) between the holes may be appropriate; if underlain by sand fill or gravel, 6-ft (2-m) spacing may be adequate. A sound procedure is to drill holes about 6 ft apart and observe whether the liquid is forced out of adjacent holes. If not, more holes should be drilled until there are enough to provide reasonably complete coverage of the subslab area.
Because of the hazard of drilling through radiant-heating pipes and plumbing, or the possibility of destroying a vapor barrier, as well as the expense and hazard incurred in removing tile, parquet, etc., many termite operators treat under slabs through the foundation wall.
Horizontal Rodding. Horizontal subslab rodding involves drilling holes through the foundation at a height that allows entry immediately below the usual 4-in. (10-cm) cap of the slab. Figure 96, A, shows a 22-ft (6.6-m) iron pipe being inserted into one of these ho!es. The pipe was pushed in to a distance of 20 ft (about 6 m) with a back-and-forth motion while insecticide was flowing at 4 to 5 gal (15 to 19 l) per minute. A neoprene cone or stopper over the pipe prevented premature backflow of the liquid (figure 96, C). The figure also shows that the liquid had spread laterally for a distance of 6 ft (2 m). The lateral spread is believed to be uniformly greater than 3 ft (1 m) as the pipe is pushed in, and it is also believed that the grid pattern formed by pushing the pipe in from all sides of the foundation results in soil saturation (Spitz, 1958).
Ethylene Dibromide for Subslab Treatment. Ethylene dibromide gas moves readily under slabs in coarse gravel, but is greatly restricted in clay soils with fine particles. A high content of organic matter also restricts the movement of the gas, as in peat or muck soils. The moisture content of the soil is another important factor in retarding gas penetration. No prediction of results can be made unless much is known about these factors at the time of treatment, and in any case fumigation is not a panacea for the control of termites under slabs. The hazard to building occupants from the subslab ethylene dibromide injection must also be considered. A fumigant may find its way into the heating-system ducts beneath a slab and from there into the house (Osmun, 1958).
The attraction of subterranean termites to wood decayed by a brown-rot fungus, Lenzites trabea, was discussed earlier in this chapter. Esenther and Gray (1968), using a bait of wooden blocks infected with Lenzites trabea and immersed for 10 seconds in a 1 % solution of mirex in toluene, were able to attract subterranean termites (Reticulitermes flavipes) to the wood, and markedly suppress termite foraging activity and damage in treated areas, compared with non-treated ones. Trials were first made in southern Ontario, Canada, near the northern border of the range of subterranean termites. later research revealed that even in southern Mississippi, in an area where termites were abundant and severely destructive, mirex-attractant blocks effectively suppressed termites for 3 years. This control method appears to show considerable promise (Esenther and Beal, 1974).
In most of California and in many southern states, a complete termite inspection includes, besides the inspection of the subarea for either subterranean or drywood termites, the inspection of the attic for drywood termites. Infestations of drywood termites may be revealed by fecal pellets that have dropped from "kickout" holes in infested wood members. The pellets may be in conical piles if they have dropped only a short distance (figure 83), or they may be scattered if they have fallen from a greater distance, as from a ridgepole, rafter, or sheathing, to the floor of the attic. If infested wood is broken open, pellets may tumble out of galleries and pockets in the wood in large numbers. Fecal pellets that are seen in an attic may have been pushed up from below to the tops of ceiling joists and plates, but usually they have fallen from wood members above the place in which they are found.
Sometimes, holes ("royal cells") made by the primary pairs of reproductives can be seen. They are about 2 mm in diameter, and proceed straight into the wood at right angles to the surface. Such holes are usually sealed with wood fragments, and a small festoon of frass can often be seen hanging from them.
Once a pair of reproductives enters an attic, the insects crawl about extensively, and may bore directly into the wood or crevices between sheathing and rafters, between rafter ends and a plate or ridgepole, between plates and studding ends, between studding and siding, between building paper and studding or rim joists or, in the substructure, between mudsills and foundations, to mention only a few of the possibilities. About 90% of the infestations begin in some crack in the wood or in wood joints. The termites may also be in lumber used for construction. In southern California in the fall months, particularly in October, piles of lumber in lumberyards are often attacked by large swarms of drywood termite alates. The entrance holes of the primary pairs may be numerous, and are easily seen.
The attics of flatroofed houses may occasionally be too narrow for a man to crawl into, or they may not be accessible. In such cases, a termite operator indicates in his report what sections of the attic were inaccessible, and may recommend that an access be cut. He is not held accountable for control of termites in inaccessible areas of the attic. If the house is to be fumigated, the termites in such areas will be killed anyway.
In coastal areas, drywood termites are not found in subfloor areas as often as in attics. In areas of very high summer temperatures, as in the San Joaquin Valley of California, they are usually found only in subfloor areas, and not in attics.
Drywood termites are frequently found in windowframes, sills, and internal trim. Pellets are usually seen there by the homeowner if an infestation is present. The windowframes and sills are often so completely eaten out that they collapse and have to be replaced.
The "Drill-and-Treat" Method
Control methods for drywood termites depend primarily on the extent of infestation. If the infestation is sufficiently localized and does not appear to extend into the walls, inaccessible parts of the attic, etc., treatment may be different from that used if it is widespread and partly inaccessible. In the latter case, fumigation is the only practicable procedure. Otherwise, the "drill-and-treat" method of control may be employed.
Termite galleries are located by probing suspected timbers with a screwdriver, ice pick, or other sharp instrument. Quarter-inch (7-mm) holes are then drilled into the infested wood members at about 1-ft (30-cm) intervals to provide access to the galleries. An insecticide dust such as "Kali-dust" (50% calcium arsenate) may be blown into the holes by means of a "Kaligun" (figure 97), or a liquid fumigant may be injected. An ounce (28 cc) of dust is enough for 15 to 30 holes; too much dust may plug the galleries and decrease the effectiveness of the treatment. The holes are plugged after treatment.
When a liquid fumigant is used, about one-third tsp (2 ml) of the liquid, usually a mixture of ethylene dibromide and DDT in a petroleum solvent of high flash point, is used per hole. An entire gallery system usually requires 0.5 to 2 tbsp (7 to 30 ml) of liquid. A 1-qt (1-l) fumigant injector pressurized with a small CO2 bomb is commercially available, but liquid fumigants can also be satisfactorily applied with a small gun oiler or other low-pressure oilcan. When treating wood into which it is not desirable to drill, 1 or 2 coats of ethylene dibromide solution, applied with a paintbrush, will often be effective. Ethylene dibromide should be employed only by experienced pest control operators because of the hazards involved in its use.
Insecticide dust blown into drywood termite galleries travels a great distance in them and in addition, is widely distributed by termites carrying the dust on their tarsi. Some termite operators believe that the gas released by liquid fumigants is more widely distributed than dust throughout the system of galleries contacted by injected liquid.
Examination of the records of 4 termite operators in southern California revealed that in many houses treated for drywood termites, live termites were again found in a few years (figure 98), particularly if the drill-and-treat method had been used (Ebeling and Wagner, 1964). Even though some termites survive a treatment, it is not a total failure, for infestation and future damage may be greatly diminished by expert operation.
The use of a proprietary pentachlorophenol emulsion (Woodtreat-TC) for control of subterranean termites was just discussed under "Treatment of Wood Substructure." This emulsion can also be applied with a paintbrush or caulking gun for the control of drywood termites. In attics, heavy wrapping paper or several layers of newspaper should first be laid down to catch any dropped emulsion that otherwise might soak through the ceilings of the rooms below. The old fecal pellets can be vacuumed, or other newspapers can be spread under the treated areas to catch any pellets that fall from the treated wood. Such pellets would indicate an incomplete treatment, and the areas should be retreated.
An application of Woodtreat-TC, is more rapid and foolproof than the drill-and-treat method, because in the latter, galleries are sometimes hard to detect. They may be stopped up, and dust or fumigants may not fully penetrate the gallery system. The Woodtreat can be brushed on beyond the limits of the suspected area, because the time and expense of doing so are negligible.
Treatment for drywood termites sometimes involves application of Woodtreat-TC to painted wood. Some paints are not penetrated effectively, and thus painted surfaces should be roughened or scratched until there are breaks showing bare wood before applying the emulsion. Very few paints, with the exception of certain marine products, will adhere effectively to areas treated with Woodtreat-TC until at least 12 to 18 months after treatment. Treated areas should then be cleaned and sanded lightly before painting.
Fumigation is universally recognized as the most effective treatment for drywood termites. Frame houses must be covered with a gastight tarpaulin, usually of 7-oz nylon coated with rubber, neoprene, or plastic (figure 99). Stucco structres with flat roofs are generally sealed with a special gastight paper over outer doors, windows, vents, and wherever the fumigator believes a satisfactory seal can be obtained without the labor of tarping. (Fumigating should be done only by licensed pest control operators.)
The procedures for house fumigation for drywood termites were first developed with HCN or methyl bromide (Gunn et al., 1947), and the treatment was first shown to be effective against Incisitermes minor (Pencille, 1947; Hodel, 1949). HCN was soon discontinued for house fumigation because of the fire hazard. Sulfuryl fluoride was found to have certain advantages, and was recommended for both Incisitermes minor and Cryptotermes brevis in Hawaii (Stewart, 1957, 1966; Bess and Ota, 1960). Minnick et al. (1972) demonstrated that the findings of Bess and Ota with reference to the success of sulfuryl fluoride as a fumigant for C. brevis in Hawaii were applicable for this species under Florida conditions.
The fumigating gases currently most commonly used in the United States are methyl bromide, sulfuryl fluoride, and acrylonitrile formulations. Methyl bromide is generally used at 2 lb per 1,000 cu ft (0.91 kg per 28 cu m) of building space, and sulfuryl fluoride at 1 lb per 1,000 cu ft (0.45 kg per 28 cu m). Acrylonitrile with 30% carbon tetrachloride and 30% chloroform is used at 2 lb (0.91 kg) per 1,000 cu ft for drywood termites and 3 lb (1.36 kg) per 1,000 cu ft for powderpost beetles. The gas is generally allowed to remain for 24 hours, but there are no rigid rules as to gas dosages or the period required for fumigation. Both are influenced by such factors as temperature, air movement, porosity of the soil under the building, etc. Much depends on the discretion and judgment of the fumigator.
The gas most often used is methyl bromide. In recent years, sulfuryl fluoride, said to be even more penetrating and effective against both Incisitermes minor and Cryptotermes brevis than methyl bromide (Stewart, 1957, 1966; Bess and Ota, 1960; Bess, 1971), and having the added advantage that it is not necessary to remove any furnishings containing foam rubber from the house, has been used increasingly for fumigating drywood termites. Currently, methyl bromide costs much less, but rubber products containing more than a minimal concentration of sulfur must be removed, otherwise they will give rise to mercaptan odors that may persist for years. Foam rubber is the worst offender, and is present in some of the pads under carpets. Some kinds of leather are susceptible, and shoes, for example, must be removed from the building. The fumigator generally uses sulfuryl fluoride rather than having to remove foam-rubber carpet padding, etc., because of the labor involved in such removal.
On the basis of experiments with sulfuryl fluoride in over 100 buildings, Bess (1971) concluded that a fumigator can guarantee a fumigation against Cryptotermes brevis, provided gas readings are obtained so that the ounce-hour-exposure (OHE) factor can be determined and an adeduate exposure assured. In about 90% of the buildings, a dosage of 1 lb per 1,000 cu ft (0.45 kg per 28 cu m) was sufficient to provide an adequate OHE, but in several buildings it was inadequate. Fumigations could be done with confidence only when gas-concentration readings were made. Under the climatic conditions of Honolulu, Hawaii, it was found that the OHE should be higher during the winter and spring than during the summer and fall, and should be increased under unusual circumstances, such as when extensive plywood paneling, cabinets, or moist timber were present. In some cases, additional fumigant may be introduced into parts of a building where it is especially needed.
One of the great inconveniences of fumigation is that the occupants of a building, including pets and plants, must stay out for a day or two. The gas must remain in the building for an extended period to be effective, and the building must be thoroughly aerated afterward.
If the entire building is not to be fumigated, and some furniture or other movable wooden objects are infested with drywood termites or powderpost beetles, the articles may be taken to a fumatorium for treatment (see figure 119, later in this chapter). Some pest control operators and, in California, the County Agricultural Commissioners' offices (County Departments of Agriculture) have fumatoria.
The drill-and-treat or Woodtreat-brushing methods already discussed provide residual protection only for the limited areas treated, and fumigation provides none whatever. The average rates of reinfestation following treatment are shown in figure 98. One of the suggested solutions to this problem has been the pressure treatment of all framing lumber, instead of only the sills, the latter being the present practice. The increased cost of construction has been estimated to be only 2% (Coaton, 1948; Hunt, 1949). The necessity for treating cut ends, notches, and bored holes at the time of construction to prevent points for termite entry makes the pressure treatment of all framing lumber an impractical procedure, in the opinions of many people. Some investigators have advocated brushcoating the lumber (Wolcott, 1955) or spraying the rough framing of houses (Hunt, 1959) or the accessible wood of finished buildings with wood preservatives. Glue-line treatment of plywood panels has been found to be effective (Ebeling, 1968). None of these approaches to the problem have been followed in commercial practice, apparently.
In the late 1950's, highly sorptive clays, diatomites, and silica gels or aerogels," dusted onto wooden blocks, were found to protect them indefinitely against drywood termites. Insects are protected from a lethal rate of desiccation by a very thin film of lipid, usually a hard wax, that covers their entire epicuticle. Certain sorptive powders with high specific surface and sufficiently ]arge pores and with not too great a sorptivity for water can absorb sufficient epicuticular lipid to cause a lethal rate of water loss (Ebeling and Wagner, 1959a, b; Wagner and Ebeling, 1959; Ebeling, 1961, 1971).
Certain silica aerogels possessing a monomolecular layer of ammonium or magnesium fluosilicate have unique physical properties, including an electrostatic charge that results in efficient deposit on dusted surfaces and efficient "pickup" by insects crawlin over them. To be insecticidal, enough silica aerogel must be deposited on a surface so that an insect crawling over it will pick up a lethal quantity on its lower body surfaces; for termites, a barely visible film suffices.
Silica gels of lowest bulk density and of greatest porosity are called aerogels. The silica aerogel referred to here, known as Dri-die 67, has a density of 4.5 lb per cu ft (2 kg per 28 cu dm) as packed. Dri-die 67 has a very low sorptivity for water, even at high humidities. It has advantageous physical properties for dusting confined spaces and voids, and possesses an insecticidal efficacy far superior to that of hundreds of other powders tested, including many silica gels and aerogels. In this book, wherever the term "silica aerogel" is used, it refers only to powders or dusts having physical characteristics and insecticidal efficacies approximately equivalent to those of Dri-die 67.
Application of Silica Aerogel During Construction. If silica aerogel is dusted onto wooden blocks, even in such small quantities that termites placed on them can survive for 2 weeks, the insects do not feed on the wood, although non-treated wood is vigorously attacked. Silica aerogel is generally used at 1 lb per 1,000 sq ft (0.45 kg per 93 sq m) in dusting attics for prevention of Incisitermes minor. The alates generally gain access to a house via the attic, and crawl about extensively before attempting to bore into the wood. In crawling about, they soon pick up a lethal dose of desiccating dust, and may die in as brief a period as 2 hours (Ebeling and Wagner, 1959a, b).
The dust is usually applied from the crawl hole that provides access to the attic with an electric blower that generates an airstream of high velocity or with a water-type fire-extinguisher, commercially available for the application of insecticide dusts (figures 100, 101; figures 32, 33, chapter 3). Fire-extingtushers are filled with dust to within a few inches of the top, and then pressurized to 100 psi. Because they are equipped with a Schraeder valve, they may be pressurized at a service station or by other sources of air or gas, such as small electric air compressors or tanks of nitrogen, provided the latter are equipped with the appropriate type of outlet. They can discharge a high volume of dust at high velocity and, like the electric blowers, can dust the average attic from the crawl hole. The unusually light weight of silica aerogel results in an even distribution of dust throughout the attic and into its extremities.
The success of attic dusting for prevention of infestation by drywood termites during recent years, as confirmed by many termite operators, suggests the advisability of blowing the dust into attics at the time of construction. As stated before, records supplied by the California Structural Pest Control Board showed that out of 325 houses built in new tracts in California in 1956- 57, 10.8% were infested with drywood termites 11 years later (Ebeling, 1968). With these colonies already established in the tracts, the rate of infestation can be expected to increase with age. Practically all houses in the older residential areas of southern California are, or have been, infested with drywood termites.
In addition to attic dusting, the protective film should be extended to include wall voids and other enclosed spaces. This is best done during construction, when holes can be made in the plaster lath about 4 ft (1.2 m) above the floor and between every 2 studs (Ebeling and Wagner, 1964). The holes may be made with a sharp pick or a half-in. (13-mm) drill. Three grams of silica aerogel are blown into each hole (figure.101). If the plaster lath is already perforated, there is no need to drill holes. Even with perforated lath, which is currently rarely used, the bulk of the dust settles on the interior of the void or falls to the floor plate. If insulation is present, we recommend that the dust be discharged downward, between the plaster lath and the paper that encloses the insulation, to be deposited principally on the floor plate. When "dry-wall" construction is used over conventional wood-stud framing, the sheets of "sheetrock" must be installed horizontally, which provides a horizontal juncture of sheets 4 ft (1.2 m) above the floor. Holes through which treatments are made are drilled at this juncture and between all studs.
In "dry-wall" construction with metal studs, such as used in concrete and steel construction for partition walls, the dust is blown into a hole drilled at the vertical juncture of 2 sheets of "sheetrock." Because there are no solid studs to prevent lateral movement of the dust, a single hole is enough for a horizontal distance of about 10 ft (3 m). Tape is routinely applied to cover the junctures of the sheets of dry wall; the holes are thus covered before the wall is painted.
For dusting houses, 3 grams of silica aerogel have been applied per interstud void. For dry wall on metal studs, 9 g were blown into every hole drilled (i.e., between the 2 sheets of sheetrock). This usually amounted to about 1.25 lb per 1,000 sq ft (0.57 kg per 93 sq m) of floor space. Combined with attic dusting (1 lb [0.45 kg] per 1,000 sq ft), the treatment should provide much protection against drywood termites for the life of the building. This treatment should also be effective for complete or partial prevention of other cryptobiotic pests, such as cockroaches, silverfish, fungus beetles (Lathridiidae), psocids, odorous house ants, Pharaoh ants, carpenter ants, bees, woodwasps, cluster flies, face flies, tropical rat mites, bird mites, and spiders.
Silica aerogel repels cockroaches, and although it serves the purpose of eliminating the dusted areas as harborages and breeding places for these insects, it is not so effective as boric acid dust for eliminating cockroaches that have entered a building without contacting dusted areas. Cockroaches will repeatedly enter protected areas dusted with boric acid, and these act as traps (see Chapter 6). Boric acid is recommended in wall voids, etc., at least in kitchens, bathrooms, and food-storage areas where cockroaches can be expected to be the principal pests. A film of boric acid will also prevent infestation of wood by drywood termites (Reierson, 1966).
Application of Silica Aerogel After Construction. Most of the silica aerogel that has been used to date has been applied long after construction was complete, usually following a fumigation for drywood termites and for the purpose of preventing reinfestation. Silica aerogel has no effect on termites already in the wood-they must be controlled by fumigation or one of the localized treatments that have been described. Following treatment, most termite operators recommend attic dusting with silica aerogel to prevent reinfestation. If some of the drywood termites survive treatment or are undetected, which is quite likely if one of the localized treatments has been used, attic dusting is justified to limit the proliferation of surviving colonies. In the fall, a certain percentage of the termites (the alates) in existing colonies can fly to other parts of a building and start new colonies; this would be prevented on any wood surface with a film of silica aerogel. Also, termites that gain access to the attic after treatment, via vents, under shingles, or through cracks, would likewise be prevented from entering the dusted wood.
Some pest control operators are still using conventional insecticide dusts as they did before fluorinated silica aerogel became available. Tests have demonstrated that the toxicant soon loses its insecticidal efficacy, particularly in the high temperature of the attic, and after a few weeks the only remaining deterrent to termites is the diluent, which is relatively inefficient. The great superiority of the silica aerogel Dri-die 67 to 5% chlordane dust in its ability to knock down and to kill drywood termites (Incisitermes minor) in attic dusting, not only immediately after application, but particularly a month later, has been repeatedly demonstrated (Ebeling and Wagner, 1959b). Likewise, Minnick et al. (1972) found in laboratory tests that a Dri-die 67 deposit killed all drywood termites (Cryptotermes brevis) in 1.5 hour, regardless of the exposure time, whereas a 20-minute contact with a 1% dieldrin spray residue resulted in complete mortality of the termites in 14 hours. The particular advantage of the silica aerogel is that it is inorganic and not subject to decomposition, and should protect the dusted wood against termite attack for the life of the building.