Pests of House Plants
Growing plants within the home or an adjacent hobby greenhouse is now so common that the pests that attack these plants may provide a substantial part of the interest in urban entomology. This increasingly important subject was given special attention by Johnson and Smith (1970), and much of the following discussion is based on their recommendations.
The house-plant pest problem can be minimized if cut flowers and new plants brought into the home or greenhouse are examined to determine if pests are present so that they can be immediately eliminated. Isolation of new plants for a few weeks, to make sure they are free of pests, is further insurance against their spread. Infestations by such soil pests as springtails, psocids, and earthworms can be prevented by using sterilized soil for potting. Plants in planters or window boxes are most easily treated for pests if they are kept in pots and placed at a proper depth in sand, soil, or peat. They can then be readily removed for treatment, or replaced if necessary. Treatment for pests is most effectively done at the first signs of infestation or injury.
Spraying with insecticides can sometimes be avoided by washing plants with soapy water and a soft brush or cloth, using 2 teaspoonfuls of a mild detergent to 1 gallon (4 L) of water. Aphids, mealybugs, and scale insects can be removed from broad-leaved plants this way. Some pests can be removed from potted plants by washing them with a spray of lukewarm water from the sprinkler that is commonly an attachment on kitchen sinks.
If only a few lightly infested plants are involved, the pests can be removed by hand. Aphids, mealybugs, and scales can be controlled by removing or by simply wetting the insects with a cotton swab dipped in rubbing alcohol. Sprays may be applied with a hand atomizer or compressed-air sprayer (figures 9 and 14, chapter 3), but it is sometimes more convenient to dip the plants into the spray mixture. Turn the plant upside down and immerse it for a few seconds. A slit cardboard disk to fit around the stem can be used to prevent loose soil from spilling out when the pot is inverted. Aerosol sprays in pressurized cans with pushbutton sprayer tops are available in stores selling garden supplies. Always read the label on the container to make sure the spray mixture was really prepared for use on plants, and be sure to follow carefully the printed directions for application.
When choosing pesticide formulations (see chapter 2, under "Insecticides"), remember that sprays or dips made from emulsifiable concentrates wet and cover foliage and pests better than those made with wettable powders, but are more likely to discolor or burn the leaves or flowers of tender plants. Diluted emulsifiable concentrates should be shaken or stirred occasionally, and preparations made from wettable powders even more frequently, to maintain a uniform mixture of insecticide in the water. If the spray beads up on the leaves and does not readily wet the plant, one-half teaspoonful of a mild household detergent per gallon (4 L) of spray or dip will improve wetting and reduce visible residues. A guide for mixing sprays or dips is given in table 7. Precautions that should be observed in the use of pesticides are given near the end of chapter 2.
Earthworms may be seen on the surface during the day if the soil is wet, otherwise they emerge only at night. Their castings or droppings are good indications of infestation. They sometimes eat parts of plants growing close to the soil surface.
Their continuous tunneling may injure root systems, or loosen young plants in the soil. By removing organic material from the carefully prepared soil mix used for potting plants, earthworms deteriorate its physical and chemical composition. This is the opposite of their usual role in nature, in which they aerate the soil and increase its organic content. They can be controlled by drenching the soil with a dilute emulsion of two-thirds teaspoonful of a 45% chlordane emulsifiable concentrate in 1 gallon of water.
Many homes have a screened or glassed-in patio or an adjacent lathhouse or greenhouse to which pests that require damp locations, such as slugs, snails, sowbugs, pillbugs, millipedes, and springtails, are attracted. Among the slugs, the gray garden slug, Deroceras reticulatum (Müller) (figure 344) is the species most commonly encountered in California. This and other species, as well as the European brown snail, Helix aspersa Müller, are discussed more fully in chapter 12. Granules with 3 to 7% of metaldehyde, scattered about or placed in little piles in infested areas, after watering or after a rain, give a convenient and effective control of slugs and snails; they will .kill on contact or when eaten. Scattering the granules appears to be the better procedure when the gastropods are widely distributed, as in a garden or flower bed, but placing them in small piles may be advantageous where these pests are concentrated in restricted areas, as in a greenhouse.
These are small, oval crustaceans, 7 to 13 mm long, and usually gray in color. The two groups named are similar in appearance (figure 341, chapter 10). Sowbugs scurry for cover when disturbed, and pillbugs roll up into a ball. Both usually hide in loose soil or under vegetable debris, boards, sacks, or pots on damp ground during the day, and are active at night. Although they normally feed on decaying vegetable matter, they sometimes eat roots and tender plant parts, especially those of bedding plants and seedlings. Their hiding places should be eliminated as far as possible. A number of common pesticides can then be sprayed under boardwalks and benches, along foundations, and in other infested areas. (See chapter 10, under "Control of Sowbugs and Pillbugs.")
Mites are very tiny arthropods, usually no larger than the head of a pin. The adults have 4 pair of legs instead of the 3 pairs possessed by insects. Some species are ectoparasites of man. Others are among the most important of the plant pests, and commonly infest house plants.
Tetranychus urticae Koch, the twospotted spider mite (figure 345), is the "red spider" most commonly encountered on house plants. All spider mites are pests of cultivated plants, particularly under dry and warm conditions. They are oval in shape, greenish, yellowish, or reddish, and are so small that they are barely visible without the aid of a magnifying glass.
Spider mites are found mainly on the undersurfaces of leaves, but in heavy infestations a small percentage may be seen on other parts of the plant. When the mites are very numerous, a silky webbing may be found covering infested areas and extending from leaf to leaf to cover the entire plant. Mites can be seen crawling over this webbing. Spider mites suck out plant juices. The removal of chlorophyll results at first in whitish or yellowish speckled areas on the upper surfaces of leaves, and eventually in a more uniformly bronzed or yellowed discoloration, possible defoliation, and stunting or even death of the plant. For plants that are sufficiently sturdy, a forceful spray of water may be enough to break up the webbing and dislodge the mites. This should be followed within 2 days by a dip or spray with malathion or dicofol (Kelthane®), with special attention to the undersides of the leaves. Tender plants may be treated with the dip or spray without the preliminary water spray. Several applications may be required, at weekly intervals.
Brevipalpus obovatus Donnedieu, the privet mite, is the principal species involved as a pest of house plants. False spider mites are flat, oval, and dark red, and are too small to be seen easily with the unaided eye. With the aid of a hand lens, active mites and bright red eggs can be readily seen, mostly on the undersides of leaves, along veins or over the entire leaves. Infested plants may become stunted and defoliated. Control can be obtained effectively by spraying or dipping with a dicofol or diazinon (Spectracide®) mixture, adding one-half teaspoonful of a mild household detergent to improve wetting. When spraying, be sure to wet the undersides of the leaves.
Steneotarsonemus pallidus (Banks), the cyclamen mite, may be seen with a magnifying glass in protected places on young, tender leaves, young stem ends, buds, and flowers. The adults are tiny, oval, amber or tan-colored, glistening, and semitransparent. The even smaller immature forms are milky white, and the eggs are oval and pearly white. The mites can spread from plant to plant where the leaves of different plants come in contact, or they can be transferred on hands or clothing. Their attacks cause the leaves of infested plants to become twisted, curled, and brittle, while the buds become deformed and often fail to open. If they do open, the flowers may be deformed and streaked. The injured leaves, buds, and flowers are often blackened. African violets (Saintpaulia ionantha) develop small, twisted, excessively hairy leaves that will soon die under a heavy infestation. Injured plant parts should be removed, and the infested plants, pots and all, should be immersed in water at 110° F (43° C) for 15 minutes. Two or 3 applications of dicofol spray at 10-day intervals will also give successful control.
Springtails (figures 336 and 337, chapter 10) can be troublesome in the soil in flower pots and in home greenhouses, as well as in houses where no plants are grown but where there are moisture problems. They are tiny,. slender, whitish or darker insects, and get their common name from their ability to jump (see chapter 10). They may become abundant in moist places where there is much organic material, for they normally feed on algae, fungi, and decaying vegetable matter. However, they may also chew on seedlings or the tender parts of plants, particularly near the ground level. They may be controlled by spraying the soil surface, pots, saucers, shelves, and affected parts of plants with chlordane or malathion.
Psocids (figures 334 and 335, chapter 10), also known as barklice or booklice, are soft, yellowish white to gray insects, oval and somewhat flattened, ranging in length from 1.2 to 5 mm, and may be winged or wingless, depending on the species (see chapter 10). They can sometimes be seen in large numbers on soil or on flower pots and benches, especially in undisturbed locations in the home greenhouse. They may also be found in or on old books and papers in slightly damp places, and on damp and moldy cereals. They normally feed on fungi or dead organic matter, and apparently do not injure living plants. Nevertheless, they can become nuisances because of the large numbers that sometimes develop. For control, spray the infested areas with chlordane or malathion.
House plants are sometimes infested by several species of thrips. They are small, slender insects, some species being scarcely visible to the unaided eye. The young nymphs are whitish to yellow or orange, and are not very active. Those of the greenhouse thrips, Heliothrips haemorrhoidalis (Bouché) (figure 346), carry droplets of black liquid excrement on the tips of their abdomens. Adult thrips may be tan, brown, blackish brown, or black, and tend to fly, leap, or run when disturbed. Both nymphs and adults have conical, rasping sucking mouthparts, and cause a typical injury characterized by irregular or streaked silvered areas, speckled with little black dots of excrement. They may destroy buds or blossoms, and will whiten, curl, or deform leaves. For control, the plants may be dipped into or sprayed with malathion emulsions or suspensions.
Aphids usually infest the undersides of leaves, or are clustered about tender leaves and the stems of flower buds. A few species feed on roots. They suck out the plant juices, and "sooty-mold" fungus sometimes grows on the sticky, sugary honeydew they excrete. The plant may be stunted, and its leaves will be curled and distorted. These pests may be easily removed by hand rubbing, washing off the plants, or by using an alcohol swab.
Infested plants may also be dipped into malathion or lindane spray mixes, or treated with aerosols designed for direct plant application.
These soft-bodied insects, with a covering of white, powdery wax, are about 4.5 mm long when full grown (figure 347). Like aphids, they are usually found at rest along veins on the undersides of leaves, but sometimes they can be seen crawling about slowly. Their eggs are laid in loose, cottony masses of wax. These are among the honeydew-excreting insects, and are therefore responsible for some of the discoloration caused by the "sooty-mold" fungus. Mealybugs suck plant juices, and the constant drain on the sap supply for which they can be responsible results in slow growth of the plant as well as wilting between waterings.
The plants may be sprayed or dipped in a malathion mixture with one-half teaspoonful of mild detergent added per gallon (4 L), for the waxy mealybugs are difficult to wet. These insects can also be hand rubbed, washed off, or killed with an alcohol swab. Soil mealybugs can be treated by dipping the infested plant and pot in dilute malathion emulsion.
Scale Insects (Coccidae and Diaspididae)
There are two types of scale insects, unarmored and armored. The females of the unarmored or soft scales (Coccidae) are oval to nearly circular and flat to nearly globular, and are sometimes partially covered with wax. Common examples are the brown soft scale, Coccus hesperidum L. (figure 348), and the hemispherical scale, Saissetia coffeae (Walker). Many species become stationary when fully developed. The males usually pupate under a small, thin, transparent, flat scale, and emerge as tiny winged adults. The females of the armored scales (Diaspididae) are flat, elongate to circular, and have a thick, protective shell or armor above and a very thin layer beneath the body. Common examples are the oleander scale, Aspidiotus nerii Bouché, and the greedy scale, Hemiberlesia rapax (Comstock) (figure 349). Once the motile first-instar nymphs settle down on a plant, the females never move again. The males transform to winged adults as with the unarmored scales. Scale insects suck sap from the plant, resulting in retarded growth and even defoliation. The unarmored scales, but not armored scales, excrete honeydew, which results in the growth of "sooty-mold" fungus and attracts ants.
Infested plants may be sprayed or dipped in malathion, adding one-half teaspoonful of a mild detergent per gallon (4 L) to increase the wetting capability of the spray. Scales are more difficult to control than most pests of ornamental plants, and several applications at intervals of 3 or 4 weeks may be required to kill them all. The scales may remain attached to the plant after they die, but their dry bodies can be readily distinguished from the juicy ones of the living insects. If only dabbed with alcohol.
Whiteflies are very small insects, the adults having 2 pairs of wings which, along with the body, are covered with a fine, whitish, powdery wax. The nymphs and pupae are soft, oval, flat, and resemble the unarmored scales. The principal house whitefly, Trialeurodes vaporariorum (Westwood) (figure 350), is about 1.5 mm long, with a yellow body and immaculate white wings. The nymphs are oval, thin, flat, semitransparent, and pale green, and bear a fringe of wax rods or filaments. Both nymphs and adults suck sap from foliage, weakening the plant, and the nymphs excrete honeydew, like the unarmored scales they resemble. Whiteflies can be controlled by dipping them in or spraying them with emulsions or suspensions of malathion, rotenone, or lindane. A special effort should be made to wet the undersides of leaves, where most of the whiteflies wail be located. Good control has been obtained with repeated applications of pyrethrin-containing aerosols (F. S. Morishita, correspondence).
These moth larvae vary in size from the barely visible, newly hatched young to some that may be 40 to 50 mm or more long. They may be of various colors, depending on the species, and some are covered with dense hair. They are chewing insects that feed on leaves, buds, or flowers, or may even cut off young plants near the soil level. Dark pellets of excrement may be found on the foliage or on the soil beneath infested plants. Hand picking may be practical to control the insects, or the plants may be sprayed or dipped into malathion or pyrethrin emulsions or suspensions. Cutworms (Noctuidae) that hide in the soil, where they usually remain during the day, can be controlled by drenching the soil with malathion.
Fungus gnats are small, slender, delicate, gray or dark-gray mosquito like insects, most species being 3 to 6 mm long, with a few as long as 12 mm or more. They are attracted to light, tend to gather at windows, and are nuisances in the home. The larvae are slender and whitish, with dark heads, and live in damp soil. Large quantities of decaying vegetable matter in damp soil are conducive to heavy infestations. The larvae often feed on the roots and crowns of plants, causing them to be stunted, discolored, and possibly defoliated (see chapter 10). They can be controlled by drenching the soil surface with a dilute chlordane emulsion prepared with the emulsifiable concentrate. Gnats flying about in a greenhouse can be killed with common household aerosols or dichlorvos resin strips.
Some species of ants are attracted to the sugary excretions (honeydew) of aphids, mealybugs, whiteflies, and unarmored scales. Some can dig up and carry away newly planted seeds or small seedlings, or injure plant roots by their burrowing activities. Sprays applied to control the honeydew-excreting insects should also control ants. If not, the infested soil may be soaked with chlordane, as recommended in this chapter for the control of earthworms.
Fig. 344. Gray garden slug, Deroceras reticulatum.
Fig. 345. Twospotted spider mite, Tetranychus urticae.
Fig. 346. Greenhouse thrips, Heliothrips haemorrhoidalis. Adults, nymphs, and "egg blisters" (arrows), with exit holes. (From Ebeling, 1959.)
Fig. 347. Longtailed mealybug, Pseudococcus longispinus.
Fig. 348. Twig heavily infested with various stages of the brown soft scale, Coccus hesperidum.
Fig. 349. Greedy scale, Hemiberlesia rapax.
Fig. 350. Greenhouse whitefly, Trialeurodes vaporariorum.
1996; 2002© Entomology UC Riverside