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Plant Pest Identification

Here are several well-known houseplant pests. Some can do extensive damage over time, and others are more of a nuisance than anything else. Contrary to common misperceptions, insects don't kill a plant overnight. Often plants show no symptoms initially, but over time, symptoms such as leaf discolor, stunted growth and malformed flowers may show up. Sudden death in a plant is usually caused by other factors such as over- or under-watering, incorrect lighting and sudden temperature changes.

The key to managing pests is inspection and early treatment. Inspect every new plant arrival and isolate it for a few weeks. Check it often for signs of pests and treat promptly.

Spider Mites


Twospotted spider mite and eggs.

Spider mite webbing on
Norfolk Island pine

The two-spotted spider mite (Tetranychus urticae) is one of the most common houseplant pests. Unlike whiteflies and mealybugs, the twospotted spider mite can survive year-round outdoors in some areas, going dormant during the winter. Original infestations of houseplants may arise from plants kept outdoors, from mite migration from outdoor plants, or by acquiring infested plants from a greenhouse. Once established on a plant, spider mites can crawl short distances or be carried or blown to other plants.

Spider mites feed on plant sap, producing small wounds on the outer plant cells which appear as white flecks. In the beginning of a spider mite infestation, these injuries occur as small patches on the leaf underside near the base of leaf veins. As damage progresses, the leaves have a generalized “off” color that appear as a graying or bronzing. Spider mites also make webbing and in very high populations the webbing may be very visible. Often heavily infested leaves drop prematurely. Common hosts include ivies, dracaenas, figs, hibiscus, Norfolk Island pine, and scheffleras.

Spider mites have a short life cycle that can be completed in one to two weeks under favorable conditions. Approximately five eggs may be laid daily by a mature female. Eggs hatch within a couple of days and the newly emerged nymphs feed almost continuously.

Management. Spider mite control can be difficult. Heavily infested plants should be disposed of whenever possible since they serve as a source of new infestations. Fundamentally important is providing optimal growing conditions as spider mite problems can be aggravated by a poor environment.

The most effective controls include washing and increasing humidity around the plant. Small plants can be repeatedly washed with a jet of water from a shower or kitchen sprayer. This forces off the mites and eggs. If done over the course of several weeks, mite populations should be reduced to low levels.

Twospotted spider mite is difficult to control with pesticides. Horticultural oils are probably the most effective spray. Bifenthrin, found in many houseplant insecticide preparations, can also be effective for spider mite control. Insecticidal soaps are marginally effective.

In large indoor plantings, using predator mites may be considered. Several species of predator mites are available and can be used successfully to manage mites on greenhouse grown tomatoes, cucumbers, and some ornamental plants.

Mealybugs


Mealybugs on coleus

Mealybugs are soft-bodied insects that suck sap from plants. This weakens the plant and causes leaves to shed. Mealybugs also excrete large amounts of sticky honeydew. Coleus, cactus, lantana, hoya, jade, and poinsettia are among the plants mealybugs prefer. Root-infesting mealybugs are associated with African violet and gardenias.

When full grown, most female mealybugs produce a large amount of cottony material in order to lay hundreds of eggs. The eggs hatch within a few days. The newly emerged insects, known as crawlers, move about the plant. Oftentimes, infestation of new plants occurs during this movement. Mealybugs mature in apporximately two months.

Mealybugs may infest different areas of the plant. Although populations on foliage are most visible, most mealybugs infest roots and some are predominantly associated with roots. Some species tend to move to roots when growing conditions are less favorable, but return in high populations on stems and leaves when plants are actively growing.

Management. Swabbing individual mealybugs with alcohol is useful for control. Dilute alcohol sprays (25 to 50 percent concentration) or insecticidal soaps can also be used. However, before applying these treatments be sure to test a small area because some plants may become injured. Alcohol and soaps are strictly contact sprays with no residual action. Root-feeding stages of mealybugs will be missed with these treatments.

Sprays of pyrethroid insecticides (tetramethrin, bifenthrin, permethrin) are usually effective for mealybug control. The soil-applied systemic insecticides disulfoton and imidacloprid can also be effective.

Biological controls for mealybugs include a species of ladybird beetle (the “mealybug destroyer," Cryptolaemus montrouzeri) and a type of parasitic wasp (Leptomastix dactylopii). However these are expensive and difficult to use effectively which limits their application primarily to large indoor plantings where using insecticides is not desirable or possible.

 Scales

Brown soft scale
 
Honeydew produced
by brown soft scale
 
Armored scale

Several scales, particularly the brown soft scale (Coccus hesperidum) and the hemispherical scale (Saissetia coffeae) may attack houseplants. These occur on many kinds of plants but problems are most frequent on ficus, citrus, ferns, and ivies. Heavy infestations of soft scales result in large amounts of sticky honeydew which can create serious nuisance problems. Sustained infestations can cause die back.

The brown soft scale is found on both twigs and leaves. Eggs hatch underneath the cover of the mother scale continuously over a period of several weeks. The newly hatched “crawlers” have limited mobility and usually move short distances in search of feeding sites. After the scale crawlers settle to feed, they begin to produce the characteristic protective body covering and excrete honeydew as waste. The scales continue to grow over the next two to four months rarely moving in later stages.

Armored scales are much less common than soft scales. Typical species are generally round in shape, often with a distinct central spot. Many cause spotting around the feeding site. They can also cause die back when abundant. Unlike soft scales, they do not produce honeydew and, except for a very brief period after egg hatch (crawler stage), are immobile. Horticultural oils are the most effective treatment for armored scales. Systemic insecticides provide poor control.

Management. Soft scales can be difficult to control since their protective covering largely prevents contact insecticides from being effective. However, spray oils are the most effective treatments for scales. Alcohol and soap sprays may also provide some control of scales, particularly crawler and very young, poorly protected stages.

Where infestations are not widespread, scales can be killed by rubbing or picking them off. Fairly minor disturbance of the settled scale can break its mouthparts, causing it to starve. Scales killed in this manner, or by soaps or oils, may remain in place and appear similar to living scales.

Crawler stages are susceptible to most houseplant insecticides. However, insecticides must maintain coverage throughout an entire generation of the insect (two to four months) to eliminate further infestation. Short persisting insecticides, such as pyrethrins and resmethrin, need reapplying at least once per week. Longer persisting treatments, such as bifenthrin and permethrin are effective for scale control when used at longer intervals. Soil applied systemic insecticide imidacloprid should be effective for most soft scale infestations.

Aphids


Chrysanthemum aphids

Aphids develop as pests on several types of houseplants including ornamental peppers, hibiscus, chrysanthemums, and many garden plants and herbs. Aphids also feed on plant sap and excrete large amounts of sticky honeydew. In high populations they cause wilting and distortions of the new growth.

Aphids common on house plants include the green peach aphid, potato aphid, cotton/melon aphid, and chrysanthemum aphid. Populations increase rapidly as generations can be completed in two to three weeks.

Management. Since aphids are exposed on the plant, periodic washing with water and contact sprays of insecticides is effective. However, most aphids adapted to greenhouse culture, particularly the green peach aphid, are highly resistant to insecticides due to long-term exposure. Imidacloprid is particularly effective for aphids. Insecticidal soaps and pyrethroid insecticides may also be effective.

Biological controls for aphids have had modest success at best. Some aphids are best controlled by a small predator midge, Cecidomyiia aphidomyza. Mass releases of green lacewing eggs, which hatch into larvae that prey on aphids and many other insects, can also be effective

Thrips


Thrips and associated damage

Thrips are extremely minute insects, usually less than 1/16 inch when full-grown. They have elongated bodies and are usually brown, although pale and dark forms can be common. Adults can fly and their very small size allows them to easily penetrate most screening.

Thrips are usually recognized by the damage they cause. Light, irregular silvery areas on the leaf surface develop around the feeding site. Tiny dark spots of excrement appear at the feeding area. This feeding may somewhat resemble spider mites, but wounds produced by thrips are larger and more silvery. Thrips infesting buds and developing flowers produce distortion and scarring of petals.

Perhaps most important is the ability of thrips to transmit certain virus diseases to plants. Tomato spotted wilt is a very serious disease, primarily affecting vegetables but also many ornamental plants. The closely related virus impatiens necrotic spot similarly has a very wide host range, particularly among ornamental plants.

Management
. Many strains are resistant to insecticides and their habits make them even harder to control. Egg stages are inserted into plant parts and later immature stages (sometimes called “pupae” and “prepupae”) develop in soil where they are inaccessible to sprays.

Pyrethroid insecticides control some thrips (onion thrips) but not the most common types. Exposed stages are controlled with soaps or oils. Adults are attracted to sticky traps, particularly yellow or pale blue.

Where problems with tomato spotted wilt or impatiens necrotic spot occur it is critical to remove and destroy all infected plants immediately since they can serve as a renewing reservoir of the disease.

Fungus Gnats


Fungus Gnat Adult

 

Fungus gnats (Bradysia species) are small, dark colored flies that jump and fly across the soil surface. Adult gnats are commonly seen as they collect around windows. Fungus gnats cause little or no injury to house plants but create a serious nuisance problem. Problems are most common during winter and early spring. Since these insects develop in potting soil, virtually any houseplant can be a host for fungus gnats.

Adults live for a few days and lay in soil cracks and around the base of plants. The pale colored larval (“maggot”) stage feeds on fungi and decaying organic matter. They also feed on root hairs of the plant which causes reduced plant vigor. A generation can be completed in one month.

Management. Fungus gnats are native insects and are common in lawns and garden soils. Reinfestation from these outdoor sources is very likely and difficult to prevent completely. However, fungus gnat populations can be reduced to levels that are not a serious nuisance. Since fungus gnat larvae feed primarily on decaying plant materials, changing soil moisture conditions is the most important step to take. Watering should be limited so that the soil surface can dry between waterings.

Some adults can be captured on sticky cards. However, as they are very short lived they may have laid many eggs before capture. Adults can also be suppressed by sprays of pyrethrins and pyrethroid insecticides. However frequent application is required to reduce adult abundance and egg laying, particularly when using insecticides of short persistence.

Larval control is more effective. The biological insecticide Bacillus thuringiensis var. israelensis or (H-14 strain) is highly effective as a soil drench. Also some neem products are labeled for soil drench purposes and can control fungus gnat larvae. Unfortunately neither of these products is commonly sold in Colorado and may be more readily acquired via mail order.

Whitefly


White Fly Adults

Greenhouse whitefly (Trialeurodes vaporariorum) is a common pest of several houseplants such as poinsettia, ivy, Hibiscus, and Lantana. Greenhouse-grown vegetables, such as tomatoes and cucumbers, are also frequently infested. Damage is caused from the insects sucking sap from the plant. Heavily infested plants may drop leaves prematurely and have reduced vigor. During feeding, whiteflies also excrete sticky honeydew that detracts from the appearance of the plant.

Winged adults are the stage most commonly seen. They are somewhat gnat-like and covered with fine white wax. Adults lay eggs on the leaves. These eggs hatch in three to seven days to produce the immature nymph stage. Most feeding injury to the plant occurs by the nymphs. Whitefly nymphs are scale-like in shape, translucent color, and fairly immobile. Feeding on the leaf undersides, whitefly nymphs are often inconspicuous and easily overlooked. After feeding for two to four weeks, whitefly nymphs change into an inactive “pupal” stage. This stage typically lasts one to two weeks, before adults emerge. Adults may be active and lay eggs for two months if conditions are favorable.

The greenhouse whitefly is a tropical insect that is incapable of overwintering outdoors in Colorado. Freezing temperatures or a relatively short host-free period eliminates greenhouse whitefly. Although most houseplant problems originate from the purchase and movement of infested plants, some whiteflies can reproduce and infest new plantings during the warm summer months.

Management. Yellow sticky cards or tape can be used to trap adult whiteflies. On small plants, adult whiteflies can be eliminated by vacuuming.

Insecticides containing pyrethrins or related insecticides (tetramethrin, resmethrin, sumithrin) are the most effective chemical controls for adult whiteflies. Horticultural oils, neem insecticides, and insecticidal soaps may control nymphs on leaves. The systemic insecticide imidacloprid is highly effective against greenhouse whitefly.

 

Useful Links:

Ohio State University, Yard and Garden Section