Prepared by Janet McLeod Scott, HGIC Horticulture Specialist, Clemson University. (New 12/07.)
When a houseplant looks less than healthy, most often it is the result of improper care. Factors such as too much or too little water, light, heat or fertilizer can cause many plant problems. However, in some cases the problem results from a pest infestation. Several insects and other pests feed on houseplants. These pests most often come into the home on newly purchased plants or on those that have been outside for the summer.
The best way to control insects and related pests on houseplants is through prevention, as it is almost always easier to prevent a pest infestation than to eliminate one. There are several precautions that you can take which will decrease the chances of having to deal with a pest infestation of your houseplants.
The first step in control is to isolate any plant suspected of being infested with a pest. Keep the plant separate from other houseplants until the pest is completely controlled. This process may take several weeks or more.
Before looking for a chemical solution to a pest problem on houseplants, there are several effective control alternatives that should be considered. However, do not expect the problem to be solved with one application. Some of these alternatives require persistence on the part of the indoor gardener, but they can give good control.
If non-chemical control methods have failed, and the plant is valuable, a stronger pesticide may be necessary. Before choosing a pesticide, it is important to identify the pest accurately. In general, a single pesticide will not kill all kinds of pests. Some pesticides are only effective against certain pests or certain life stages of particular pests. In addition, it is important to understand that more than one application of a pesticide is often necessary for control. When possible, alternate the pesticide used from one application to the next as some pests develop resistance quickly.
Houseplant insect sprays can be obtained at gardening centers. Only a few pesticides are labeled for use indoors on houseplants. Before using a pesticide indoors, be sure that the label specifies that use. You may want to treat your plant outdoors and then bring it inside after the pesticide has dried completely. If you take plants outdoors to treat, make sure that weather conditions are mild.
Typically, a pesticide label will include both a list of plants for which the pesticide is recommended as well as a list of plants that are known to be sensitive to the pesticide. Symptoms of pesticide injury on plants include distortion of leaves and buds, yellowing of leaves, spotting of leaves or flowers, and burn along the leaf edges as well as total burn. When damage occurs, it often becomes visible within 5 to 10 days, sometimes sooner. In general, the damage does not kill the plant.
As always, before purchasing and using any pesticide, be sure to read all label directions and precautions, and then follow them carefully.
Aphids: Aphids are small, soft-bodied, pear-shaped insects about 1/16 to 1/8 inch long. They are usually green but may be pink, brown, black or yellow. Some aphids have a woolly or powdery appearance because of a waxy coat. Adults may or may not have wings.
Aphid adults (winged adult in center) and immatures
Alton N. Sparks, Jr., University of Georgia
Aphids are usually found feeding on new growth or the undersides of leaves. Some feed on roots. They suck plant sap, resulting in yellowing and misshapen leaves. In addition, growth may be stunted, and new buds deformed. As aphids feed, they excrete a sugary material, called honeydew, which makes leaves shiny and sticky. Sooty mold fungi may grow on the honeydew, producing unsightly dark splotches on the plant’s surfaces.
Control: With minor infestations, handpicking, spraying with water or wiping the insects with a cotton swab dipped in rubbing alcohol may be practical. Insecticidal soap* may also be used. In most cases the treatment will have to be repeated multiple times.
Mealybugs: Mealybugs are small, pale insects, related to scales. They are about 1/8 to 1/4 inch long and move very sluggishly. The adult females cover themselves and their eggs with a white, waxy material, making them look cottony. Some have waxy filaments that extend beyond their bodies. Nymphs (immature forms) hatch from the eggs. Once they begin to feed, the waxy coating starts to form. Nymphs look like adults only smaller. The wax on mealybugs helps repel pesticides and makes them somewhat difficult to control.
US National Collection of Scale Insects Photographs Archive
USDA ARS, www.insectimages.org
Mealybugs are most commonly found on the lower surfaces of leaves and in leaf axils (where the leaf attaches to the stem). One species feeds on the roots. They suck plant sap, causing stunted and distorted growth and sometimes plant death. Like aphids, mealybugs excrete honeydew, providing the opportunity for growth of sooty mold fungi.
Control: Light infestations can be controlled by removing individual mealybugs by hand or by wiping each insect with a cotton swab dipped in rubbing alcohol. An insecticidal soap* may also be used. With a heavy infestation, it may be necessary to discard the plant.
Spider Mites: Mites are not insects but are more closely related to spiders. Since they are extremely small, plant damage is typically the first sign of their presence. A silky web is often seen with heavier infestations.
Spider mites with webbing
David Cappaert, Michigan State University, Bugwood.org
Twospotted spider mite adult
David Cappaert, Michigan State University, Bugwood.org
Both spider mite adults and their immature forms damage plants by sucking plant sap. Damage includes light-colored speckling on the upper surface of leaves, and results in a plant with an overall faded look. If the mites are left unchecked, leaves become bronzed or yellowed, and the plant dies.
Control: Spray sturdy plants forcefully with water, including the undersides of leaves, to dislodge mites and break up their webs. Plants also can be sprayed with an insecticidal soap* or miticide. It is often necessary to spray once a week for several weeks to control mites.
Scales: Several species of scales are pests on houseplants. Scale insects can be divided into two groups: armored scales and soft scales. An armored scale secretes a waxy covering that is not an integral part of its body. The covering can be scraped off to locate the insect living beneath it. In contrast, the waxy covering that a soft scale secretes is an integral part of its body.
Scales are unusual insects in appearance. Adults are small and immobile with no visible legs. Scales vary in appearance depending on age, sex and species. Some are flat and appear like fish scales stuck to a plant. Others look like waxy, colored masses. They range in size from 1/16 to 1/2 inch in diameter. They are usually found on stems and the undersides of leaves, but may be found on upper surfaces as well. Scales feed by sucking plant sap. Their immature forms, called crawlers, are mobile and also feed by sucking plant sap. Like mealybugs, the armored scale insects excrete honeydew (with resulting sooty mold problems). Soft scales do not excrete honeydew.
Armored scale adult
US National Collection of Scale Insects Photographs Archive,
USDA ARS, Bugwood.org
US National Collection of Scale Insects Photographs Archive
USDA ARS, Bugwood.org
Control: Early infestations of scales can be removed by scraping with a fingernail. Adult scales are relatively protected from insecticides by their waxy covering. Their crawlers are susceptible, however.
Whiteflies: Whiteflies are not true flies, but are more closely related to scales, mealybugs and aphids. They are very small about 1/10 to 1/16 inch long. They have a powdery white appearance and resemble tiny moths. When at rest, the wings are held at an angle, roof-like over the body. The immature stage is scale-like and does not move.
Central Science Laboratory, Harpenden Archive, British Crown
Both the adults and their immature forms feed by sucking plant sap. The damage that they cause is similar to that caused by aphids. The infested plant may be stunted. Leaves turn yellow and die. Like aphids, whiteflies excrete honeydew, which makes leaves shiny and sticky and encourages the growth of sooty mold fungi. When plants that are infested with whiteflies are disturbed, the whiteflies flutter around for awhile before settling again.
Control: Wash the plant. Flying adults can be vacuumed out of the air. Spray the plant thoroughly with insecticidal soap*.
Thrips: Thrips are tiny, slender, yellowish to blackish insects with fringed wings. They are typically found on leaves and between flower petals. At less than 1/16 inch in length, the adults are very difficult to see without a magnifying lens. Blowing lightly into blooms and leaves causes thrips to move around quickly, making them easier to see.
Andrew Derksen, University of Florida, Bugwood.org
Both adults and nymphs (immature stage) feed by scraping surface cells to suck plant sap. Leaves fed on by thrips will often take on a silvery or speckled appearance similar to damage caused by mites. Leaves may drop early. When thrips feed on flower buds, the flower may die without opening. Flowers may be streaked or distorted as a result of feeding.
Control: Rinse leaves with water. Spray with insecticidal soap*.
Leafminers: Leafminers are the larvae (immature worm-like stage) of a large number of different insects. The larvae feed between the upper and lower leaf surfaces. Leafminer damage appears as a winding, discolored trail or an irregular blotch within the leaf. Although damage from these pests is unsightly, it is rarely serious.
John A. Weidhass, Virginia Tech, www.insectimages.org
Control: Remove and destroy any leaves showing leafminer damage.
Beetles: Various kinds of beetles and their larvae feed on houseplants. They may enter the home when houseplants are brought inside at the end of summer, or they may enter through some opening. They have chewing mouthparts.
Control: Remove and destroy the beetles.
Caterpillars: Caterpillars are the larvae of butterflies and moths. They range in size from about 1/8 inch to 2 or more inches long. Their color varies according to species with gray, brown, and green being common, as are mottled and striped colors. They may be smooth or have spines, hairs or bumps along their bodies.
Butterflies and moths lay their eggs on the undersides of leaves of plants that have been outdoors. Stray moths that have gotten into the home can also lay eggs on houseplants. When the eggs hatch, the caterpillars can be quite small, but grow with each molt (process of shedding the skin).
Caterpillars have chewing mouthparts. Some feed openly on leaves, buds and flowers and can eat large portions of the plant in a relatively short period of time. Others bore into stems to feed. A good indication that caterpillars are causing the damage is the presence of frass (fecal pellets) on leaves and under the plant.
Control: Remove and destroy caterpillars and eggs.
Fungus Gnats: Adult fungus gnats are delicate in appearance and about 1/8 inch long. Often they can be seen running across or flying near the soil surface under a houseplant. They are weak flyers and are attracted to light. The adults do not feed on houseplants but can be a nuisance to people. In severe infestations they are often seen in large numbers on nearby windows.
Johnny N. Dell, Retired, www.insectimages.org
The whitish larvae (immature forms) of fungus gnats have shiny black heads and can grow as large as ¼ inch. The larvae generally feed on decaying organic material or fungi growing in the soil. The larvae of some species will also feed on roots. This feeding is especially damaging to very young plants. With older, established plants, the initial sign of an infestation is that the plant loses its normal healthy appearance. A heavily infested plant may lose leaves as a result of the feeding of larvae on its roots.
Indoors, fungus gnats are most often a problem when potting soil that is rich in organic matter, such as peat moss, is used to grow plants. It is especially a problem when overwatering occurs.
Control: For plants that can tolerate it (i.e. most houseplants, especially during winter), allow soil to dry between watering. Dry conditions will kill the larvae.
Springtails: Springtails are tiny insects about 1/5 inch long that inhabit the soil. They vary in color but are usually white or black. They are wingless, but can jump. Their presence is usually a sign of overwatering.
Susan Ellis, www.insectimages.org
While springtails normally feed on decaying organic matter, they will chew on seedlings or tender plant parts. Damage is usually minimal. In large numbers, they can be a nuisance.
Control: For plants that can tolerate it (most plants), let the soil dry between watering.
Root Ball Pests: Houseplants taken outdoors during the summer may have their root balls infested with pillbugs, millipedes and slugs. These houseplant pests may cause minor feeding damage to root systems. They are generally found along the exterior of the root ball in small cavities carved from the potting mix.
Control: The plant container can be gently removed to inspect for pillbugs, millipedes and slugs, which simply can be scraped away.
*Note: Some houseplants, including ornamental ivy, maidenhair fern, dieffenbachia, schefflera, crown of thorns, chrysanthemum, Easter lilies during bud formation, asiatic or oriental lilies, jade plant, begonia, fuchsia, zebra plant, impatiens and certain palms, are sensitive to insecticidal soap, and it should not be applied to them. In addition, insecticidal soap should never be applied to houseplants outdoors in direct sunlight or to plants under drought stress. For other plants, test insecticidal soap first on a small part of the plant before treating the entire plant. Symptoms of injury may take at least 48 hours to appear.
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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.