Houseplant Horrors: Insect Infestations

Mealybugs attack a purple basil plant (Ocimum basilicum).
When insects grow to infestation stage, it's best to discard the
plant and its soil. Photo by J.S. Corser, Durham Co. Master Gardener.

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.

Prevention

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.

• Provide a plant with the growing conditions that it needs so that it is more likely to grow vigorously. Stressed plants tend to be more susceptible to pests.
• Before buying or bringing a plant indoors, always check it and its container for signs of pests.
• A plant that has been outside for the summer, especially one sitting on the ground, may have pests that have crawled in through the drainage holes. Take the plant out of the pot to examine the soil.
• Isolate new plants from plants already in the home for six weeks to ensure that any pest brought in will be less likely to spread.
• While plants are isolated, carefully examine them for signs of pests or damage on a regular basis of about once a week. Pay particular attention to the undersides of leaves where pests are most often found. Using a 10X magnifying lens will make it easier to see small pests and also immature pest stages. Infestations are often much easier to control if caught early.
• When repotting a plant, use commercially prepared potting soil rather than soil from outdoors, which can be a source of pests.
• Washing smooth-leaved plants every two to three weeks discourages pest infestations and also improves the appearance of foliage. Small plants can be inverted and swished in a bucket of tepid (lukewarm) water. To prevent loss of soil, cover it with aluminum foil or plastic wrap. Large plants can be hosed down gently, or upper and lower surfaces of leaves can be wiped with a soft, wet cloth. Large plants can also be rinsed in a tepid shower.
• Since cut flowers from the garden can be a source of pests, keep them separate from houseplants.
• Pests of houseplants can enter homes from outdoors, so make sure that screens and doors fit well.

Non-Chemical Control

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 only an isolated portion of the plant is infested, as occurs with leafminers, remove and destroy the infested parts. If the roots are infested, take a cutting and start a new plant. Be sure to start with a clean pot and sterile potting soil.
• Early infestations can often be removed by handpicking.
• Use a cotton swab dipped in rubbing alcohol to wipe off insects such as aphids and mealybugs. Scale insects may need to be scraped off with a fingernail.
• Spraying a sturdy plant with water will remove many pests. Be sure to spray all plant surfaces. Repeated water sprays help control spider mites.
• Spraying the plant with an insecticidal soap can often eliminate a pest infestation in its early stages. Insecticidal soaps are contact insecticides and are only effective when they make direct contact with insects. Once the soap solution dries, it has no effect against pests. Insecticidal soaps are most effective against soft bodied insects and related pests, such as aphids, mealybugs, immature scales (crawlers), thrips, whiteflies and spider mites. Since pests may be hidden or in the egg stage, it often takes more than one treatment to eliminate them. See Table 1 for examples of products and additional comments about insecticidal soap sprays.
• If the plant is severely damaged and is not a valuable one, the best and simplest solution may be to discard the plant and its soil and start with a new plant.

Chemical 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 garden centers and farm supply stores. 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. Spraying insecticides outdoors prevents over-spray from contacting furniture, drapes or carpet.
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.

To read more in depth about controlling major pests like: aphids, mealybugs, spider mites, whiteflies, pillbugs, millipedes and slugs, see the rest of this article:
http://www.clemson.edu/extension/hgic/pests/plant_pests/indoor/hgic2252.html

Revised & pesticides updated by Joey Williamson, HGIC Horticulture Agent, Clemson University 03/14. Originally prepared by Janet McLeod Scott, HGIC Horticulture Agent, Clemson University. New 12/07.