Several species of scale insects, mealybugs and whiteflies are commonly found on plants in the home or greenhouse. All are sap-feeding insects that can weaken plants and cause poor, stunted growth. Death of infested plants occurs only in severe cases.
Houseplant insects may create an annoyance caused by large quantities of a sweet, sticky liquid waste product called honeydew that is excreted as the insects feed. Honeydew can make a sticky, shiny mess on the plant and nearby furniture and floors. A black fungus called sooty mold may grow on the honeydew.
Scale insects have a tan to brown shell-like covering or scale that protects the insect's body. Scales may be from 1/16 to 1/4 inch in diameter and are usually found on the stems and/or leaves. Some scales are hemispherical in shape, while others are oval and flat. Mealybugs appear as white tangles of cotton on the leaves or stems. A common location is the slim, protective gap at the junctions of stems and leaves.
Houseplant insects are difficult to control. There is no easy, simple, one-shot cure. One possibility is to pick off individual scales and mealybugs or gently scrub (or rub) the insects loose from the leaves and stems. This is a laborious task that works only on small, large-leafed plants. Dabbing each insect with an alcohol-soaked cotton swab is another possibility on lightly infested plants.
Sprays can be used for houseplant insect control. Success will depend upon thoroughness and persistence. Insecticide sprays (aerosols or hand pump sprayers) made just for houseplants are available at garden centers. Formulated active ingredients include insecticidal soaps, pyrethrin, rotenone, resmethrin and acephate. You can substitute a mild dish washing detergent for commercial insecticide soaps. Use a dilute solution of 1 Tbs of detergent per quart of water. Soap sprays can be applied with a sprayer or a soft cloth used to wash infested leaves and stems. Insecticides must be applied thoroughly, repeatedly and persistently (weekly for a month or more) to get good control.
Granular insecticides that you add to the soil of infested houseplants seem to have very limited effectiveness and their use is discouraged because of toxicity concerns. On those plants that regrow after pruning, removing the heavily infested stems and treating the remainder is a possibility. Finally, unless the plant is particularly valuable, many people find it best to throw away infested plants before the pests spread to other houseplants.
This article originally appeared in the April 6, 1994 issue, p. 45.