Scale Insects on Houseplants

Scale insects are a common pest of houseplants and can be found stunting and deforming plants throughout the year. Any of several different species of scale insects may be found on plants in the home or greenhouse. They all have in common a waxy or shell-like covering over the body of the sessile insect.

Most scales are tan or brown and vary from 1/16 to 1/4 inch in diameter. Scales may be found on the stems and/or leaves. Some scales are hemispherical in shape, while others are oval and flat.

Scale insects feed on plant sap sucked thorough a slender stylet that remains inserted constantly into the plant tissue. Sap loss may cause poor, stunted growth. Death of infested plants is possible in severe cases. More commonly noticed than the scales is the large quantity of a sweet sticky liquid called honeydew that is excreted by the scale insects. 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 are often difficult to control. There are several well-known remedies that can be tried in an attempt to eliminate scales from a houseplant. However, there is no easy, simple cure for a scale infestation. One possibility is to pick off individual scales or gently scrub (or rub) the scales loose from the leaves and stems. This is a laborious task that works only on small, large-leafed plants. Dabbing each scale with an alcohol-soaked cotton swab is another possibility on lightly infested plants.

Sprays can be used but success will depend upon thoroughness and persistence. Aerosol or hand pump insecticide sprays made just for houseplants are available at garden centers. Formulated active ingredients include insecticidal soaps, pyrethrin, rotenone, resmethrin and acephate. You can use a mild dish washing detergent in place of the 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 used with a soft cloth while washing 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 in the home is discouraged because of toxicity concerns. On those plants that regrow after pruning, remove the heavily infested stems and treat the remainder of the plant. 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 November 10, 1995 issue, p. 148.

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