By Donald Lewis
Entomologist
Iowa State University Extension

It is common to want green living plants around us in our homes during the cold and gray days of winter. Unfortunately, the indoor winter environment is tough on plants, and insect pests can quickly make the situation worse. Scale insects, especially, can increase unnoticed on houseplants through the fall and winter, until you suddenly face a sticky situation or a stunted or dead plant.

Several species of scale insects commonly infest plants in the home or greenhouse. Most have a tan to brown shell, or scale, that covers and 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 undersides of leaves. Some scales are hemispherical in shape, while others are oval and flat.

Scale insects suck the plant's sap through a slender needle-like feeding tube that remains inserted constantly into the plant tissue. The end product of this all-liquid diet of sugar and water is a large quantity of sweet, watery waste that the insect must excrete. The excreted liquid is called honeydew which can make a sticky, shiny mess on the plant leaves and on nearby furniture and floors. 

The honeydew is often the first symptom noticed when a house plant is infested with scales. Other symptoms of sap loss include reduced or stunted growth and possibly death of infested plants in severe cases. Provide good consistent care and regular water to promote health and vigor and to reduce the impact of insect feeding.

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, resmethrin, permethrin, bifenthrin and plant oil extracts. 

Insecticides must be applied thoroughly, repeatedly and persistently (weekly for a month or more) to get good control.  On those plants that regrow after pruning, remove the heavily infested stems and treat the remainder of the plant.

We no longer advise use of ordinary dishwashing detergent in place of commercial insecticidal soap products. "New and improved" and concentrated liquid detergents with "grease cutters" and other ingredients have a high potential for being harmful to the plants.

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 possible exposure to children and pets.

Finally, unless the plant is particularly valuable, many people find it best to throw away infested plants before the pests spread to other houseplants.

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