Insect and mite pests can appear on houseplants at any time of the year. However, now would be a good time to check houseplants for scales insects, mealybugs and spider mites. See pamphlet Pm-1595 for details.
Several commercial insecticide products are available for treating insect pests on houseplants. One of the most popular is insecticidal soap, because of its low toxicity to household occupants. You can use a mild dish washing detergent in place of the commercial insecticidal soaps ("potassium salts of fatty acids"). 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 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 May 3, 1996 issue, p. 69.