With a Minnesota winter at its best, we are in “houseplant season” which is the time of the year when we turn to our houseplants to satisfy our itching green thumbs. As a result of giving them our attention, we are likely to find any problems that may exist. I’ve been receiving calls from houseplant enthusiasts asking about the sticky substance that they are finding on the leaves of some plants, as well as on the surface of the table or floor beneath them. This sticky substance is called honeydew and indicates the presence of scale insects, spider mites or mealy bugs. These insects and their relatives are usually found on the undersides of the leaves along the veins, on the stems of the plants, and in the axils of the leaves. They cause injury to the plants by using needle-like mouthparts to pierce plant tissue and feed on the plant’s sap. All three of these pests excrete honeydew, which is a sticky, clear substance made up of excess sugars that are not digested by the pest. In addition to the presence of this sticky substance, the leaves of the plants often turn yellow and drop or, in the case of spider mites, the leaves may become stippled with light-colored spots. Adult scale insects are soft-bodied insects without wings or legs. They are very small and are usually protected by a waxy, shell-like covering. They are usually tan or brown and resemble the spore cases found on the undersides of the fronds of certain fern plants. Mealybugs are also small, soft-bodied insects, but are easily recognized by their white, cottony appearance. Of the three pests mentioned here, spider mites are the most serious pest of houseplants. Spider mites are not true insects, but spider relatives. They are difficult to see with the naked eye. To verify spider mite presence, place a sheet of white paper under discolored or sticky leaves, then tap the leaves and watch for tiny moving creatures on the paper. As soon as any of these pests are detected, isolate the infested plants. Wash off light infestations of these pests on plants with smooth leaves using a soft cloth and a mild soap solution made up of 1/2 teaspoon of mild dish soap mixed in a quart of water. It is important that the soap does not contain any degreasing chemicals. Scale insects or mealybugs can be removed from plants like African violets with fuzzy leaves by using a cotton swab dipped in rubbing alcohol. Heavily infested plants may require the application of insecticidal soap or a chemical insecticide. Resmethrin or tetramethrin are two insecticides available for control of scale insects and mealybugs. Because the waxy coverings of scale insects are impervious to insecticides, add a few drops of liquid detergent to the insecticide to enhance penetration. Applying decachlorobis (Pentac) to the infested plants can control spider mites. At least two or three repeat applications of any of these chemical products every 10 to 14 days may be necessary to satisfactorily control the pest. Whenever insecticides are used, it is important to carefully read and follow all directions and precautions printed on the label. Unless heavily infested plants are especially valuable, it may be advisable to discard them instead of using heavy applications of a chemical product.
