JoAnne Skelly: A hitchhiking pest

Just when you thought your summer battles with insects were over, I have to remind you about insect infestations on houseplants. Your houseplants may be at risk if you recently brought outdoor container plants into the house. Usually containerized plants pick up hitchhiking insects while they are outdoors and these travelers start breeding when brought into a warm interior

Here is an example: With the onset of cold weather, a friend brought a lemon tree into the house from outdoors. I noticed that the leaves looked spotted and dusty. Some leaves had a black residue on them. At first, I thought spider mites, but as I looked with a magnifying glass, I realized I was seeing scale insects. Scales are sap-sucking insects with tiny straw-like mouthparts. They are relatives of aphids, leafhoppers, whiteflies and mealybugs. And, like aphids and their other cousins, most scales leave a sticky residue called honeydew on plant surfaces after they feed. The black deposit I noticed is actually a sooty mold fungus that thrives on the sugary residual.

Scale insects do not look like other insects and are immobile for most of their life. Adult females are usually circular or oval without wings, a head or other visible body parts. They look like tiny —1/16-inch — tan to brown turtle shells or plate-like covers collecting along the veins on the upper and lower surfaces of leaves and on stems. Some scales are white and covered with a cottony type substance.

Controlling scales is difficult. Their shell-like cover protects them from pesticides. The young are born under the female’s shell and then move out to find their own feeding site. At this crawler stage, they are susceptible to pesticides. Once scale crawlers find their feeding spot, they stay there, building their own protective shells. They reach maturity in 65 days and may reproduce all year indoors.

Control methods start with isolating the plant away from other plants and then washing each leaf on the top and bottom surfaces to remove all the shells. Or, the scales can be picked off by hand or wiped off with a cotton swab dipped in alcohol. Some insecticides are designed specifically for houseplants. You cannot use an insecticide indoors unless the label says it is safe for that purpose. Any management strategy must be repeated persistently weekly for a month or more to get the little buggers under control. Discarding the plant may be the easiest option.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at


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