Tiny insect, cottony maple scale, causing big annoyance in Douglas County
When looking into your trees, does it appear that someone has glued popcorn to the branches? Worse yet, when you park your car in the shade, do you come back to find sticky sap covers it? Do you find that same sticky substance all over your lawn, windows and patio furniture? The cause of this annoyance is a tiny insect known as cottony maple scale. Right now, Douglas County is experiencing a heavy infestation of this pest.
Though cottony maple scale prefers maple trees and boxelder, it can affect almost any hardwood tree or shrub. No matter what type of tree it decides to invade, the appearance and life cycle of this scale is much the same.
Scale insects can be identified by a hard shell that protects them from predators and the weather. The female overwinters as a small, brown scale less than 1/8" long. They attach themselves to the bark of small branches and twigs. In the spring, as soon as the sap of the tree starts to flow, the female grows rapidly and begins to lay her eggs in a cottony mass of wax that she is able to secrete.
Each white egg mass contains anywhere from 1,000 to 1,500 eggs. Eggs hatch into crawlers in late June and July. They migrate to the underside of leaves and feed along the veins and midrib by withdrawing sap from the plant parts. They spend the remainder of the summer feeding on leaves. In late summer, mature males emerge and mate with immature females. The males then die a few days later. Just before leaf drop in the fall, mated females move back to the branches and twigs for overwintering. There is one generation per year.
Tree damage is caused when heavy populations of cottony maple scale withdraw sap from the tree. Premature leaf loss, stunted growth and branch dieback may be seen. The scales may also produce large quantities of a sappy substance called honeydew. Honeydew may be seen covering our cars, sidewalks, lawns and windows. Though cottony maple scales are not known to kill full grown, healthy trees, it may be possible that sustained, severe populations could cause serious injury to already weakened and stressed trees. Only in unusual circumstances will the death of a tree be seen.
In most years, cottony maple scale populations are controlled by weather and natural enemies. Lady beetles, wasps and flying parasites are all natural predators of the scale. Because natural controls are usually very effective, applying chemicals is not recommended. Insecticides may kill the natural predators and parasites and could prolong the outbreak and create a more serious problem.
If the occurrence of cottony maple scale is severe and spraying is necessary, timing is key. Inappropriate timing is not only ineffective, but may be counterproductive. Scales are usually noticed when mature females are present, which is not the appropriate time to spray insecticides. Scales need to be controlled as crawlers before their waxy, protectant cover is produced.
For more information on cottony maple scale, visit www.extension.umn.edu/garden/.