Roger Patterson of rural Allegheny County writes: "Late this summer I have noticed what appear to be tent caterpillar tents in my lilac bushes. I'm fairly certain these are not the tent caterpillars we normally get in the spring. These late summer munchers are white and hairy. Their tents are similar, and they seem to kill the branches they inhabit. Can you identify these caterpillars and suggest how to control them other than spraying insecticides?"

You're seeing nests of the fall webworm, the caterpillar of a small white, nondescript moth. Webworms are common and native to much of the U.S. Many trees --apples and cherries, persimmons and walnuts -- can be victimized. The extent of infestations varies from year to year.

Unlike eastern tent caterpillars, which appear in the spring and weave their webs in the crotches of trees, webworms appear in late summer and build their webs on the outer tips of branches. Tent caterpillars eat young vigorous leaves, while webworms eat leaves that have already done most of their work for the tree. As summer winds down, plant growth slows, so leaf loss in late summer is less serious than when it occurs earlier in the season.

Web building begins when the first eggs hatch, usually in early July. Females lay as many as 400 eggs on the leaves of favorite foods and when the caterpillars hatch, they begin building the web. Over the course of four to six weeks, the caterpillars molt six times and grow to about 11/2 inches in length.

Then they leave the nest, drop to the ground and find a protected place to pupate during the fall and winter. In late spring, adults emerge from their cocoons, mate and die. The adult moth lives only a few weeks.

To protect themselves from predators, webworms stay inside their tents and eat the leaves enveloped by the silky shelter. This is another difference between webworms and tent caterpillars, which leave their tents to eat fresh leafy growth.

Burning the tents is a popular control technique, but it can damage trees and trigger brush or forest fires. Instead, open webs with a stick to give predators such as cuckoos, tanagers, orioles and vireos access to the caterpillars. If you have just a few webs, simply prune and destroy them.

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