Roger Patterson of southwestern Pennsylvania, 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. and occur in red- and black-headed forms. Many trees, from apples and cherries to persimmons and walnuts, can be victimized. The extent of infestations varies from year to year, but fall webworm outbreaks are as predictable as bird migration, the fall rut of white-tailed deer and winter snows.
I'm seeing the same wispy silken clouds in the woods here on the ridge. Though nests of fall webworms are conspicuous, the small white moth that results rarely attracts attention. In fact, the moth that arises from a fall webworm is so nondescript it doesn't even warrant a name. A Field Guide to Moths: Eastern North America simply calls it the "fall webworm moth."
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, and leaf loss is much less serious than when it occurs in spring or early summer. Though fall webworms are never a pretty sight, neither are they particularly troublesome.
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 skeletonize the leaves and begin building the web. Over the course of four to six weeks, the caterpillars molt six times and grow to about 1 1/2 inches in length. Then they leave the nest, drop to the ground and seek refuge under a rock or a slab of bark. There they pupate and spend the fall and winter. In late May or June, adults emerge from their cocoons. After mating, the females lay their eggs, and the adults die. The adult moth stage lives only a few weeks.
To protect themselves from hungry birds and other predators, webworms stay inside their webs and eat the leaves enveloped by the silky shelter. This is another difference between webworms and tent caterpillars -- tent caterpillars leave their tents to eat fresh leafy growth.
If it sometimes seems that evidence of fall webworms appears overnight, that may be the case. Webworms, which are recognized by long white or pale yellow hairs on each body segment, work all night long and after the colony grows to a certain size, the web grows quickly. A large nest may be three or four feet long and encase several branches.
For those who wish to control webworms, there are several options. A popular, though dangerous, treatment is to burn webworm nests. I discourage this choice because it can damage trees, cause brush or forest fires and kill people. Instead, open webs with a stick to give predators such as cuckoos, orioles, tanagers, and vireos access to the caterpillars. These birds are among the few that eat hairy caterpillars.
Chemical treatment with insecticides is also possible, but I don't recommend it. Webworms are most active late in the season, so the damage they inflict is mostly cosmetic. Defoliation of at least 20 percent of a tree's leaves is necessary before plant health declines.
If you just have a few webs, simply prune the tented branches and destroy them. Because webworms usually don't cause permanent damage, I recommend a laissez-faire approach.
Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or by e-mail via my website, http://scottshalaway.googlepages.com.