UNH Cooperative: Fall Webworms or Tent Caterpillars?

By Tina Savage

There are numerous insects that use silk to build shelters. The two mentioned in the title are often confused with each other, but with information about their life cycles and preferred habitats they can be easily identified. First of all let's take a look at the eastern tent caterpillar. The full grown larva is approximately 2 inches long and has sparse hairs. They are black with white and blue markings and have a white stripe down the back. The adult moths are reddish-brown and have two whitish stripes on the wings. There is a single generation of eastern tent caterpillar per year. Egg masses over winter on twigs of trees. In the spring, about the time that buds open, the larvae hatch and feed for two days before weaving their tent. As the larvae develop they add layers to the tent. Early in the summer, they spin a cocoon, pupate and emerge as adults. After mating, the females deposit egg masses on the twigs of trees, where the developing larvae remain until hatching the following spring. The fall webworm is a general feeder on nearly all trees except conifers. After over wintering as a pupa, adults emerge in late spring or early summer. After mating, females lay masses containing several hundred eggs on the undersides of leaves. Emerging larvae can be in two forms, either yellowish green with a black head or tan with a red head. Larvae begin to hatch in late July to early August. Both colors have numerous, long gray hairs and paired wart-like black spots down their backs. Larvae gather in masses and produce the silky web that surrounds and protects the colony. Feeding occurs inside the web and the web is expanded to include new feeding areas. After their last molt, larva leave the colony and can be found wandering anywhere on the host plant. They crawl to a protected place, spin a cocoon and pupate. The adults may take two forms, either all white or white with black spots. In some areas of the country there are several generations per year, here there is only one. Fall web worms, while unsightly, consume leaves late in the summer. The damage to the leaves occurs when the host tree is about to drop its leaves anyway so, as long as the host tree isn't already stressed, no serious damage will be done. Eastern tent caterpillars do most of their damage early in the growing season and a severe infestation can defoliate the host tree and cause the stress that may lead to the loss of the tree. Early detection of these pests is important to avoid defoliation of the host trees. For fall webworm, there are three alternatives: 1. Live with the problem. The host tree is seldom harmed because of the time of year when active feeding occurs. There are also approximately 75 natural enemies that parasitize and prey on the fall webworm. 2. Prune out the webs. Fall webworm "nests" are usually at the end of branches and are easily removed and destroyed. Tent caterpillar "nests" are located in the crotches of trees and can be scraped out with a stick. 3. Apply insecticidal sprays. If chemical control cannot be avoided, treatment is recommended when the webs first appear. Early treatment is preferred, partly because small larvae are more susceptible to the sprays and large webbed nests are somewhat water proof and difficult to penetrate with sprays. With any pesticide, read and follow the label directions. Eastern tent caterpillar will respond to two of the previous three options. Letting nature take its course may lead to the destruction of the host tree, so options two and three above also apply to the eastern tent caterpillar. Never burn webs, burning will damage the tree as much or more than the larvae feeding and open burning is restricted in most communities. The similarities among these insects may lead to misidentification, especially when describing what you see to others. The differences pointed out should make identification easier. Consider all the factors you can, identifying features on larva and adults, trees affected and time of year. Careful consideration of these factors should lead to proper identification through the process of elimination. University of New Hampshire Cooperative Extension Carroll County, 75 Main Street, P.O. Box 860, Center Ossipee, N.H. 03814, telephone 539-3331.