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Fall Webworm

June 27, 2001
Fall webworm, Hyphantria cunea, is appearing in southern and central Illinois as young, small nests or webs on trees and shrubs. Fall webworm has two generations per year in the southern portion of Illinois, with the first one occurring now. There is only one generation per year in the northern portion of the state. The second generation that normally occurs from late summer into fall is usually more numerous and more destructive; however, by that time, trees are preparing to shed their leaves.

Fall webworm feeds on over 120 different species of deciduous trees. Favorite hosts include ash, birch, black walnut, crabapple, elm, hickory, maple, oak, pecan, and sweet gum. Fall webworm generally doesn't feed on conifers. In June, adult females fly and lay between 200 to 500 white eggs on leaf under-sides. Adults are 2-inch-wide white moths with brown wing spots on the forewings. They also have tiny spots of red or orange at the base of their front legs.

Eggs hatch into caterpillars that feed for about 4 to 6 weeks. Young caterpillars tend to skeletonize leaves (which means they remove all leaf tissue except the veins), whereas older caterpillars consume the entire leaf. The caterpillars are pale green to yellow in color, with or without black spots, and are covered with long, white hairs. Older caterpillars are from 1 to 1-1/2 inches long. They build large, protective nests (webs) that are usually found on the ends of branches, and they hide in these nests in large groups to avoid natural predators such as birds. The nests increase in size as caterpillars continue to feed, and heavily infested trees can be completely covered with nests. Severe early season feeding not only causes aesthetic injury but also weakens trees and increases their susceptibility to woodboring beetles. Fall webworm overwinters as pupae in loosely webbed cocoons.

Fall webworm management consists of physical removal and/or the use of pest-control materials. On small trees, prune out and destroy nests. Be sure to prune plants to maintain their aesthetic appeal. Scout trees regularly so that you can detect fall webworms early; the removal of small nests has minimal impact on a tree's aesthetic quality. Treat first-generation fall webworm caterpillars with pest-control materials, including acephate (Orthene), Bacillus thuringiensis kurstaki (Dipel or Thuricide), carbaryl (Sevin), chlorpyrifos (Dursban), or spinosad (Conserve). Bacillus thuringiensis kurstaki must be applied early when caterpillars are small and before they construct large nests. Use high spray pressures to break up the nests to get the pest-control material inside to the caterpillars and the leaves that they are feeding on. Second-generation caterpillars may not warrant spray applications because, at that point, trees will be dropping their leaves. Not spraying will help preserve natural enemies such as parasitoids and predators. (Raymond Cloyd)

Author: Raymond A. Cloyd


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