Fall webworm has two generations per year in the southern half of Illinois with the first one occurring now. The generation that occurs from late summer into fall is usually more numerous and thus more destructive; by that time, however, trees are preparing to shed their leaves.
Fall webworm feeds on over 120 different species of deciduous trees. Favorite hosts include crab apple, ash, oak, elm, maple, hickory, sweet gum, and black walnut. Fall webworm generally doesn’t feed on conifers. In June, adult females fly and lay between 200 to 500 white eggs on leaf undersides. Adults are 2-inch-wide white moths with brown wing spots and tiny spots of red or orange at the base of their front legs.
Eggs hatch into caterpillars that feed for approximately four to six weeks. Young caterpillars tend to skeletonize leaves (that is, 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. 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, it also weakens trees and increases their susceptibility to wood boring beetles. Fall webworm overwinters as pupae in a loosely webbed cocoon.
Management consists of physical removal and/or the use of insecticides. 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 will have minimal impact on a tree’s aesthetic quality. Treat first-generation fall webworm caterpillars with materials such as acephate (Orthene), Bacillus thuringiensis kurstaki (Dipel), carbaryl (Sevin), chlorpyrifos (Dursban), or spinosad (Conserve). Do not treat crab apples with Orthene. Bacillus thuringiensis kurstaki should be applied early when caterpillars are small and before they construct large nests. Use high spray pressures to break up the web and get the insecticide 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. This approach helps preserve beneficial insects such as parasitoids and predators.