Fall webworm, Hyphantria cunea, is appearing throughout many parts of Illinois as small to large nests or webs on trees and shrubs. Fall webworm has two generations per year in the southern and central parts of Illinois, with the second-generation occurring now. There is normally only one generation per year in the northern half of the state. Fall webworms are quite noticeable in August and September, with silk webbing up to 3 feet long enclosing the ends of branches. The caterpillars, leaving bare branches with dirty webbing attached, consume leaves inside the silken web.
Though aesthetically unappealing, this feeding is not harmful to tree health as leaves will drop soon anyway. During this time of year, the most effective control entails pruning out the webs enclosing the caterpillars. If an insecticide spray is needed, use enough pressure to penetrate the water-resistant web.
Fall webworm feeds on more than 120 different species of deciduous trees, including 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 each lays 200 to 500 white eggs on leaf undersides. Adults are white moths with 2-inch wing spans, with brown spots on the forewings and 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 ones consume the entire leaf. Caterpillars are pale green to yellow, some with black spots, and are covered with long, white hairs. Older caterpillars are 1 to 1-1/2 inches long. They build large, protective nests or webs, typically on branch ends; 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. Severe early season feeding not only causes aesthetic injury but also weakens trees, thus increasing susceptibility to wood-boring beetles. Fall webworm overwinters as pupae in loosely webbed cocoons.
Managing fall webworm consists of physical removal and/or the use of insecticides. Infestations on small trees can simply be pruned out and the nest destroyed. (Be sure to prune plants in a manner that maintains their aesthetic appeal.) Scout trees regularly so that fall webworm populations can be detected early; removing small nests has minimal impact on a tree’s aesthetic quality. Treat first-generation fall webworm caterpillars with recommended insecticides, such as acephate (Orthene), Bacillus thuringiensis kurstaki (Dipel or Thuricide), carbaryl (Sevin), or spinosad (Conserve). Do not treat crabapple trees with Orthene. The bacterium, Bacillus thuringienis kurstaki, must be applied early when caterpillars are small and before they construct large nests. The caterpillars must ingest this material for it to kill them. Use high-pressure water sprays to break up the nest to get the insecticide inside where the caterpillars reside and onto the leaves where they feed. Second-generation caterpillars may not warrant spray applications. Not spraying will help preserve natural enemies such as parasitoids and predators.