Fall webworm, Hyphantria cunea, is evident in southern Illinois and parts of central Illinois as young, small nests or webs on trees and shrubs. Fall webworm has two generations per year in southern Illinois, the first one occurring now. There is typically only one generation per year in the northern portions of Illinois. The second generation, which occurs from late summer into fall, is usually more numerous and potentially more destructive—however, by that time, trees are preparing to discard their leaves.
Fall webworm feeds on over 100 different species of deciduous trees, including ash, birch, black walnut, crabapple, elm, hickory, maple, oak, pecan, and sweet gum. Fall webworm typically doesn’t feed on conifers, such as pines, firs, and spruce. In June, adult females fly around and are capable of laying up to 500 white eggs on leaf undersides. Adults are white moths with brown spots on the forewings. They are about 2 inches long with tiny red-orange spots at the base of the front legs.
Eggs hatch into caterpillars that feed for about 4 to 6 weeks, depending on environmental conditions. The young caterpillars tend to skeletonize leaves, removing all leaf tissue except the veins, whereas older caterpillars consume the entire leaf. Caterpillars are pale yellow-green to white with black spots (but sometimes without) and are covered with long white hairs. Older caterpillars are 1 to 1-1/2 inches long. The caterpillars build large, protective nests or webs that are typically found on the ends of branches. They hide in the nests in large groups, or congregations, to avoid natural predators such as birds. The nest grows in size as the caterpillars feed, and heavily infested trees can be completely covered with nests. Severe early-season feeding by large populations of fall webworm can ruin the aesthetic quality of trees and shrubs. In addition, plants may be stressed enough to increase their susceptibility to wood-boring insects. Fall webworm overwinters as a pupa in loosely webbed cocoons.
Managing fall webworm involves physical removal and/or the use of insecticides. Prune out and destroy nests on small trees (but avoid ruining the aesthetic appearance). Scouting trees regularly will help detect early infestations, so that pruning will not impact the aesthetics of trees and shrubs. First-generation fall webworm may be treated with acephate (Orthene), Bacillus thuringiensis var. kurstaki (Dipel), carbaryl (Sevin), and/or spinosad (Conserve). Insecticides containing Bacillus thuringiensis var. kurstaki as the active ingredient must be applied early, when caterpillars are small and before they construct large nests. Remember—the material has to be consumed by the caterpillar to be effective. Initially, use high-pressure sprays to dislodge the nests so that the insecticide can get inside to the caterpillars that are feeding on the leaves. Second-generation caterpillars typically don’t warrant spray applications because trees will soon be losing their leaves.