Fall webworms live communally in silk tents, which they construct at the ends of branches in over 100 species of hardwood trees. Often, there is just a tent (Figure 1) or two on scattered trees, and the only impact is unsightly webbing. However, occasional outbreaks can result in significant defoliation and the temporary annoyance caused by large numbers of wandering hairy caterpillars as they leave the tents to pupate. Healthy deciduous trees generally can withstand substantial late season defoliation without significant consequence, so no control measures are needed, even if they are practical. Natural enemies can help to bring populations back to normal levels.
Fall webworms feed for about 4 weeks. Initially the small, hairy caterpillars are pale yellow with a double row of dark spots along their backs. They feed on surface tissue, leaving veins intact and giving leaves a skeletonized appearance (Figure 2); they eat entire leaves. The larger stages are covered with long whitish hairs that grow from black and orange warts scattered along their bodies. Fall webworm caterpillars may be active into September.
Fall webworm control may be justified to protect new trees in a landscape or those weakened by other stresses. If practical, physically remove and destroy the caterpillars and their tents. Insecticides containing Bt or spinosad provide selective caterpillar control, but they work best against small caterpillars, and spray coverage can be a limiting factor if trees are large.
By Lee Townsend, Extension Entomologist