Forest Pests: Insects, Diseases & Other Damage Agents

The elm spanworm periodically severely defoliates hardwoods throughout the eastern United States and Canada. Ash, hickory, and walnut are preferred, but elm, oak, cottonwood, and various other species are also heavily attacked. Yellow-poplar and sycamore are rarely attacked. Infestations may defoliate vast areas of mixed hardwoods in a relatively short time. Heaviest defoliation often occurs in mountainous terrain along ridgetops. Although one defoliation usually has little impact, two consecutive summers of defoliation can cause dieback and even mortality through invasion of secondary pests.

After overwintering, the eggs hatch in May or June, Initially, the larvae feed only on the lower surfaces of the leaves, producing a "shot hole" effect, but later they consume the entire leaf. During the early feeding period, they produce silky threads by which they often are wind-dispersed over large areas. The full-grown larvae are about 2 inches long. Color appears related to population density; in high populations, larvae are black with rust-colored head capsules; in low populations, they are green or yellow. Mature larvae construct silken cocoons among partly eaten leaves or in secluded areas such as bark crevices. In about 10 days, the new adults emerge and mate. The adult moth is white with a wingspan of about 1½ inches. Females lay eggs in compact groups on the undersurfaces of branches. There is one generation per year.

Larva(e); Instar V, light phase, indicative of low density population Photo by Arnold T. Drooz, USDA Forest Service	Adult(s); male and female moths under an aster leaf in the field Photo by Arnold T. Drooz, USDA Forest Service
Egg(s); on pignut hickory branch Photo by Arnold T. Drooz, USDA Forest Service

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