Forest Pests: Insects, Diseases & Other Damage Agents

The elm spanworm is found in most areas of the Eastern United States and adjacent Canada. The caterpillars feed on elm, ash, hickory, and oak but rarely on yellow-poplar. In the past this insect has been serious defoliator of shade trees in large cities; more recently, severe and widespread outbreaks have occurred in forested areas of the southern Appalachians and Connecticut.

Spanworm eggs are laid in irregular, single-layered, compact masses on the underside of twigs or large branches or on tree trunks. They are bright yellow green when first laid in the summer but darken to a dull olive or gray in winter (fig.4).

In the spring, egg hatch occurs about the time tree buds begin to unfold. The manner and location of feeding change as the larvae grow. Young larvae feed on the underside of leaves, causing a shothole effect. As they mature, larvae eat all leaf material between the major veins. Full-grown larvae are about 50 mm (2 in.) long and are usually dull, slate black with a rust-colored head (Fig. 5). Color variation occurs in mature larvae in response to increased population levels. Mature larvae pupate in loosely constructed silken cocoons between partially consumed leaves, on branch tips, in bark crevices, or on understory vegetation (fig.6).

The adult moth, which emerges in July, is pure white and has a wingspan of 30 to 37 mm (1 to 1.5 in.) (fig.7).