In severely defoliated stands, cocoons may be spun on exposed branch tips, in bark crevices, or stumps in the undergrowth. The pupal period varies from 9 to 17 days. Adults emerge in late June through July. They are on the wing at night. If an infestation is close to a urban area, male moths may fly to lights in large numbers frequently described as resembling a snowstorm in the summer.
All damage is caused by the larval stage of the elm spanworm.
As soon as egg hatch occurs, larvae begin to feed on the underside of leaves, causing a shothole effect. As larvae mature, they eat all leaf material between the major veins. Larvae are capable of completely defoliating shade trees and large areas of mixed hardwood forest during outbreaks. If necessary, a registered insecticide should be applied when larvae are small. Where possible, prune small twigs that are infested with masses of eggs.