Outbreaks of this insect occur periodically throughout much of North America. It is the principal defoliator of aspen in parts of Canada and the United States. Maple decline in Canada and the Northeastern United States has been linked to 2 to 3 years of severe defoliation by the tent caterpillar. Repeated defoliation seems to predispose the trees to later decline. Fore example, extensive decline and mortality in Vermont has occurred on 30,000 acres (12,000 ha) of sugar maple defoliated three or more times during the outbreak of 1978 to 1982.

The major effects of forest tent caterpillar are reduced growth and crown dieback. Mortality usually is uncommon. In northern hardwood forests, the insect primarily defoliates sugar maple and black cherry. However, forest tent caterpillars can survive on a variety of other species such as aspen, yellow birch, basswood, beech, and ash.

Following heavy defoliation by the tent caterpillar, sugar maple refoliates in 5 to 6 weeks. Still, sap quality (percent sugar) and sap yield (volume of sap per tap) are reduced significantly the following spring. If extensive dieback occurs, sap yield may be reduced for several years.

Dieback and mortality often are greatest in stands thinned immediately before, during, or right after an outbreak, so thinning should be postponed for at least 3 years after defoliation ends. This will allow surviving trees to recover from the stress of defoliation. Also, severely affected trees will be identified by the degree of crown dieback. Dying or severely weakened trees should be salvaged before they become significantly stained or decayed.

Sugar maple die back following defoliation by the forest tent caterpillar often is most prevalent on sites that also suffer from drought. Recently, sugar maple in south-central New York was devastated following severe defoliation by the tent caterpillar. The infestation became evident in 1980 and peaked in 1982 after approximately 200,000 acre (81,000 ha) were heavily y infested. Many overstory sugar maples were killed following a single year of complete, or nearly complete, defoliation. Recent damage surveys indicate as much as 95-perecent mortality in numerous stands, and many of the surviving trees continue to decline. This rapid and unusual mortality (i.e., it occurred following the first year of heavy defoliation) provably resulted from the combined effects of the defoliation and the severe drought that occurred the same year.

The stout-bodied adult o the forest tent caterpillar is a buffbrown moth with two dark brown, oblique bands near the center of each front wing. When extended, the forewings span 1.0 to 1/5 inches (25 to 37 mm) from tip to tip. Adults may be present from late June through July, and frequently congregate around outside lights at night. The full-grown caterpillar is approximately 2.0 inches (50 mm) long. It has conspicuous body hairs and a series of whitish keyhole or footprint-shaped marks on its dark back (Fig. 26). Narrow orange lines and relatively wide pale blue stripes extend along each side. The gregarious caterpillars congregate during midday on silken mats spun on the tree bole or at the base of a large limb. Unlike many of its relatives, the forest tent caterpillar does not construct a true tent. The overwintering eggs are laid in compact masses that encircle small twigs. The cylindrical glossy egg band (Fig. 27) is te stage commonly sampled for survey purposes.