Fall Cankerworm (Alsophila pometaria) Spring cankerworm (Paleacrita vernata)

U.S. Department of Agriculture, Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State and Private Forestry, Forest Insect and Disease Management., Broomall, PA. p. 123, illus.(USDA Forest Service, Northeast Area State and Private Forestry Publication. NA-FR-4)

Young larvae ("inchworms") of both these defoliators skeletonize new leaves of host trees. Older larvae consume entire leaves, leaving only the midribs. Tree mortality rarely occurs until after 3 years of heavy defoliation. Infestations are generally short-lived, lasting 2 to 3 years. Preferred hosts appear to be apple and elm, but stands of hickory, maple, ash, yellow birch, beech, basswood, boxelder, cherry, and oak have also been severely defoliated.

Full-grown fall cankerworm larvae have three pairs of false legs on the abdomen, vary from light green to dark brownish green, and are about 1 inch long. Spring cankerworm larvae have two pairs of false legs on the abdomen, are reddish to yellowish brown, yellowish green, or black, and are 3/4 to 1¼ inches long. The life cycles of both pests are similar except for the overwintering stages; fall cankerworms overwinter as adults and eggs, while spring cankerworms overwinter as pupae. There is one generation per year in each species.