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Spring and Fall CankerwormsPage 1of 3 Spring and Fall Cankerworms Mark E. Ascerno and Jeffrey Hahn Copyright © 2003 Regents of the University of Minnesota. All rights reserved. Cankerworms, or inchworms, are common shade tree defoliators in Minnesota. Hosts include elm, apple, hackberry, basswood, oak, boxelder, maple, and ash trees as well as shrubs growing beneath heavily infested trees. Cankerworms go through natural cycles with periods of abundance ranging from two to seven years (with an average of four years) followed by periods of low populations lasting 13 to 18 years. However, spring and fall cankerworms have not had a significant population outbreak since the late 1970s, although isolated infestations have occurred since that time. One explanation for this may be the reduced number of elm trees in Minnesota. As the population of elm (the preferred host for cankerworms in Minnesota) continues to decrease due to Dutch elm disease, the number of remaining hosts may not be suitable for supporting large populations of cankerworms. Thus, it's difficult to determine when the next population outbreak may occur, or whether one will ever occur again. Identification Spring and fall cankerworms look very similar. Adult females of both species are wingless, giving them a spider-like appearance (Figure 1). They are both grayish brown and about 5/16 of an inch long. Males have grayish brown wings and an average wingspan of 1 1/8 inches long. There are slight differences in markings and size between the two species that Figure 1: Adult female spring cankerworm. are difficult to detect. The larvae also look very similar. Full-grown cankerworms are about 1 inch in length and can vary in color. Spring cankerworms range from yellow-green to brownish to blackish. They often have a white stripe that runs along the side of its body from the head to the posterior. Fall cankerworms range from light green to dark green to black. You can distinguish the two species by counting the number of prolegs (false legs) on the back half of the abdomen. Fall cankerworms have three prolegs while spring cankerworms have only two (Figure 2). Figure 2: Mature spring cankerworm larva. Life History Spring and fall cankerworms feed at the same time of year, often on the same trees, causing the same kind of damage. They start to feed on buds and expanding leaves right after egg hatch in the spring. As they feed on leaves, they create small BB-sized holes. As the larvae become larger, these holes expand until all tissue between the veins is consumed. Despite the similarities in their feeding, there are differences in their life cycles. Fall Cankerworm -- This species emerges from the soil as an adult moth, generally in late October after a hard freeze. The male moths emerge a few days before the females. Mating takes place as the females crawl up tree trunks in search of small twigs on which to glue neatly ordered egg clusters. The males and females die shortly after the eggs are deposited. http://www.extension.umn.edu/distribution/horticulture/DG0876.html6/5/2009