MEET THE FALL CANKERWORM
By Arthur V. Evans
During a recent warm spell on the heels of New Year’s Day, a small collection of somberly hued moths gathered at my front porch light. I posted a picture of one of these moths on my entomology page on Facebook, and it was immediately identified as the fall cankerworm, Alsophila pometaria (Harris), a moth in the family Geometridae. The caterpillars of geometrids are collectively called inchworms. Adult fall cankerworms present a striking example of sexual dimorphism. Males are fully-winged, while the females are wingless. Native to North America, fall cankerworms are found from Nova Scotia to South Carolina, west to western Alberta, Colorado, Kansas, and California.
Adults are typically active in fall and early winter. Females lay batches of 50-200 carefully aligned and upright eggs on small twigs and branches. Before leaving, they cover their eggs with scales from their abdomen. Upon hatching in late spring, each caterpillar descends from its egg on a single silken strand and is dispersed by the wind.
The ravenous larvae consume leaves and young fruits of many kinds of deciduous tree and are especially fond of maple, oak, and elm. Young larvae “skeletonize” patches (cankers) on the undersides of leaves by eating only the leaf tissues between the small veins. Older larvae consume nearly all leaf tissues and leave only the major veins behind. After 4-5 weeks of feeding the caterpillars reach maturity and lower themselves on to the ground via silk strands to enter the soil to pupate.
Large numbers of fall cankerworm larvae can defoliate trees and seriously damage fruit trees. Most trees and shrubs can withstand the onslaught, but mortality is possible if the plants are already stressed from drought and other adverse conditions. Check with your local nursery or extension agent for information on effective controls for fall cankerworms.
Cranshaw, W. 2004. Garden insects of North America. The ultimate guide to backyard bugs. Princeton University Press, Princeton, NJ. 656 pp.
Johnson, W.T. and H.H. Lyon. 1994. Insects that feed on trees and shrubs. Second edition with corrections. Cornell University Press, Ithaca, NY. 560 pp.
Powell, J.A. and P.A. Opler. 2009. Moths of western North America. University of California Press, Berkeley, CA. 369 pp.
© 2011, A.V. Evans