Cankerworms are caterpillars of several North American moths that have wingless females. Cankerworms feed on the buds and leaves of trees in early spring and can be a major pest.
Elm, oak & apple are favorites. However, wide varieties of tree species may be susceptible.
There are two species of cankerworms that cause defoliation: spring and fall cankerworms. Wingless adult females deposit eggs on trunks or branches of host trees, one in the spring and one in the fall. For both species, larvae hatch in the spring at bud-break. They enter the buds to feed and first skeletonize the leaves as they expand. Then they continue to devour almost the entire leaf.
The chewing from the cankerworms causes skeletonized leaves, which lead to early defoliation. If several successive years of defoliation from cankerworms occurs there can be a decline, sunscald, dieback, and secondary pathogens due to stress.
*Photo credit: Mike Raupp - professor at the Entomology Society of America