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.
Spring & Fall Cankerworms Life Cycle:
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.
- 4-6 week feeding period
- Color varies: green to black; Length: up to 1 inch long
- Easily blown by the wind from tree to tree
Damage by Cankerworms:
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.
Cankerworms Treatment & Control:
- Treatment program using Bacillus thuringiensis formulations (biological control) or other pesticides. Continue until populations are appropriately reduced or gone.
- Trunk banding to trap wingless females helps in reducing the overall population attacking the tree. This is most effective with a stand-alone tree that is not in contact with adjacent trees.
*Photo credit: Mike Raupp - professor at the Entomology Society of America