Cankerworm caterpillars are called loopers because they have a gap between their three pair of legs and appendages called prolegs near their tail end, so that they “inch” along as they crawl. There are fall and spring cankerworms – the main differences being when the female emerges from the ground & lays her eggs and the number of pairs of prolegs on the caterpillars. Fall cankerworm caterpillars have three pair of prolegs & spring cankerworms have two pair. Fall cankerworm adult females emerge from the ground beginning in November. The Spring cankerworm female emerges from the ground in late winter or very early spring. Both of these females are wingless moths, so must crawl up the trees to mate & lay eggs.
Eggs of both the spring & fall cankerworms hatch about the same time as the buds swell and leaves begin to emerge. They feed on the leaves until about mid-May and then drop from the trees on web-like strings to return to the soil where they pupate and will emerge again as adults in fall & late winter.
Most years these caterpillars are just a nuisance from their frass or caterpillar poop that drops from the trees and then as they drop from the trees on their web-like strings and land in your face and hair. In years where there are large numbers of these caterpillars, they can defoliate even large trees. Trees that are defoliated for multiple years in a row can be weakened and may die.
In our area, trees can be banded beginning around this time of year with sticky material to prevent the females from climbing up the trees to lay eggs. Tree banding is done by wrapping a narrow band of cotton batting or fiberglass insulation around a tree trunk then wrapping this material in plastic or tape. A sticky material, like Tanglefoot™, is applied to the plastic to catch female moths as they climb. The batting helps fill bark grooves to prevent moths from getting under the band. Timing for installing the tree bands is ideally after leaf fall and after we have had the first hard freeze or 3 days with temperatures below 30°. The banding should be checked periodically because the bands may become covered with female moths or debris allowing later moths to crawl over the bodies of their unfortunate comrades continuing up the tree to lay their eggs. Banding should be removed from the trees by May 1.
Favorite host plants for these caterpillars include apple, ash, beech, birch, boxelder, cherry, dogwood, elm, hickory, linden, maple, and oak. Oaks are the tree you see most commonly wrapped in our area. Most local hardware stores, farm stores, and garden centers usually have the supplies needed for banding your trees.