Commonly known as the “inchworm,” there are actually two species of cankerworms that attack fruit and ornamental trees: the “fall” and “spring” cankerworms. Their names refer to the season during which their eggs are laid, although the eggs of both species will hatch about the time buds on trees begin to open in spring. As a result, both species occur together in mixed populations and can often be found in the same tree.
Both species’ young larvae feed on the buds and unfolding new leaves of host trees in spring, devouring all but the midrib of the leaf and often defoliating the entire tree. Trees suffering a heavy infestation will usually produce a second crop of leaves after the initial spring defoliation, but its overall vitality may be diminished. Therefore, it is advisable to fertilize affected trees and accompany the fertilization with sufficient watering to restore the tree to health.
The cankerworm larvae are often seen dropping by silken threads from host trees before complete defoliation is noticed. Other infestation signs to watch for are their droppings on cars, sidewalks, and driveways. Favored trees are elm and apple, but several species of oak, cherry, hickory, honey locust, maple, ash, beech, linden, birch, pecan, and hawthorn are also affected.