There are two types of cankerworms common to Michigan, fall and spring cankerworms. The biggest difference between the two species is that the fall cankerworm female moth lays its eggs in the fall, usually after the first hard frost and the spring cankerworm female moth lays its eggs in the spring, usually after the first major spring thaw. Both species eggs are lain closely together in rows around small twigs and begin to hatch in early May, usually between the 1st and 15th depending on the weather.
Both species have larvae that are about one inch long when they are fully-grown and range in color from light green to brownish green to black with white stripes. The larvae have the ability to spin down on silken threads, which allows them to be blown around to other trees when looking for a meal. Trees commonly attacked by cankerworm larvae include apple trees and other fruit trees, Ash, Elm, Oak, Hickory, Maple, and many other shade trees. The first noticeable signs of an infestation are small holes (often referred to as "shot holes") in the new leaves. As the larvae continue to feed, the holes enlarge until only the leaf veins remain. Natural enemies (parasites, predators and diseases) usually keep cankerworms in check. During severe outbreaks, a cycle usually lasting two to three years, trees may be completely stripped. Trees stripped of all their foliage often re-leaf by early July, however their growth is slowed and their resistance to disease and other insect attack is weakened. After several years with severe defoliation (50% or greater), dead branches may develop, usually in the upper part of the tree's crown. Tree decline may be followed by tree death, especially when trees are drought stressed two years in a row.
What Can Be Done?
All too often people do not notice these insect pests, or other insect pests for that matter, until they have caused severe damage and are a complete nuisance. Knowing the life cycle of the insect will allow you to be pro-active with your plant health care. Preventing egg laying and larval feeding by mechanical means such as tree banding and egg mass destruction can help prevent epidemic population levels. To help alleviate the stress of defoliation (greater than 30%), trees should receive adequate water throughout the growing season. When rainfall is not sufficient supplemental irrigation is recommended.