Cankerworms are the larvae of a moth and feed on the foliage of deciduous trees. According to the Penn State College of Agricultural Sciences, dogwood is one species preferred by this troublesome pest, which can defoliate entire trees in certain extreme cases. Detecting cankerworm problems early is essential to saving the life of your dogwood bush.
Cankerworms are also known as loopers, inchworms and measuring worms, and their most distinctive trait is their widely spaced sets of feet, one at the extreme front of the body and the other at the extreme back. This causes them, when moving, to "inch" their back feet toward their front, causing their bodies to fold and rise in the middle, forming a shape like a loop. Cankerworms are green in color with either a white or a black stripe running the lengths of their bodies.
There are two types of cankerworm, both of which affect dogwood bushes similarly, according to the University of Massachusetts Extension. Spring cankerworms are named because the adult moths emerge in the spring. Adult fall cankerworms, as the name suggests, do not emerge until the fall. Caterpillars of both species hatch in late spring.
The University of Massachusetts Extension describes initial cankerworm damage as causing dogwood leaves to appear "tattered." Larvae consume the entire leaf except for the midrib and large leaf veins and may defoliate an entire bush. According to the Penn State College of Agricultural Sciences, two seasons of defoliation can kill a bush, especially when other harsh conditions, such as drought, cause additional stress to the plant.
After eggs hatch in the spring, the larvae of both types of cankerworms begin feeding and drop to the ground, where they pupate until fall for fall cankerworms or the following spring. Adults emerge, mate and die shortly after laying eggs. According to the University of Minnesota Extension, larvae produce silk threads that carry them from tree to tree. This puts dogwoods growing beneath heavily infested trees at particular risk.
Insecticides control cankerworm populations, although according to the University of Minnesota, they are only effective if applied while caterpillars are small and damage is minimal. Damage to a dogwood cannot be reversed once the caterpillars are large, and insecticides may actually cause them to drop from the tree, pupating into adults that breed the next season's infestation. For an environmentally sound solution, apply Bacillus thuringiensis, a bacterial pesticide that targets only caterpillars and not beneficial insects or other wildlife.