During the past month, a saga has unfolded in parts of Anne Arundel County. It is the tale of the disappearing leaf. Spring is the time when leaves expand and grow to do the important work of making food for plants. Of course, a very useful byproduct of photosynthesis is the oxygen we breathe, so be sure to thank a tree today. Unfortunately, for thousands of trees, particularly oaks, instead of getting larger inch by inch, leaves have been disappearing inch by inch down the gullets of cankerworms a.k.a. inchworms.
We first met the fall cankerworm on January 9, 2006 in the Bug of the Week episode A Moth in Winter. The name fall cankerworm is a bit confusing. It refers to the fact that the eggs of this species are laid in the late fall rather then the season in which the caterpillars feed. You may recall that during the freezing days of winter, this bizarre, wingless moth laid eggs on twigs and branches. With the return of warm weather in early spring, these eggs hatched. The emergence of the cankerworm brood is timed precisely to the appearance of young leaves on the tree where eggs were deposited. This synchrony between egg hatch and leaf expansion ensures a banquet of tender, nutritious leaves for tiny inchworms. Young leaves are the ideal food for inchworm growth. As oak leaves age, their nutritional content declines due to lower levels of protein and higher levels of noxious chemicals.
At first, damage caused by the feeding of inchworms was inconspicuous, consisting of small shot-holes in leaves. Over the past three weeks as tiny inchworms grew, millions of caterpillar jaws consumed acres of leaf tissue leaving trees shredded or in some cases completely denuded. After cankerworms stripped the foliage from their natal hosts, they descended on silken threads to maples, cherries, and blueberry bushes below, where the onslaught continued.
Most trees will tolerate this insult. Many oaks have already begun to send out a second flush of leaves. These leaves replace those consumed by the inchworms and resume the important business of photosynthesis. If no further stressful events occur such as drought or a second wave of defoliators such as orange striped oakworms, then trees should recover nicely. However, several years of repeated defoliation at the mouths of caterpillars can weaken trees and predispose them to attack by more deadly insect pests such as borers or killer diseases such as fungi.
Many locations in Anne Arundel County are into a second year of unusually high levels of cankerworms. I have seen isolated pockets of cankerworm defoliation in Howard County as well. When will the plague end? No one knows for certain but, historically, populations of defoliating caterpillars rise for a few years and then collapse to innocuous levels. Often natural enemies of the caterpillars such as birds, other insects and spiders, rodents, or diseases contribute to the decline. Changes in the quality of leaves they eat may also decline through time. Lower quality food results in fewer eggs produced by mother moths and populations shrink. Unfavorable weather conditions may also play a role. If you find fall cankerworms on your oaks now, it is likely too late to do much to stem their damage. Applications of insecticides will serve mostly as revenge because they are done feeding and most caterpillars have moved to the ground to pupate. Pesticides applied to leaves at this time will not harm them. However, if you have a tree that has been defoliated, consider irrigating it if our early season drought continues into the summer. Also, mark a date in your garden calendar in April of 2007 to inspect your oaks for the early signs of inchworms.
For more information on cankerworms and inchworms, please visit the following web site: