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April 14, 1999
Crabapples, honey locust, elms, oak, linden, beech, and other trees that appear to be leafing out late or unevenly may have cankerworm infestations. These loopers, or inchworms, are slender green to brown caterpillars that grow to about 1 inch long. A reduced number of prolegs on the abdomen causes the caterpillars to move in a looping motion. Eggs hatch at bud break, and the caterpillars eat the expanding leaves of the host. Check for cankerworms by striking the tree branch. Caterpillars will fall off the leaves and be suspended by silk threads.

Control cankerworm caterpillars with Bacillus thuringiensis kurstaki (Dipel, Thuricide) or another labeled chemical insecticide. Cankerworms are less obvious than they once were. Before the Dutch elm disease epidemic, when American elms lined the streets, outbreaks of these caterpillars would be very noticeable because they would strip these trees of all foliage in the spring.

Toward the end of May, mature caterpillars will drop down on silk strands to the soil to pupate. There are two species of cankerworm. The fall cankerworm emerges as a moth in November and December to lay eggs, while the spring cankerworm emerges to lay eggs in late February and early March. The males are usually the earliest and last moths to be seen. The females are wingless, grayish, 1-inch long insects that climb the host tree to lay their eggs. Both species of cankerworms are present as caterpillars in the spring. (Phil Nixon)
Author: Phil Nixon


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