Soon after the tree leaves begin to open they can be attacked by cankerworms. And as we all know, the worms are back again this year. The slender green or brownish "measuring" worms prefer elm, basswood, boxelder and apple trees, but will attack most other trees and shrubs.

The eggs of both the fall (green color) and spring (brown color) cankerworms hatch in the spring. The worms feed for 3 to 4 weeks and then drop from the trees or crawl down the trunks. They reach a length of about 1 inch before leaving the trees and burrowing into the soil when they enter the pupal stage. Soon after the first frost in the fall, moths of the fall cankerworm emerge, mate and lay eggs in trees.

The best time to spray trees for cankerworm control is just when the young worms begin to feed and before leaves are badly damaged. Cankerworms are spread primarily by wind. The wind blows small larvae on their silken webs to other trees. Several chemical insecticides: Malathion, Sevin, and Methoxychlor; as well as the bacterial insecticide, Dipel, may be used for control. Other insecticides labeled for leaf feeding caterpillars such as Tempo can also be used. The active ingredient in Tempo is available in Bayer Advanced Garden Multi-Insect Killer.  When using insecticide, mix and apply according to label directions.

A tree that is completely defoliated usually leafs out again 2 or 3 weeks after the worms stop feeding. If the tree is in good condition one such defoliation doesn't kill it. However, dry conditions may put additional strain on the trees. If your trees are completely defoliated, you may give it an additional amount of water to help it re-leaf.

This page was last updated April 2003

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