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Cankerworms (06/13/19)

Cankerworms are back in force after a weak showing last year. Both spring and fall cankerworm larvae are out now, and are feeding voraciously on their preferred elm, basswood, boxelder, and apple.

Cankerworms are back in force after a weak showing last year.  Both spring and fall cankerworm larvae are out now, and are feeding voraciously on their preferred elm, basswood, boxelder, and apple.  They may also attack other trees and shrubs.  They feed on buds and expanding leaves, starting with small round holes and eating all the tissue between leaf veins as they grow.   

Cankerworm eggs were laid in trees last year, and many have recently hatched into larvae.  The larvae are slender, greenish to brownish “inch worms” and you may see them feeding on leaves or you may see them hanging from slender silken threads and traveling with the help of wind and gravity to a new food source.  They will feed for 3 or 4 weeks then drop out of the trees and burrow into the duff where they will pupate.  Fall cankerworm moths will emerge after the first frost, mate, and the flightless female will crawl up a tree trunk to lay her eggs.  Spring cankerworm moths will emerge next spring to do the same thing.

Trees that are completely defoliated will usually leaf out again within two or three weeks.  If the tree is in good condition, it will be able to tolerate at least two years of complete defoliation.  Chemical treatment can be effective when the insects are still small - less than one-half inch long.  Treatment is not effective after you can see severe damage.  Some active ingredients of insecticides labeled for homeowners include: carbaryl, cyfluthrin, imidacloprid, esfenvalerate and permethrin. Biorational pesticides include Bacillus thuringiensis var. kurstaki (Bt), insecticidal soap, spinosad and pyrethrin.  When using insecticides, mix and apply according to label directions.

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Lezlee Johnson

North Dakota Forest Service

Forest Health Manager

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