Inchworm, inchworm it's time to go

Cankerworm larvae damage leaves as severe infestations spread to multiple varieties of trees.

Spring and fall cankerworm are defoliating trees like this plum. Although, the cankerworm prefers the leaves of other trees, there are so many this year they are eating whatever leaves they can find.

Fortunately for area trees, the march of the munching little green inch worms has almost ended.

The spring and fall cankerworm larvae, which arrived in northeast North Dakota in early June, should be done feeding by the end of the month, said Lezlee Johnson, North Dakota Forest Service forest health manager. Though the cankerworm larvae prefer basswood, elm, boxelder and apple trees, this year they fed on all types of trees, Johnson said.

The cankerworm larvae are slender, greenish to brownish in color and can be seen hanging from silken threads, which help them travel to new food sources.

Though the spring cankerworm and fall cankerworm defoliate trees, they don’t damage them unless the trees have been defoliated several other years or are weakened by something else, such as drought, Johnson said.

The cankerworm feed on buds and developing leaves, beginning with small round holes, then eating all the tissue between the tree veins as they grow.


“Healthy trees can handle that,” Johnson said.

If it seems as if there are more cankerworms this year, it is because there are.

“Cankerworms are cyclical, and we don’t have a predator,” Johnson said. “Last year, there was a very light infestation. The only place they were reported were north and west of Bismarck. This year, it’s widespread throughout the whole state, and they’re very severe.”

Cankerworms don’t have any predators and, by the time the worms start damaging trees, it’s too late to spray for them, according to Johnson.

“When you see the insects are bigger than half an inch, it’s too late to treat,” she said.“But if they’re a half an inch or under, you have a lot of options.”

Active ingredients of insecticides homeowners can use include carbaryl, cyfluthrin, imidacloprid, esfenvalerate and permethrin. Biorational pesticides include Bacillus thuringiensis, insecticidal soap and pyrethrin.

When the cankerworms are finished defoliating trees, they drop out of them and burrow into the plant litter and pupate. Fall cankerworm moths emerge after the first frost, mate and then the flightless female crawls up a tree trunk to lay her eggs. Spring cankerworm moths will emerge in the spring and do the same thing.


Ann is a journalism veteran with nearly 40 years of reporting and editing experiences on a variety of topics including agriculture and business. Story ideas or questions can be sent to Ann by email at: or phone at: 218-779-8093.
What To Read Next
Get Local