What is Cankerworm?

Cankerworms are caterpillars that feed primarily on the leaves of American elms, Manitoba maple and Green ash in May and June. We often call cankerworms Loopers, inchworms or measuring worms. Cankerworms spin down from trees on silken webs.

Winnipeg has two types of cankerworms, the fall cankerworm and the spring cankerworm Both look similar and cause damage in the spring when the leaves come out.

Both the spring and fall cankerworm are about 2.5 cm long when fully grown. They range in color from light green to brownish green with a dark stripe down the back.

The fall cankerworm adult, which is a moth, lays its eggs on trees before winter. It usually does this after the first severe autumn frost. The eggs hatch into cankerworm larvae in the spring. The spring cankerworm adult, also a moth, lays its eggs on trees in the early spring. It lays its eggs after the first major spring thaw.

Sign and Symptoms

Small holes in the new leaves are often the first sign of damage. As the cankerworm larvae eat, the holes become larger until only the leaf veins remain. During high populations, cankerworms can completely strip trees. Leaving them vulnerable to insect and disease.

Prevention

The fall cankerworm adult, which is a moth, lays its eggs on trees before winter. It usually does this after the first severe autumn frost. The eggs hatch into cankerworm larvae in the spring. The spring cankerworm adult, also a moth, lays its eggs on trees in the early spring. It lays its eggs after the first major spring thaw.

Watering & Fertilizing

Your trees can be kept healthy by watering and fertilizing. Click here to learn more about watering.

Control - What you need an arborist to do

Biological Control

Apply an organic biological product Bacillus thuringiensis var. kurstaki (Btk) to control cankerworm larva. The product is applied to the leaves of the tree, once feeding begins the caterpillar dies with 2-3 days. The product is rainfast (resistant to water wash off) within 2 hours.

Chemical Control