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Insect Control

Cankerworm Information

What are cankerworms?

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.

Is there more than one type of cankerworm?


Photo credit:
Chris Hsia under license CC BY 2.0

Winnipeg has two types of cankerworms, the fall cankerworm (Alsophila pometaria Harris) and the spring cankerworm (Paleacrita vernata Peck). Both look similar and cause damage in the spring when the leaves come out.

What do cankerworms look like?

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.

What is the life cycle of a cankerworm?

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.

What does cankerworm damage look like?

Small holes in the new leaves are often the first sign of damage. As the cankerworm larvae eats, the holes become larger until only the leaf veins remain. During high populations, cankerworms can completely strip trees.

Will my trees be damaged?

Most trees grow back their leaves by early July. However, their growth is slower and they are less able to fight potential new diseases and other insect attacks.

How can I control cankerworms?

Adult control

The best way you can control adult cankerworms is to band your trees before larval cankerworms become a problem. Banding your trees with Tanglefoot is an environmentally acceptable way to keep adult cankerworms from climbing the tree to lay eggs. You should band your trees by mid-March to control the spring cankerworm and by mid-September (or before the first hard frost) to control the fall cankerworm . Encourage your neighbours to also band their trees because cankerworms can travel on their silken threads as larvae.

Larval control

A tree is normally very tolerant of some defoliation and control efforts are unnecessary when only a few larval cankerworms are present. However, at times when the cankerworm populations are high, it may be necessary to control them with the use of biological products or chemical pesticides. Biological products like Safer's BTK™ Biological Larvicide contain the active ingredient Bacillus thuringiensis var. kurstaki (Btk) to control cankerworm larva. Malathion® formulations are a chemical alternative used to control cankerworms. These products can be found at most local hardware stores. All Pest Control products purchased and used must be registered with Health Canada and contain a Pest Control Product (P.C.P. or PCP) Number on their label. Once the cankerworms are hanging on their silken threads, contact chemical control is the only possible alternative. Whenever you use a pesticide, always follow label directions.

Last update: July 20, 2020
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