 Cankerworm Information
What are cankerworms?
Is there more than one type of cankerworm?
What do cankerworms look like?
What is the life cycle of a cankerworm?
What does cankerworm damage look like?
Will my trees be damaged?
How can I control cankerworms?
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.
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Is there more than one type of cankerworm?
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.
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What do cankerworms look like?
Spring cankerworm
Fall cankerworm
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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.
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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.
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Cankerworm damage
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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.
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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.
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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.
An excellent resource on banding trees is the Coalition to Save The Elms, a non profit organization. If you require assistance
in having your trees banded, contact the Coalition at 204-775-6642 or
visit their Coalition to Save The Elms website.
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 Bioprotec® ECO and Safer's BTK™ Biological Larvicide contain the active ingredient Bacillus thuringiensis var.
kurstaki (Btk) to control cankerworm larva. Malathion® EC50 (Malathion) formulations are a chemical alternative used to control cankerworms. These products can be found at most local hardware stores. 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.
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