Cankerworm Update

Picture of a cankerworm
An image of a cankerworm. Source: Gary Yokoyama, The Hamilton Spectator.

The City of Burlington and many other municipalities throughout the Greater Toronto Area have noticed significant tree leaf damage caused by cankerworm, a native insect. Cankerworm occurs on a natural cycle of every eight to 12 years in forested areas. Burlington’s last outbreak was in 2008. Cankerworms are often referred to as inchworms or loopers and can be either green or brown. They are often confused with gypsy moths because they infest some of the same types of trees, at the same time.

As part of the city’s Integrated Pest Management (IPM) strategy, staff will continue to monitor cankerworm populations throughout 2017 and are meeting with other municipalities, who are experiencing similar outbreaks. If populations reach critical levels that threaten forest health, the City of Burlington will take action on city-owned woodlots and valleys. At this time, no chemical treatment is planned by the City of Burlington.

Frequently Asked Questions:

What is the lifecycle of cankerworms?

The caterpillars feed on leaves during the spring from April to mid-June. Then, they move into the ground for the next six months to mature into adult moths. Between November and December, the moths emerge from the soil and lay their eggs in the tree canopy.

Will cankerworms kill my tree?

Cankerworms will not kill healthy trees. Healthy, vigorous trees will grow new leaves later in the summer.

What kinds of trees are affected by cankerworms?

Cankerworms are most commonly found on hardwood trees such as oaks, maples, elms and ash.

What should I do if I have cankerworms on trees on my private property?

Cankerworm damage can be controlled on your trees in an environmentally friendly way in the late fall. Install a sticky band around the main stem of the host trees to trap the female moths and prevent them from climbing up the tree to lay their eggs. Female cankerworm moths do not fly and must climb up the nearest tree to lay their eggs. If you are banding trees on your property, make sure you band all trees that are susceptible to the insect. Tree banding should start in late October and can be removed by late December.

Banding a tree to control cankerworm damage.
Banding a tree to control cankerworm damage. Source: Craig Saunders, CBC.

Are there natural predators to cankerworms?

Yes, natural organisms such as parasitic wasps, birds, mice, ground beetles and bacteria usually keep cankerworm population levels down. However, serious infestations do sometimes break out, usually every eight to 12 years in forested areas. Mild winters, such as this past winter, can lead to an increase in cankerworm populations.

For more information please visit Natural Resource Canada website.


Content for this post was provided by the City of Burlington’s Forestry staff. For more information about the city’s forestry operations, please visit

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