Yuck! The caterpillars are back! (And a few tips to get rid of them!)

Jun 3, 2025

Whether we like it or not, the forest tent caterpillar (Malacosoma disstria Hubner) is making a comeback, and we're starting to see it in our trees.  

This caterpillar is indigenous to our forests and, generally speaking, poses no threat to our trees, despite the extensive defoliation it leaves in its wake. In fact, it is the most widespread defoliating native insect in Canada.

This caterpillar only attacks hardwood trees. Even when the trees are severely affected, they can withstand an infestation, which lasts no more than three to six consecutive years. Normally, in the course of the same year, the stripped trees will produce new leaves. Infestations are generally short-lived. As it's only just beginning, you can expect to see the presence of the pesky pests for a few weeks.

If you want to try and get rid of the caterpillars in your home, here are a few tips:

• Use a biological insecticide, Bacillus thuringiensis (B.t.), sold under various brand names. Caterpillars that ingest it die within a few days. This biological insecticide is preferred, since it only affects caterpillars and is composed of a bacterium already found in nature. It is not harmful to other animals, including fish and birds, plants or humans.
• Try an insecticidal soap (one teaspoon of blue dishwashing soap, one tablespoon of vegetable oil mixed in one litre of water). You can spray directly on the caterpillars and they will die.)
• You can also use your hose to clean your porch, house and outdoor furniture.
• If you see caterpillars in groups, you can use a Shop-Vac to suck them up. Make sure to empty the contents of the Shop-Vac in soapy water.

Although their large numbers make them repulsive, these caterpillars are not harmful and pose no risk to humans or pets. They are more of a temporary aesthetic problem that will soon disappear.

We would also like to advise you that our green spaces teams are hard at work trying to keep the play and rest areas caterpillar-free and usable. We thank you for your patience, if certain areas have a significant presence of caterpillars. We'll get there as soon as we can!

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CONTACT: Public Works Department, 506.739.2103