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Caterpillar Hanging by a Thread

Have you noticed the little caterpillars in the forests? Here are some facts and observations from Antonin St-Jean, head of Les amis' Conservation Services.
As many of you have noticed over the past few weeks, a surprising phenomenon is occurring in the wooded areas in southern Quebec and Ontario, and more particularly on Mount Royal. Thousands of small hairy caterpillars are hanging from silk threads or feeding in the canopy. They are caterpillars of the Bombyx Disparate (Lymantria dispar), an exotic species from Eurasia also known as the Gypsy moth. For more information, see this detailed fact sheet on the website of our partner, Space for Life:

Just like the famous forest tent caterpillar (Malacosoma disstria), a native species also called tent caterpillar, its infestation is cyclical, increasing and waning over the years. These cycles allow the forest and the caterpillars to maintain a balance and survive over the long term. However, since we are dealing with an exotic species, there is still much to be learned about population fluctuations in North America, particularly in the context of climate change. Mild winters may allow more eggs to survive, leading to a higher infestation rate the following year. 


At the end of last summer, we had observed, on the trees, thousands of brownish egg masses covered by the dead females.In a forest environment such as Mount Royal, there are unfortunately no actions to be taken for the moment, other than monitoring the degradation caused and targeted monitoring of reproduction in certain sectors of the park. When this phenomenon occurs from time to time, the consequences are mainly aesthetic and it does not usually harm healthy trees that are growing well. However, if the situation occurs several years in a row, some tree species could be seriously affected or even die. Generally, the affected trees will produce a second set of leaves during the summer. Nevertheless, the damage on Mount Royal is significant, particularly in the areas known as The Upperfell and The Crags, where the defoliation rate of some trees is about 75 %. Our team, as well as the City of Montreal, is closely monitoring the situation and will continue to watch over the health of this precious forest.

Antonin St-Jean, Head, Conservation Services

Photo : Insectarium de Montréal (Maxim Larrivée)


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