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2021 Gypsy Moth Outbreak

5 pairs of blue dots followed by 6 pairs of red dots


2021 Gypsy Moth Outbreak Causes Severe Defoliation

By Jacquie Bede
Dept. of Plant Science, McGill University


I am sure you have all noticed the fallen leaves and the rain-like sound made by the gypsy moth caterpillars at the Arboretum this summer. This year’s outbreak and the devastation caused by these voracious caterpillars has been astounding. They have defoliated numerous trees, with oak, larch, apple, aspen and birch as their favourites. Although these are their preferred food sources, other tree species can be affected as well when food becomes scarce.

Defoliation alone will not kill a tree but it is harmful because it limits the tree’s ability to photosynthesize and store important resources like sugars to make leaves for the next year. The tree is weakened and therefore more vulnerable to illness. We are currently experimenting spraying bio insecticide Btk to reduce defoliation in young trees.

The gypsy moth caterpillars are quite stunning, particularly the older larvae. From their head going down their back, they are adorned with five pairs (one on either side) of blue spots followed by six pairs of bright red spots. Their spots and sides are covered with tufts of setae, a hair-like structure. I should mention here that though the caterpillars are not “toxic”, the setae can be irritating and can cause rashes.

There are two closely related gypsy moth species (Latin name: Lymantria dispar) that destroy forests in North America. The information on them overlaps and can be quite confusing, but one subspecies, the European gypsy moth (Lymantria dispar dispar), was introduced to North America over 150 years ago for silk production. Obviously, this was unsuccessful, and the insects inadvertently escaped. A related subspecies (Lymantria dispar asiatica) was brought over probably multiple times, by ship. As invasive or alien species to North America, both gypsy moth subspecies have few natural predators or parasitoids here.

You may be wondering, what has caused the outbreak this year? Insect outbreaks happen regularly. The insects are always present in the environment, but at certain yearly intervals, the conditions are right and the population explodes. Because the gypsy moth is an invasive species, it has few natural enemies. Birds will eat them but tend to avoid them because of the setae. There are some parasitoids or viruses that can infect the caterpillar, but winter temperature is the most critical factor in gypsy moth survival. Although the eggs can overwinter, sub-zero winter temperatures (< -25oC) play an important role in gypsy moth control. Last year’s relatively mild winter likely contributed to the population explosion this summer. I will never complain about the cold again!


Gypsy Moth Life Cycle


Gypsy Moth Control:

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