The Destructive Forces of Forest Tent Caterpillars

The Destructive Forces of Forest Tent Caterpillars

This past weekend at the cabin in northern Minnesota, we witnessed the destructive capabilities of a munching army of forest tent caterpillars. Masses of them covered every surface, their hairy bodies writhing in search of leaves. If you stood still and listened, you could hear the sounds of them – crunching on leaves and dropping feces to the ground. They would descend from the trees and land on your clothes, or creep their way up you if you were brave enough to sit down amongst them. It was difficult to go outside the cabin without tracking at least one back in.

Outbreaks of forest tent caterpillars (Malacosoma disstria Hubner) such as this one occur periodically in northern regions. According to the Forest Service, large outbreaks in the Lake States typically last for around 3 years and then diminish. Forest tent caterpillars (FTCs) prefer broadleaved trees, such as oaks and aspens.  We noticed that the FTCs paid particular attention to basswoods; entire trees had the majority of their leaves removed. The remainders of the leaves resembled skeletons, as they basically consisted of the stem and the center vein of the leaf.

There is one generation of FTC per year. Larvae are small, black, and hairy, and appear around the time of the emergence of leaves in the spring. Colonies of larvae stay together as they molt and grow; over time they develop blue lines along the sides of their body and a series of white spots on their back. During molting periods and when resting the larvae congregate together in a silken mat in the crowns of trees. As they grow older, the location of the mat that they create may move lower in the crown or onto the trunk. When full grown, the caterpillars are roughy 2 inches long.

The adult moths emerge from cocoons roughly two weeks later, and are tan-colored and nocturnal. They live for approximately 5 days and deposit egg masses around twigs during this time. Larvae will emerge from these egg masses the following spring.

Their populations can be negatively impacted by early summer weather, a cold or damp spring, parasitic wasps and flies, starvation, and viral disease. In Minnesota, the native large gray parasitic fly, Sarcophaga aldrichi, becomes more abundant in response to FTC outbreaks. This fly is harmless, but irritating. It is known as “the friendly fly” because it will land on anything, including people, and is difficult to shoo away.

If you have difficulty simply ignoring the writhing masses of FTCs that have invaded your property, you can try to control them by removing and destroying egg masses from trees. My personal notion is that sometimes we can just live with and tolerate other species instead of eradicating them simply because we find them irritating. The FTCs make for some good stories, at least!

– Carolyn

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