What are natural controls for the forest tent caterpillar?

The forest tent caterpillar is a native insect and has evolved in the forest ecosystem for thousands of years. Natural control mechanisms have also evolved which help to keep outbreaks from seriously damaging forested areas.

A natural control mechanism that causes population collapse is starvation induced by the caterpillars’ feeding. During the early stages of an outbreak, the trees have enough foliage to support the increasing number of caterpillars (FTC).

After a year or two of complete defoliation, the large number of caterpillars needs more foliage than is available. Starvation typically kills 75-95 percent of the caterpillars.

Late spring frosts that defoliate the trees have much the same effect. However, frosts hard enough to cause complete defoliation to all tree species at the same time are not common. Defoliating frosts force the young caterpillars to wait 7-10 days for refoliation. Frost is the only factor that can cause a collapse during the first years of an outbreak.

Another significant natural control occurs near the end of the outbreak cycle. A parasitic fleshfly native to Minnesota, Sarcophaga aldrichi, kills many FTC pupae in their cocoons. Although the fly often plays a significant role in the collapse of an outbreak, its population often increases to the point of becoming a nuisance to people.

Predatory beetles, ants, tree bugs, spiders, and small animals and birds feed on caterpillars and pupae, but the extent of their control is not known. Bacteria, fungus, protozoan and virus diseases become important late in the outbreak cycle. This is commonly due to the weakened state of the larvae as low-level starvation begins and is enhanced by the constant contact of the larvae with each other. Cool, wet spring weather also plays a role by slowing down the development rate of the insects while making disease transmission easier. A non-stinging wasp, Itoplectis conquisitor, is another important parasitoid of FTC pupae.

Although homeowners may want to use insecticides to protect trees and preserve their appearance, the DNR encourages people to first consider the type of insecticide and its effectiveness, and discourages the use of treatments that may pose any environmental concerns. Insecticides such as Bacillus thuringiensis var kurstaki (Btk) can be effective against defoliation when applied while the caterpillars are small and the DNR strongly recommends its use over other insecticides because of its environmental safety. Btk is a natural occurring bacterium that has no effect on birds, people, other animals and most insects.

The DNR provides technical advice to homeowners and land owners interested in treating their vegetation. More information about the biology and management tips for forest tent caterpillars can be found on the DNR website.

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