Forest Tent Caterpillar

The forest tent caterpillar is a native defoliator of a wide variety of hardwood trees and shrubs. The FTC’s range in North America extends from coast to coast and from the tree line in Canada to the southern states. These insects feed primarily on aspen and birch trees in northern Minnesota and on basswood and oaks in central and southern Minnesota. The only hardwood not regularly fed on is red maple. In fact, FTC will even eat tamarack foliage during outbreaks. It is often mistakenly called the armyworm.

Forest Tent Caterpillar Outbreaks

Widespread outbreaks of FTC occur at intervals of ten to twenty years and are three to five years in duration. Locally, outbreaks normally last for two to three years. Widespread outbreaks occurred in Minnesota in 1922, 1937, 1952, 1967, 1978, and 1990. Outbreaks commonly begin over large areas simultaneously, primarily the result of weather preceding the outbreak. Population buildups can be triggered by a cool winter followed by a warm spring. Cool springs and warm winters usually accompany a population collapse.

During outbreaks, Forest Tent Caterpillars can number from one to four million caterpillars per acre. They create an extreme nuisance to people living or vacationing in forested areas. Young caterpillars spin threads and fall from the trees onto picnic tables, patios, and people causing serious annoyance.
In the forest, defoliation from FTC usually causes little damage to tree health. Most trees develop a second set of leaves after attack, but these leaves are noticeably smaller and tend to cluster near the branch tips. The second year after the collapse of an outbreak, 80% of the trees have normal sized leaves. FTC defoliation reduces tree vigor, but vigor recovers within a few years of the population collapse.

The FTC 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.