Life Cycle and biology

The forest tent caterpillar has one generation per year. It goes through four stages to complete its life cycle.

Egg masses

Eggs of the forest tent caterpillar are laid in a mass wrapped around the twigs of the trees and shrubs. Occasionally, egg masses are found encircling cables and ropes. New egg masses are covered with dark brown glue-like material. They are blended with the host, and look like a swelling of a twig from a distance. Each egg mass contains up to several hundred eggs. Egg masses are laid in late June to mid-July, and stay throughout the winter until warm temperature breaks their dormancy in the following spring.

New (unhatched) forest tent caterpillar egg masses on the twigs of aspen

Egg masses are covered with a frothy glue-like material

Larvae

In northern Wisconsin, the caterpillars hatch from egg masses by late April to mid-May. Once they hatch, caterpillars (larvae) continue devouring leaves for 5-6 weeks. Feeding peaks in early June and completes by mid to late June.

Larvae of forest tent caterpillars on oak leaves

After caterpillars completely defoliate the tree where they hatched, hungry caterpillars start to migrate to search for more food. City objects and buildings are often covered with migrating caterpillars, causing a nuisance to humans.

A parking meter in downtown Rhinelander, covered with young caterpillars

Despite the name, forest tent caterpillars do not form a tent. However, they produce silken mats on branches and trunks to congregate, and lay silken trails as they move.

Pupae

Full-grown caterpillars spin silken cocoons and pupate inside. Cocoons are constructed in rolled leaves, bark crevices, or any protected places, such as on the eaves and windowsills of houses.

A cocoon of forest tent caterpillar in a rolled leaf

Cocoons of forest tent caterpillars on the eaves

Adult

The adult is a light-brown moth with two dark brown lines on its wings. It is about the size of a quarter to half-dollar coin. About 10-14 days after they pupate, adult moths emerge. In the northern Wisconsin, the moths of forest tent caterpillars are seen from late June to mid-July. The moths are attracted to lights at night. Because of this habit, during outbreaks, many moths are found flying in well-lit places such as gas stations and parking lots where lights are kept on at night. Once the adult moths emerge, they mate, lay eggs, and die within a week.

Moths of forest tent caterpillars near the light