Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

Forest Tent Caterpillar (Malacosoma disstria)

Several Forest Tent Caterpillars on a branch
Credit: Edgar T. Jones, 1984

Forest Tent Caterpillar - the moth
Credit: Terry Thormin

Egg case of the Forest Tent Caterpillar
Credit: Edgar T. Jones, 1989

INTRODUCTION

The Forest Tent Caterpillar can become so numerous some years that it becomes dangerous to travel at normal speeds on some highways as the squashed caterpillars make the road surface very slippery.

IDENTIFICATION

The caterpillar, which is familiar to most Albertans, is blue with black, red and white markings. The white marks form a series of spots, each surrounded by black, going down the middle of the back. Mature caterpillars may get up to 5.5 cm long. The moth has light yellowish or reddish brown wings with two darker bars crossing each forewing. Wingspans range from 2.3 to 3.7 cm.

DISTRIBUTION

In Alberta this species is largely restricted to the Boreal Forest, Aspen Parkland and Mountains where there are hardwood trees. It is found throughout virtually all of North America.

TIME OF YEAR

In Alberta the caterpillars first become active around mid-May, just about the same time that the trees are leafing out. They will continue through until late June or early July when the last of the caterpillars pupates. By early July the first adults appear, and although the adults live only five to ten days, they can be around into early August because of differences in the timing of hatching and pupation.

HABITAT AND HABITS

Forest Tent Caterpillars feed on a wide variety of host plants. In Alberta, the most common host plant is Trembling Aspen, but many species of native and ornamental trees and shrubs will also be attacked. Populations tend to be cyclical, erupting every 6 to 16 years, and severe infestations may last 2 to 4 years. During these years they can cause considerable defoliation to trees, but seldom does this result in serious damage or death. The defoliation of one's favorite tree or shrub is certainly unsightly, and the caterpillars can become a nuisance and even dangerous, when sidewalks and roads become greasy from their squashed bodies. Later, when the moths emerge and are attracted to lights, these can also be a problem. It is worth mentioning that this species does not make a tent, but rather spins a mat of silk on the trunks or main branches of the trees on which they rest when not feeding.

SIMILAR SPECIES

In Alberta the only other tent caterpillar is the Prairie Tent Caterpillar, Malacosoma californicum. This species does make a tent on which the caterpillars rest. There are two subspecies of this species in Alberta, M. californicum pluviale, which occurs in the boreal forest and foothills, and M. californicum lutescens, which occurs in the prairies and aspen parkland. The caterpillar of pluviale has very little blue in it and is generally black with yellow and white mottling. The caterpillar of lutescens is rather similar to the caterpillar of M. disstria, but has a solid white line down the middle of the back. The moths of both subspecies are very similar, and differ from M. disstria in having the front wing bands being white instead of dark. Where M. disstria overlaps with M. californicum it is generally the more commonly encountered.

COMMENTS

There is no doubt that these caterpillars can become numerous enough that they are a major nuisance and occasionally quite destructive. We recommend that people try to limit their numbers without resorting to spraying. This can be done several ways. When the leaves are off the trees the egg masses are more easily seen. They can be scraped off the branches with a dull knife, or on smaller branches, the whole branch can be cut off. Because the eggs often have parasites, it is suggested that rather than dispose of the egg mass, you put it in a place where it won't be covered in snow and away from the trees or bushes. Egg masses that are still on the branch could be easily duct taped to a pole or fence. This prevents the caterpillars from surviving, as they cannot find food immediately, but allows the parasitic wasps that have been developing in the caterpillar to emerge and parasitize other caterpillars. When caterpillars hatch in the trees or bushes, hose them down with cold water as soon as you see them. Young caterpillars are particularly susceptible to this treatment. At later stages caterpillars probably can be dealt with individually. If you feel that you have no option but to spray the caterpillars, we recommend that you start by looking up your local community's web site. They probably have a section on pest control that will point you in the right direction.

Note: We cannot provide advice on controlling insect pests. If you need more information beyond what has been provided here, we suggest you either search further on the web, or talk to a local exterminator.

[Royal Alberta Museum Home Page] [Invertebrate Zoology]

For more information on this bug, please contact the Bug Room.
© Copyright 2000-2008, Royal Alberta Museum - ALL RIGHTS RESERVED
Last update: August 19 2004