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Overview

Notwithstanding their name, forest tent caterpillars do not build tents. They do produce, and rest on inconspicuous flat silk mats spun on branches and trunks. They also lay inconspicuous silken pathways to and from branches on which they feed. It is their close relative, the eastern tent caterpillar that forms tents.

Host plants include oak, sweet gum, birch, elm, ash, poplar, sugar maple, tupelo, and aspen. Under the pressure of high population density, they will feed on many other species of woody trees and shrubs.

Symptoms and diagnosis

Over-wintering egg cases on twigs of host trees are not particularly conspicuous. Often the first noticeable evidence of forest tent caterpillar attack is a sparse crown and the constant sprinkling of frass. When they are not feeding, they can be observed clustered on their silken mats on lower branches and trunks.

In sufficient quantities, they completely defoliate host trees and even stands of trees. Sometimes they will form wandering swarms in search of additional food sources.

Since defoliation occurs early in the year, healthy trees will usually develop a second crop of leaves and suffer little serious harm. Two or more consecutive years of defoliation can substantially affect growth and cause branches and twigs to die back.

Caterpillars grow to about 2 inches in length. They are black to dark gray and fuzzy along the bottom edge. As they age, they develop narrow side stripes of yellow, a wider side stripe of blue, and have a series of white oval spots running down their backs. These oval spots readily distinguish them from the related eastern tent caterpillar, which possesses a solid white line running down the back.

Life cycle

Forest tent caterpillars have one generation annually. Egg masses of 200 or more eggs laid the previous summer over winter and hatch in early spring. The egg masses of these forest tent caterpillars have square ends, unlike the tapered ends of the egg cases of the eastern tent caterpillars.  Caterpillars start feeding immediately and feed for 5 or 6 weeks. Mature in early to mid-June, they seek resting places on host plants and nearby structures, spinning their cocoons in rolled leaves and bark crevices, on fences, under building eaves, and in nooks and crannies everywhere. Emerging in 1 ½ - 2 weeks, they find mates, lay egg masses on host plants, and expire within the week. The moth adults are attracted to night lights, and heavy infestations can produce large numbers of moths the next morning in the vicinity of those lights. Moths have wing spreads of 1 3/8 to 1 ¾, with females usually larger than males. Wings are light cinnamon tan with two darker bands forming shallow inverted Vs when moths are at rest.

In some parts of their range, population “outbreaks” (high population densities) tend to occur periodically, with 2 or 3 years of high population density followed by several years of low density. Weather conditions and the predator-prey effect are considered possible causes for these fluctuations.  In the predator-prey scenario, as caterpillar populations increase, predatory parasites increase in numbers, eventually overwhelming the caterpillar population after 2 or 3 years, causing it to crash. At that point, the predatory parasites are unable to sustain their populations, and they crash. The caterpillar population can then expand again in the relative absence of the predatory parasites, and the cycle begins again.

Integrated Pest Management Strategies

1. Let nature take its course. Several natural control agents will normally keep populations of forest tent caterpillars within acceptable bounds. Unseasonably high and low temperatures will kill them. Ants, birds, beetles, parasitic wasps, naturally occurring bacteria, viruses, and fungi all contribute to keeping populations down. Avoid the use of most pesticides. They will be equally effective in reducing predatory insects and will therefore usually be counterproductive. Turning off night lights during the few days the moths are present may reduce the egg cases in the immediate vicinity.

2. Hand-pick egg cases. Check host plants over winter for egg cases, prune them out and destroy them.

3. Find and destroy caterpillars and cocoons. Look for congregations of caterpillars on lower branches and trunks at dusk. Pick them off or brush them off into soapy water and dispose of them. Find and destroy cocoons.

4. Maintain disease and predator populations. Look for and preserve caterpillars that appear sick or diseased. Caterpillars and cocoons with a number of small white ovals attached to them are “parasite factories” and should be protected.

5. Pesticides. Pesticides are usually not needed or recommended. Bt, a bacterial killer of caterpillars, may occur naturally or may be purchased for application to young caterpillars. It is less effective with older caterpillars.   Repeated outbreaks may require special attention and pesticides are an alternative.  Insecticidal soaps, permethrins, and pyrethrins may be needed for older larvae if removal is impractical.  Always follow pesticide labels.  Note the predator-prey scenario and the likelihood that a pesticide will adversely affect potential predators as well.

6. Maintain tree health. Make sure the affected trees are adequately watered and mulched.   Fertilizing after caterpillars are gone is an option.

Organic Strategies

Strategies 1, 2, 3, 4, and 6 are strictly organic approaches. For an organic approach to Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt), insecticidal soap, and pyrethrin products.