Discover What the Forest Tent Caterpillar Moth Really Eats – The Ultimate Diet Guide!

The forest tent caterpillar moth (Malacosoma disstria) is a widespread insect found throughout North America, known for its periodic outbreaks that can cause significant defoliation of hardwood trees. Understanding their diet and life cycle is crucial for managing their populations and minimizing potential damage. Despite their name, forest tent caterpillars do not build tents, unlike other tent caterpillar species.

Host Plants and Feeding Habits

Forest tent caterpillars are polyphagous, meaning they feed on a variety of broadleaf trees and shrubs. Some of their preferred host plants include:

Oak
Sweet gum
Birch
Elm
Ash
Poplar
Sugar maple
Tupelo
Aspen
Black gum
Cherry
Basswood
Fruit and nut trees
Madrone
Redbud
Toyon
Willow

In the Northeast, sugar maple and red oak are favored ovipositional hosts. However, caterpillars frequently leave their natal tree and consume the leaves of many other species of hardwood trees. One exception is the red maple, which is not attacked by the caterpillars and is one of the few trees to remain foliated during outbreaks.

The caterpillars start feeding immediately after hatching in early spring, coinciding with the budding of host trees. They feed for about five to six weeks, growing rapidly. The first noticeable evidence of forest tent caterpillar attack is often a sparse crown and the constant sprinkling of frass (fecal pellets). In sufficient quantities, they completely defoliate host trees and even stands of trees. Since defoliation occurs early in the year, healthy trees will usually develop a second crop of leaves and suffer little serious harm. However, extensive defoliation two or three years in a row may cause stunting and dieback.

Life Cycle and Development

The forest tent caterpillar has one generation annually. The life cycle consists of four main stages:

Egg: Egg masses of 200 or more eggs are laid the previous summer and overwinter on twigs of host trees. The egg masses have square ends, unlike the tapered ends of the egg cases of the eastern tent caterpillars. The spumaline that covers the egg mass is light colored when first deposited, but it darkens over time.
Larva (Caterpillar): Eggs hatch in early spring, and the first instar caterpillars emerge. New caterpillars are nearly uniformly black, are less than 1/8 inch long, and have conspicuous hairs. 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. Mature larvae are about 2¼” long, dark bluish with yellow lines extending along each side of the body, and keyhole or footprint-shaped white spots down the middle of the back. Caterpillars grow to about 2 inches in length. The larvae are nomadic, frequently abandoning a resting site, or biovouac, in favor of another. Frequent movement is most often related to foraging and thermoregulation. Caterpillars launch forays from their resting sites in search of food, laying down exploratory trails. The trail pheromone is deposited when the caterpillars drag the undersides of their last abdominal segments against the substrate. The first caterpillars to finish feeding typically move away from the feeding site and establish a new resting site. The newly fed caterpillars lay down recruitment trails which serve to guide the remainder of` the colony to the new bivouac. Forest tent caterpillars form colonies that move about in a line, following silk trails laid down by leaders. With each successive molt, markings of pale bluish lines along the sides of a brownish body and a top row of footprint- or keyhole-shaped, white spots down the back. Although they are called tent caterpillars, they do not spin a tent. They form a silken mat on the bark where they congregate when resting or molting. During their early stages, they congregate in the upper part of the tree. Later they congregate lower in the tree and on the trunk. Larvae usually pass through five stages. When high populations result in complete tree defoliation, older caterpillars often crawl about in search of food or a suitable place for spinning cocoons. Their tendency to crawl in single file has caused them to be called "armyworms" by some.
Pupa: Mature caterpillars seek resting places on host plants and nearby structures in early to mid-June, spinning their cocoons in rolled leaves and bark crevices, on fences, under building eaves, and in nooks and crannies everywhere. Five or six weeks after hatching, the caterpillars spin yellow cocoons of silk colored in a folded leaf, bark crevice, or other sheltered place. In these cocoons, the larvae molt into pupae. Pupae are reddish-brown and the silk cocoons are commonly formed in loosely rolled leaves. The pupa stage requires about two weeks.
Adult (Moth): 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 inches, with females usually larger than males. The forewings have two darker oblique lines near the middle. The adult moth is light brown with a wingspan of approximately 1¼”. Forewings have one broad or two thin parallel dark brown lines across the center. The female moth lays eggs in late spring. The eggs hatch in the fall and the larvae remain in the egg mass until the next spring. Adults emerge in midsummer.

Population Dynamics and Outbreaks

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.

Management and Control Strategies

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.

Here are some strategies for managing forest tent caterpillar populations:

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.
Hand-pick egg cases: Remove and destroy egg masses from branches of small trees before eggs hatch in the spring.
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. Brush off caterpillars and cocoons from houses, picnic tables or decks with a stiff broom. Use a spray of water to knock caterpillars off. Be careful not to crush too many caterpillars as they can smear and leave marks on some paints.
Maintain disease and predator populations: Look for and preserve caterpillars that appear sick or diseased. Wasps and flies can grow inside forest tent caterpillar eggs, larvae and pupae to kill them. An example is the large gray fly, Sarcophaga aldrichi, that is native to Minnesota.
Pesticides: Pesticides are usually not needed or recommended. However, insecticides may be required for shade trees, recreational areas or during continuous epidemics in valuable hardwood forests. Bt (Bacillus thuringensis), a bacterial killer of caterpillars, may occur naturally or may be purchased for application to young caterpillars. Bacillus thuringiensis (Bt) controls larvae if high-pressure spray equipment is used so that insecticide penetrates webbing and also thoroughly sprays foliage around webbing. Bacillus thuringiensis kills only caterpillars (butterfly and moth larvae) that feed on sprayed foliage. The somewhat-selective spinosad is also effective, but can adversely affect bees and certain natural enemies. Because it is toxic to bees for several hours after the spray has dried, do not apply spinosad to plants that are flowering. Other pesticides that conserve beneficial insects: insecticidal soap, spinosad (microbial pesticide) and azadirachtin (botanical pesticide). Chemical pesticides like: acephate, bifenthrin, carbaryl, cyfluthrin, diazinon, esfenvalerate, fluvalinate, lambda-cyhalothrin, malathion, permethrin and phosmet. Always follow the product's label and ensure the product is effective against forest tent caterpillar. Spray trees by mixing 3¼ pints of 57% malathion EC with 100 gallons of water (1 tablespoon per gallon) or mix 2½ pounds of 80% sevin WP with 100 gallons of water. Apply pesticides when larvae are small (1 inch or less), usually in early to mid-May. Larger larvae are difficult to kill and can continue to defoliate trees before pesticides have any effect.
Maintain tree health: Strategies 1, 2, 3, 4, and 6 are strictly organic approaches.

Impact and Management in Specific Regions

In Texas, periodic and localized outbreaks of the forest tent caterpillar (Malacosoma disstria) may occur. They are particularly fond of the leaves of oaks and gums. The major impact from these caterpillars is slower tree growth.

In wood lots, birds, rodents and even bears eat forest tent caterpillar larvae. Control measures should be implemented only in case of heavy defoliation for a number of years. In wood lots, resort areas and campsites where large areas may need treatment: aerial spraying of pesticide by aircraft is the most rapid, effective and economical methods. Spraying should not be done when breezes threaten to drift the pesticide over open water or other sensitive areas. Treat an additional strip about 400 feet wide adjacent to the area, when spraying in residential or recreational areas. This barrier strip controls migrating caterpillars. The pesticide BT is preferred since it does not harm people, birds or beneficial insects and is generally considered first in aerial spray programs. Do not use diflubenzuron near wetlands or water, as the pesticide may affect aquatic insects and other arthropods.

Distinguishing Forest Tent Caterpillars from Similar Species

It is important to distinguish forest tent caterpillars from other similar species, such as the eastern tent caterpillar and other Malacosoma species. Adult eastern tent caterpillar moths are brown with 2 light-colored bands cutting across the forewing. Sometimes, the middle band between the two lines is lighter or whitish. The caterpillars have long, pale hairs; the body is mottled and striped with blue, yellow, and black, with a whitish line running down the back. The communal larvae of that species create tents.

The western tent caterpillar, Malacosoma californicum, larvae are reddish brown with some blue spots and covered with tufts of orange to white hairs. The Pacific tent caterpillar, M. constrictum, resembles M. californicum, except more blue is visible and the larvae usually feed only on oaks. Pacific tent caterpillars produce small tents a few inches wide.

Forest tent caterpillars have distinct, white, keyhole-shaped markings along the back. Larvae feed in groups on foliage, not within a webbed nest. Also, the communal larvae of that species do not create tents.

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