Throughout the United States, various types of caterpillars defoliate trees, and one of the most destructive in South Carolina is the forest tent caterpillar (Malacosoma disstria). Despite its name, the forest tent caterpillar does not build tents like the eastern tent caterpillar, but instead will create silken mats on the bark and other surfaces when they congregate. These moth larvae are easiest to identify in mid-summer when groups of hairy, blue-striped larvae can be observed in large numbers. Forest tent caterpillars are widely distributed in North America and will feed on many species of broadleaf trees and plants.
Mature forest tent caterpillars can be found in clusters on silk “mats”.
Photo by David Jenkins, South Carolina Forestry Commission
Identification and Life Cycle
In late summer, adult females lay 100 to 350 eggs in dark, glossy masses that encircle small twigs. The eggs hatch in the following spring and tiny, black, hairy caterpillars begin feeding. As the caterpillars grow through 5 to 6 weeks, they develop characteristic markings, which include two parallel blue lines down the sides of the body and white markings down the center that resemble keyholes or boot prints. The caterpillars reach about 2 inches long when they are fully grown and retain defensive hairs throughout the larval phase.
Forest tent caterpillar egg masses are brown in color and wrapped around small twigs. In this photo, the little holes indicate the eggs have hatched.
David Coyle, ©2024, Clemson Extension
Forest tent caterpillars start out small and have distinct white markings down the center of the body that resemble keyholes or boot prints.
David Coyle, ©2024, Clemson Extension
When the caterpillars are not feeding, they gather in groups on silken mats, which they create. As the caterpillars grow, this group behavior (which is called being gregarious) diminishes. In early- to mid-summer, the caterpillars completely disperse from these groups and create yellowish silk cocoons. Cocoons are often less conspicuous as they are found within leaves or in sheltered locations, like bark crevices. After approximately 10 days, stout, yellow to brown moths emerge. The adults have furry bodies and a band of dark brown across their wings.
Adult moths are about 1” across and brown in color.
Photo by Mark Dreiling, Bugwood.org.
Hosts and Damage
Forest tent caterpillars feed on many different hardwood species, including oak, ash, sweetgum, and fruit trees. However, they may seek out other food sources, such as vegetable and fruit crops, if the larvae completely defoliate a tree before completing development. This may become a concern for homeowners with garden crops. Defoliation mostly damages the aesthetic value of trees, but repeated defoliation over multiple years can cause tree stress and occasional mortality. Defoliation by the forest tent caterpillar can also make trees more vulnerable to other damaging agents, such as drought. Forest tent caterpillar outbreaks typically last 3 to 5 years and recur about once every decade. Outbreaks eventually die down on their own, but homeowners may take action to prevent damage to desired plants and prevent aesthetic damage to property.
Tree defoliation from a forest tent caterpillar outbreak that occurred in the South Carolina Lowcountry from 2022-2024. These outbreaks defoliated thousands of acres of bottomland hardwoods as well as plantation-grown commercial hardwood species. The light areas show where caterpillars have completely defoliated host trees, while the dark green areas represent non-host trees (pines).
Photo by David Jenkins, South Carolina Forestry Commission
Management
The most common management tactic is to do nothing, as natural enemies typically regulate populations. Some common natural enemies in the Southeast include birds, wasps, and flies, including Sarcophaga aldrichi – a large grey fly that is a natural enemy of the forest tent caterpillars. While these flies do not bite, they can be a nuisance for homeowners, especially during outbreaks. For additional information on natural enemies, see Incorporating Beneficials Into the Gardener’s Toolkit.
Forest tent caterpillars are also susceptible to naturally occurring pathogens. A native virus called nuclear polyhedrosis virus (NPV) can quickly kill forest tent caterpillars when populations are high. Caterpillars infected by NPV will be slow-moving before wilting and dying. Caterpillars killed by NPV are typically found hanging in an upside-down “V” shape. Entomophaga aulicae (Ea) is a fungal pathogen that can also kill forest tent caterpillars. Pathogens such as these can be used as a biopesticide against forest tent caterpillars due to the low environmental risk and host specificity. NPV and Ea are the most effective during large forest tent caterpillar outbreaks.
When killed by NPV, forest tent caterpillars hang in an upside-down “V” shape.
Photo by David Jenkins, South Carolina Forestry Commission
For small infestations of forest tent caterpillars, egg masses and individual caterpillars can be physically removed by hand or with a strong stream of water. When caterpillars are moving in the early summer, double-sided sticky tape can be placed in a ring around the trunk or branches to prevent further defoliation. The tape will capture caterpillars as they move up and down the tree; these tapes should be changed regularly during an outbreak.
When more intense management is needed, insecticides can be used for forest tent caterpillar control. Applying bacterial-based insecticides, such as Bacillus thuringiensis (Bt) or Spinosad, to the foliage can control younger caterpillars. Other pesticides labeled for caterpillar control may be used on older caterpillars. Please consult your local Extension agent prior to use, and always read and follow the instructions on pesticide labels.
Originally published 11/24
