Forest tent caterpillar (Malacosoma disstria) is a significant defoliator of various trees and shrubs. It is indigenous to North America. Outbreaks of forest tent caterpillar occur periodically, often at 10 to 12 year intervals. During outbreaks, infested plants may be completely defoliated. Infestations that persist for more than three years can result in plant mortality.
Distribution & Habitat
Forest tent caterpillar occurs throughout the United States and southern Canada. It is particularly abundant in eastern North America.
Forest tent caterpillar infests a wide range of hosts, which vary by region. In the Northeastern United States, forest tent caterpillar infests apple, ash, aspen, basswood, birch, cherry, elm, and oak. In the Central and Midwestern United States, forest tent caterpillar favors oak and quaking aspen. In the Southern United States, plum, sweetgum, swamp blackgum, and water tupelo are common hosts. In the Western United States and Canada, trembling aspen is the preferred host. Other tree species that are frequently infested by forest tent caterpillar include cottonwood, red alder, and willow. When populations are dense, forest tent caterpillar may feed on wild and ornamental shrubs such as azalea and rose, as well as on numerous cultivated fruits and vegetables.
Newly hatched larvae are dark brown, gray, or black. Initially, the larvae are small, measuring 3 mm in length. As the larvae grow, they develop faint yellow and blue lines along either side of their body, with a row of white spots on each abdominal segment. These white spots are laden with fur-like setae. When mature, the larvae can extend up to 2 inches long. The larvae form yellow silken cocoons, which they utilize for pupation. The adults are yellow or tan moths, with a wingspan of 1 to 1 ½ inches. Adults have a thick, furry body. Two dark, slanting lines appear in the center of the forewings. The adult females lay eggs in cylindrical masses that are around ½ of an inch long. Older egg masses exhibit a dull white coloration.Life Cycle
Forest tent caterpillar produces one generation per year. The insect develops through four stages: an egg stage, a larval stage, a pupal stage, and an adult stage. Upon being laid, the eggs overwinter. Masses of larvae emerge from the eggs in spring, as bud expansion begins. The larvae are affable creatures. They establish colonies, which forage together for most of the larval stage. Some of the larvae weave silken strands that form a trail, leading the colonies to nearby foliage. These larvae are called leaders. The leaders secrete a pheremone that induces the colony to move. Once the larvae have settled on a branch, they commence feeding.
As the larvae feed, they pass through five instars. During the early instars. the larvae feed on flower and leaf buds in the upper part of the crown. Once the upper portion of the crown has been defoliated, the larvae travel to the lower part of the crown to continue feeding. During the later instars, the larvae become independent. By the fifth instar, they seldom feed together. Despite their name, forest tent caterpillars do not construct tents. Instead, they establish a silken mat on the trunk or branches of infested hosts. The larvae aggregate on the mats to rest between feeding periods, and to molt.
After five to six weeks, the larvae mature. They construct silken cocoons, often forming them in folded leaves, which are fastened together with a few thick strands of silk. The larvae envelop themselves in the cocoons to pupate. Two to three weeks later, they emerge as moths. Upon leaving their cocoons, the females secrete a pheremone that attracts the males. Once the males have located the females, they mate. The adults are nocturnal; they take flight soon after nightfall, and return to host trees to rest before dawn. After copulating, the females fly to nearby hosts to deposit their eggs. Females lay batches of 100 to 350 eggs, most in the upper portion of the crown. The eggs encircle small twigs, and are bound together with a frothy substance called spumaline. The spumaline hardens, preventing the eggs from freezing, or becoming desiccated. Within three weeks, pharate larvae form in the eggs. They overwinter within the eggs, and hatch the following spring.
Symptoms of Infestation
In winter and early spring, masses of eggs can be observed on the branches and trunk of infested plants. Once the eggs have hatched, the flowers and leaves of infested plants will be rapidly consumed. If the larvae hatch prior to leaf expansion, they will mine the buds. On sugar maples, sap production may be significantly reduced in the year following infestation. Plants that have been weakened by forest tent caterpillar may be invaded by other pests, and disease pathogens. When defoliated, many plants will produce a second flush of growth. This second flush of growth will often appear stunted. Repeated defoliation can result in widespread foliar dieback, as well as plant mortality.
- Exposure to freezing temperatures can cripple larval populations.
- When plants are defoliated before the larvae mature, the lack of available foliage may cause them to starve.
- Forest tent caterpillar has a multitude of natural enemies that help to limit its populations. Several species of flies and wasps parasitize the eggs, larvae, and pupae. Two large gray flies, Sarcophaga aldrichi and Sarcophaga houghi are often responsible for the collapse of entire colonies. The adult flies deposit larvae on the cocoons. The larvae penetrate the silk of the cocoon and the epidermis of the pupae, effectively killing them. Itopletis conquisitor is an important ichneumonid wasp parasitoid of the pupal stage. Predatory beetles, ants, true buds, and spiders feed on the larvae and pupae. Frogs, mice, skunks, and over 60 species of birds prey on the larvae and pupae.
- Forest tent caterpillar is susceptible to a bevy of bacterial, fungal, protozoan, and viral diseases, which can cripple populations.
- Early instar larvae can be plucked from the branches that they feed on. This method is most effective when populations are low.
- Infested branches can be pruned out and disposed of. Pruning is most ideal in winter, when the egg masses are visible.
- Several insecticides and a microbial insecticide are registered for use in controlling the insect. Applications should begin in spring, when the larvae first appear. Subsequent applications may be performed at 7 to 14 day intervals, as required.
Photo courtesy of Greg Hume CC-by-3.0