Insect Profiles: Eastern Tent Caterpillar (Malacosoma americanum)
Eastern tent caterpillar (Malacosoma americanum) is a species of moth that is native to North America. The insect was first observed in the United States in 1646. Eastern tent caterpillar is a member of the family Lasiocampidae. It is often mistaken for a bagworm, and sometimes erroneously referred to as gypsy moth, fall webworm, or forest tent caterpillar. Forest tent caterpillar is similar in appearance, but does not produce a tent. Eastern tent caterpillar is notable for being a social insect. The larvae form communal nests, or tents, in the branches of trees, where they feed in groups. Larval populations tend to fluctuate each year, with outbreaks occurring every eight to ten years.
Distribution & Habitat
Eastern tent caterpillar is distributed across the Eastern and Central United States. The span of its range extends into Canada, where it may be found from Nova Scotia to Alberta.
Eastern tent caterpillar commonly infests apple, crabapple, and wild cherry. Hawthorn, maple, peach, pear, and plum are also infested, albeit with less frequency.
The adults lay clusters of small, glossy eggs on the twigs of susceptible trees and shrubs. The egg clusters can measure up to 19 mm in length. The larvae are black, hairy caterpillars. A distinct white stripe trails down the back of each caterpillar. Brown and yellow lines are displayed along the caterpillars’ sides, along with a characteristic row of oval blue spots. As the larvae develop, they secrete fine strands of silk that combine to form a communal web. When mature, the larvae are 2 to 2 ½ inches long. The coccoons that the larvae weave measure about 1 inch long, and are comprised of closely woven white or pale yellow silk. The pupa are the size of the adults; they are reddish-brown, and enclosed in the silken coccoons. The adults are reddish-brown moths with two oblique, white stripes extending across each forewing. When mature, they measure approximately 60 mm in length.
Eastern tent caterpillar is univoltine; it produces one generation per year. The eggs overwinter in masses that encircle the twigs they have been laid on. Each egg mass consists of 200 to 400 eggs. The eggs hatch in early spring, as leaf expansion begins. As the eggs hatch, they reveal masses of larvae. The larvae are amongst the most social of any insect. The larvae from one egg mass will generally remain together, and work together to establish a communal web in the crotch of a tree. Larvae from multiple egg masses may unite to form one large colony.
The webs, or tents, are composed of silken threads woven by the larvae. The tents are multifunctional, serving as aggregation sites for the larvae as they feed, while safeguarding them from predators. The tents also provide the larvae with shelter on warm and rainy days. The larvae emerge in the early morning to feed on nearby leaves. During the afternoon, the larvae will often retreat into the tents to reduce their exposure to the sun. At night, the larvae will leave their tents to resume feeding.
The larvae pass through six instars as they develop. To accommodate their increasing size, the larvae contribute additional layers of thread to their tents. Once the larvae have entered their final instar, their feeding habits become more restrictive. They begin feeding exclusively at night. They also cease expanding their tents, conserving their silk for the construction of coccoons.
The larvae reach maturity in four to six weeks. Once mature, they begin to wander from their tents to construct silken coccoons. Once they have found a suitable location, the larvae commence building their coccoons. To secure the coccoons, the larvae attach them to nearby objects with a few coarse threads. The larvae then enter the coccoons, and proceed to pupate. They emerge as adult moths three weeks later. The adult moths are nocturnal. They become active during the evening, whereupon the males navigate to the females to mate. Upon mating, the males expire. The females deposit their eggs on nearby twigs, dying shortly thereafter. The eggs overwinter, and hatch the following spring.
Symptoms of Infestation
A colony of eastern tent caterpillars can rapidly defoliate small trees and shrubs. While this may temporarily reduce the aesthetic value of the host, most plants will produce a second flush of growth within two to three weeks. The silken tents produced by the larvae are one of the most conspicuous symptoms of infestation. As the tents enlarge, they become increasingly apparent on the branches of infested plants. When present, the larvae can be frequently observed crawling on infested plants, walkways, furniture, and buildings.
- Insecticides can be applied to susceptible plants to manage larvae populations. Applications should be performed as soon as the larvae hatch. Once the larvae have established their tents, they become more difficult to combat.
- In early spring, tents can be pruned out and destroyed.
- Egg masses can be stripped from trees during winter to reduce populations the following spring.
- Several natural predators, including various tiny braconid, ichneumonid, and chalcid wasps, can drastically reduce larvae populations when present.
- Maintain tree vigor to encourage recovery from defoliation.