Eastern Tent Caterpillar

5 Apr 2006

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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The great thing about spring is all of the insects reminding us how much we missed them over the winter. Around this time of year I can drive down the highway and identify some trees at the speed limit thanks to my insect friends pointing them out. I am, of course, referring to the beautiful web nests constructed in certain trees by our friend the eastern tent caterpillar.

The eastern tent caterpillar, Malacosoma americanum, is an insect native to North America. Populations fluctuate from year to year, with large outbreaks occurring every several years. Defoliation of trees, building of silk nests in trees, and wandering caterpillars crawling over plants, walkways, and roads cause this insect to be called a pest in the late spring and early summer. Species of the genus Prunus and Malus are preferred hosts, including black cherry, chokecherry, and apple. Eastern tent caterpillar nests are also found on hawthorn, maple, cherry, peach, pear, and plum.

The eastern tent caterpillar is primarily an aesthetic problem and has little adverse effect on host trees. While tent caterpillars can nearly defoliate a tree when numerous, the tree will usually recover and put out a new crop of leaves. Their tent nests can become an eyesore, particularly when exposed by excessive defoliation. The tent nests are built in the crotches of limbs and can become quite large.

Larvae cause concern when they begin to wander to protected places to pupate. They are frequently seen crawling on other types of plants, walkways, and storage buildings. Although they can create a mess when they are squashed on driveways, sidewalks, and patios, keep in mind that no additional feeding or damage is done by the wandering mature caterpillars and insecticides are generally ineffective against mature larvae.

The eastern tent caterpillar overwinters as an egg, within an egg mass of 150 to 400 eggs. These egg masses are covered with a shiny, black varnish-like material and encircle branches that are about pencil-size or smaller in diameter. The overwintering eggs hatch about the time black cherry buds begin to open in the spring. These insects appear to be social in that caterpillars from one egg mass stay together and spin a silken tent in a crotch of a tree. Caterpillars from two or more egg masses may unite to form one large colony. During the heat of the day or rainy weather, the caterpillars remain within the tent. They emerge to feed on leaves in the early morning, evening, or at night when it is not too cold. They consume the entire leaf, except the midrib. Trees sometimes contain several nests and can be completely defoliated in 2-3 weeks.

The caterpillars are hairy, black with a white stripe down the back, brown and yellow lines along the sides, and a row of oval blue spots on the sides. In 4 to 6 weeks the caterpillars are full grown and 2 to 2-1/2 inches long. At this time, they begin to wander away individually from the nest in search of protected areas to spin a cocoon. The adult moths emerge from their cocoons about 3 weeks later, in early summer. The moth is reddish-brown with two pale stripes running diagonally across each forewing. Their principal concerns after emergence from their cocoons are mating and egg laying. Since the moths have no functional mouthparts, they cannot feed and thus live only a short time. The females lay up to 2500 eggs in tight clusters coated with a waterproof foamy brown substance, and the insects spend approximately nine months of the year, including the winter, in the egg stage. The eggs will hatch next spring. There is one generation per year.

Natural enemies play an important role in reducing eastern tent caterpillar numbers in most years. Various tiny wasps frequently parasitize caterpillars. Several predators and a few diseases also help to regulate their populations. This, in part, accounts for the fluctuating population levels from year to year.

Control is not normally necessary since defoliated trees usually refoliate after being attacked. If control is desired, prevention and early control is important. Removal and destruction of the egg masses from ornamentals and fruit trees during winter greatly reduces the problem next spring. In the early spring, small tents can be removed and destroyed by hand. Larger tents may be destroyed or removed by winding the nest upon the end of a stick. Burning the tents out with a torch is not recommended since this can easily damage the tree. Young caterpillars can be killed by applying an insecticide containing, Bacillus thuringiensis (Bt). Larvae within the tents are protected beneath the webbing and are more difficult to kill with an insecticide.

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For more information about these and other landscape gardening topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.