Eastern Tent Caterpillars Are Back

Apr 11, 2017

They’re back!
Our doctors have noted eastern tent caterpillar nests so it is time for be looking in the trees.

Eastern Tent Caterpillars 2017

When mature, the large, hairy caterpillars wander from these nests, especially along fence lines. Consumption of large numbers of caterpillars by pregnant mares precipitated foal losses in the mare reproductive loss syndrome (MRLS) outbreak that peaked in 2001. MRLS can cause late-term foal losses, early- and late-term fetal losses and weak foals. Studies revealed that horses will inadvertently eat the caterpillars, and the caterpillar hairs embed into the lining of the alimentary tract (which includes the pharynx, esophagus, stomach, small intestine, and colon). Once that protective barrier is breached, normal alimentary tract bacteria can gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta.

If practical, farms should plan to move pregnant mares from areas where these trees are abundant to minimize the chance of exposure to the caterpillars. The potential is greatest when the mature tent caterpillars leave trees and wander to find places to pupate and transform to the moth stage. Please read below for more information on the eastern tent caterpillar and how to get rid of these nests.


Eastern Tent Caterpillar
ENTFACT-423: Eastern Tent Caterpillar |
by Ric Bessin, Extension Entomologist

University of Kentucky College of Agriculture
Eastern Tent Caterpillar Larva
The eastern tent caterpillar, Malacosoma americanum, is a pest native to North America.
Populations fluctuate from year to year, with outbreaks occurring every several years. Defoliation of trees, building of unsightly silken nests in trees, and wandering caterpillars crawling over plants, walkways, and roads cause this insect to be a pest in the late spring and early summer. Eastern tent caterpillar nests are commonly found on wild cherry, apple, and crabapple, but may be found on hawthorn, maple, cherry, peach, pear and plum as well.
While tent caterpillars can nearly defoliate a tree when numerous, the tree will usually recover and put out a new crop of leaves. In the landscape, however, nests can become an eyesore, particularly when exposed by excessive defoliation. The silken nests are built in the crotches of limbs and can become quite large.
Larvae cause considerable 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.
Insecticides are generally ineffective against mature larvae.
Eastern tent caterpillar nests are frequently confused with fall webworm nests. Unlike the tent caterpillar, fall webworm nests are located at the ends of the branches and their loosely woven webs enclose foliage while the tents of the eastern tent caterpillar do not. While there may be some overlap, fall webworm generally occurs later in the season.
ETC Egg Mass
The eastern tent caterpillar overwinters as an egg, within an egg mass of 150 to 400 eggs. These masses are covered with a shiny, black varnish-like material and encircle branches that are about pencil-size or smaller in diameter.
The caterpillars hatch about the time the buds begin to open, usually in early March. These insects are social; 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.
ETC Tent
The larvae are hairy caterpillars, 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. As the larvae feed on the foliage, they increase the size of the web until it is a foot or more in length. 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 cocoon is about 1 inch long and made of closely woven white or yellowish silk and is attached to other objects by a few coarser threads.
ETC Adult Female
The adult moth emerges from the cocoon about 3 weeks later. The moth is reddish-brown with two pale stripes running diagonally across each forewing. Moths mate and females begin to lay eggs on small branches. The eggs will hatch next spring. There is just one generation per year.
• Natural enemies play an important role in reducing eastern tent caterpillar numbers in most years. Caterpillars are frequently parasitized by various tiny braconid, ichneumonid, and chalcid wasps. 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.
• 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 pruned out and 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 varkurstaki. Other insecticides include carbaryl, methoxychlor, and malathion. Larvae within the tents are protected beneath the webbing and are more difficult to kill with an insecticide.

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.