Navigation

Untangling the web of the Eastern Tent Caterpillar

There is a noticeable outbreak of Eastern Tent Caterpillars (ETC) now in progress in parts of Door County, and the so-called tents of these fascinating creatures are especially visible along roadsides. The most commonly chosen tree in which these larvae construct their silky white dwelling places is the Chokecherry, along with other close members of the rose family including the Black Cherry, crabapple, peach and plum.

Naturally, they are classified as pests in that they can defoliate, and possibly kill, a tree that is in poor health, but in general they should be looked at as little more than a nuisance. Fortunately, regional outbreaks vary only from 6 – 16 years; the caterpillars do not transmit diseases to humans, don’t bite, and aren’t poisonous. In the case that a tree in good health is completely defoliated by these caterpillars, it occurs early in the season while the leaves are still fairly small and the tree has not expended a lot of energy. In the majority of instances, the tree will sprout a new set of leaves by mid-summer, although smaller than normal, and will survive nicely.

A closeup of the social group.

The eggs hatch from a single inch-long mass of 300 – 400 lain the previous July by the female ETC Moth, a small reddish-brown moth with two pale stripes running diagonally across the wings. Strangely, the very tiny larvae develop inside each egg but remain inactive there throughout the winter. They don’t hatch into one-tenth-inch-long caterpillars until the following spring, about the time that the Chokecherry leaf buds emerge.

The lives these caterpillars will lead, from the time they are hatched in around early May to approximately the middle of June, is unquestionably a community form of living. They all feed, rest and digest together, and work constructing and enlarging the nest at the same time. The days are divided into definite periods of activity. The nest construction, nearly always in the crotch of a small tree or shrub, begins when the caterpillars are roughly 20 days old. The principal direction of its daily growth is upward with a continual expansion at the top.

The white silky nests are marvelous structures that facilitate aggregation, help the caterpillars stay warm at night, and serve as communication centers where the caterpillars are alerted to new food finds. Carefully examine one of the nests and you will see that it has been built to receive early morning sun for heat. As the nest grows in size there will be different layers or compartments that vary in temperature. The colder the air temperature becomes at night the farther away from the outer surface the larvae move. The incredibly fine texture and the streamlined shape of the nest repels rain quite well.

The small specks of “dirt” you see on the inside of the nest are mainly droppings, called frass, and the shed skins of the caterpillars. Each of these creatures experiences five to six instars, each instar occurring when the caterpillar’s size has outgrown the capacity of the skin and it is shed.

The caterpillars leave the nest in the morning after the air has warmed and move to feeding sites, eat, then return to bask on the sunny warm side of the nest. Doing so absorbs heat and also helps digestion. While in the process of crawling to a feeding site they lay down pheromone trails from the nest to the food so they can find their way back. During rainy weather they remain in the nest.

A fully-grown caterpillar is about two and one-half inches long, is black with a white stripe down the back, has narrow brown and yellow lines along the sides and a row of oval blue spots on the sides, really quite an attractive creature. Their larval development lasts from seven to eight weeks. Now each goes its separate way, spins its frothy little cocoon in the litter on the ground, trunk of a tree, on the side of a mailbox or gate post, or on the side of a building.

Webby homes made by tent caterpillars are a common site around the peninsula right now.

About two weeks later the adults eclose (emerge from the cocoon). The female secretes a pheromone drawing the male to her. They mate, she lays eggs and dies. The female’s entire life cycle may not have lasted more than 24 hours. A frothy material, called spumaline, coats the eggs preventing them from drying out and also from being parasitized by small wasps. In around three weeks the tiny larvae can actually be seen within the eggs where they will remain until the following spring.

The Fall Webworm, which some of you may be familiar with, has very long silky hairs, constructs the “tent” at the end of a branch with the leaves of the plant enclosed within. Their activity may continue from May to October. By comparison, the tent caterpillars’ tent is in the crotch of a shrub or tree, seldom if ever contains leaves, and is accomplished in the late spring into early summer. The Gypsy Moth caterpillar has five pairs of blue spots followed by six pairs of red spots along its back. They do not construct tents and are easy to identify

Natural enemies play an important part in reducing ETC numbers in most years. Included are tiny Braconid, Ichneumonid and Chalcid Wasps. Two of the best bird predators of the caterpillars are the Yellow-billed and the Black-billed Cuckoos. It has been found that the tiny fiberglass-like hairs of the caterpillars break off and lodge in the lining of the cuckoos’ stomachs. After a while the birds simply regurgitate the linings and continue to gorge themselves with the juicy easy-to-capture larvae. By the way, the hairs of some hairy caterpillars (there are many species) are known to be allergenic and you should especially avoid accidentally getting them into your eyes.

Look more deeply into the life cycle of the ETC and you will discover a number of benefits to nature. They provide food for some birds, shrubs and small trees beneath larger defoliated trees receive more light and, as a result, benefit, the frass of the caterpillars breaks down and helps fertilize the soil, the cocoons on the ground provide nutritious meals for small mammals, and the moths and are eaten by birds and bats.

It’s always most fitting, when examining the life cycle of the ETC, as well as many other life forms in nature, to realize that a great deal of good usually accompanies the bad.