Eastern Tent Caterpillar: Webworm
Caterpillars defoliate trees but cause no lasting damage
The Easter Tent Caterpillar is a pest with a most remarkable life cycle.
Wander through the woods in the spring and you are likely to see trees with strange white webs in the angles of the branches. Step closer and you might see a few caterpillars basking in the sun. Take a few steps closer and you might discern that the entire ‘tent’ is full of these strange caterpillars. Before the theme music of a horror movie starts playing in your head, be assured, you are simply examining the home and remarkable society of the Eastern Tent Caterpillar.
Appearance:
Larval: The caterpillar is black with a white stripe down the back. Brown and yellow lines run down the sides next to a row of oval blue spots.
Adult: The moth is reddish-brown with a pair of pale diagonal strips on the forewings.
Social Life and Larval Development
Despite the repulsiveness of the worm and the damage they do to young trees and to the beauty of the landscape, the life and society of the Eastern Tent caterpillar is complex and fascinating. In late spring or early summer, the adult form of the caterpillar, a moth, lays eggs. The egg mass contains 200-300 eggs and can be found encircling branches that are pencil-sized or smaller in diameter. The egg masses look like ugly black fungal growths and are covered with a shiny, black varnish-like material.
Within three weeks fully formed caterpillars exist inside the eggs but they stay put until the following spring when the chew their way out of the eggs as the buds on the tree begin to expand. Immediately, the caterpillars begin constructing a tent, usually near the site of their egg case. Caterpillars from one case stay together for their entire larval cycle.
The caterpillars feed three times a day, just before dawn, mid-afternoon, and just after sunset. Every day, the size of the tent is increased to accommodate the growth of the caterpillars. The caterpillar’s tent is fascinating. Like many insects, the caterpillars appreciate the warmth of light. As they build their tent, they spin the majority of the silk on the most illuminated side of the tent.
The tent is created in layers and so it is thermally heterogeneous. Consequently, the caterpillars can adjust their temperature by moving from layer to layer. If the tent becomes too hot, they can move to the shaded side of the tent and hang from the tip of their abdomen so that they cool down more quickly.
The caterpillars do everything together including travel to their feeding site. When the caterpillars move from location to location, they leave a silk trail and a pheromone trail behind them. Existing paths are more appealing to the caterpillars so soon there are well-marked silk routes on the tree.
During the last stage of their larval period, the caterpillars feed only at night. When they reach the end of their larval period, the caterpillars leave the tent and go on independent journeys to find a place to construct a cocoon. Within two weeks, a moth emerges from the cocoon. Mating and egg-laying occur on the day the moths leave the cocoon, the female moth dies soon after.
Control
Birds do not threaten the Eastern Tent Caterpillar as only the cuckoo can tolerate their taste. However, they do have natural predators. A variety of parasites use them as hosts including: braconid, ichneumonid, and chalcid wasps
While unsightly, the damage that the caterpillars inflict on trees is not significant. Most trees recuperate easily and quickly. However, unless one likes the appearance of defoliated trees with big webs, it is desirable to remove the caterpillars. The best method of control is removal of egg masses when found. In the spring, small webs can be removed by hand. Some insecticides will work on the small larvae however; they are not effective on the worms within the tent and therefore are not particularly useful.
