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Tent Caterpillars

The tent caterpillar is one of the most conspicuous and familiar insect pests in the Northwest. Their silky, white tents can easily be seen covering the tips of tree branches. Tent caterpillars eat the foliage of many deciduous trees and shrubs, especially alder, willow, fruit trees, and roses. During a heavy infestation they will migrate and feed on many other plants.

Tent caterpillars damage plants by eating the foliage as it emerges. A healthy plant will usually grow out new leaves by summer, but its growth may be reduced, making it more susceptible to the stresses of drought, cold, disease, and other insects. For this reason, it is a good idea to monitor tent caterpillars closely and control them if necessary. Fortunately, there are effective non-chemical control methods available.

The life cycle

The egg masses are visible in winter, are grayish-brown and about one inch long. As soon as the eggs hatch in April or May, the caterpillars begin eating leaves and create "tents" - white, silky shelters that cover the forks and tips of tree branches and shrubs. They feed during daylight and return to their tents at night.

The caterpillars are easy to recognize. They have a yellow checkered pattern with a blue dotted line down their backs, and can be three inches long when mature. The caterpillars molt four times over a period of five or six weeks, then stop eating. They choose a place to spin a cocoon and lay their eggs, and moths emerge about two weeks later. The adult moths mate immediately, lay eggs, and die a few days later; the eggs winter over until spring, when the whole process begins again.

What to do

While the creatures are unpleasant, they are not harmful to people. Natural predators include many garden birds, which eat the caterpillars. Other predators are wasps and ground beetles. Homeowners who see infestations on their property should watch the number of tents forming and monitor the leaf damage. Individual tents can be removed by pruning in the early morning and evening, when the temperature is cool and caterpillars are inside them (a pole pruner is useful for high branches). Put pruned nests in a bucket of soapy water or seal them in a plastic bag and crush it. Do not remove large branches or perform excessive pruning as a means of caterpillar control - you may do more damage to the tree by pruning than the caterpillars would do by eating the leaves.

Another method of control is to hand-strip egg masses, or prune them out of plants during the winter.