Tent caterpillars are a serious nuisance in western Washington. Periodic outbreaks result in masses of unsightly tents in trees, defoliation of deciduous trees, and a high level of aggravation for people trying to control the wriggling masses of larvae. These caterpillars attack a wide variety of plants. Their preferred hosts are alder, apple, ash, birch, cherry, cottonwood, willow, fruit trees, and roses. During heavy infestations, the tent caterpillars will migrate and feed on many other plants.
Trees infested with several tents are often totally defoliated. A single occurrence rarely kills a tree, but it does reduce growth and makes the tree more susceptible to other hazards such as drought, freezing, or disease. A healthy tree which has been attacked will usually grow new leaves by midsummer.
Forest tent caterpillars (Malacosoma disstria Hubner) do not form a true tent despite their name. Rather, they spin silken mats on tree branches or trunks. The mature caterpillar is about 2 inches long. Its body is blue with black spatters and has white, footprint-shaped markings.
The western tent caterpillar (Malacosoma californicum pluviale Dyar) is often the most numerous in western Washington. Its orange and black markings are familiar to many people. This species spins tents on the tips of branches. The eggs hatch in early spring just as the new buds break in April or May. The young larvae begin feeding in groups. The larvae of both species molt (shed their skins) four times during their 5- to 6-week growing period.
As the caterpillars mature, they begin to feed in small groups or singly. Just before they spin their cocoons in mid-June, they crawl about looking for a protected place in plants or on structures to attach their cocoons. The adult moths emerge in approximately 7 to 10 days. The moths are stout-bodied and light brown. They often fly in clusters around street or porch lights on summer evenings. After the moths mate, the females lay 100 to 350 eggs in a froth-covered band around small twigs or branches of host trees. The eggs mature in 3 weeks but do not hatch until the following spring.
Tent caterpillars are primarily a nuisance. They do not transmit diseases to humans, do not bite, and are not poisonous. During years when large numbers of these caterpillars hatch, they can cause slippery roads and walks when they leave the trees.
Benefits of a caterpillar outbreak can be numerous in a natural setting. While caterpillars are distasteful to most birds, some birds feed on them. When alders and other trees are defoliated, the shrubs and trees below receive increased sunlight, giving some of them a boost in growth. The eaten leaves pass through the caterpillar's body and emerge as little pellets which can break down easily, returning nutrients to the forest floor. Pupae provide nutritious meals for small mammals, and moths are eaten by birds and bats.
Where trees are crowded or stressed, the defoliation could be a life and death matter. Weak trees may die; healthy trees will leaf out again. In a natural setting, surviving trees can prosper in the absence of competition.
Healthy ornamental trees and shrubs should survive even serious defoliation. Trees which have been under stress (excess cold, heat, crowding, drought, flooding, etc.) may succumb and require more protection.
Tent caterpillars have numerous enemies. One is a tachinid fly which parasitizes the larvae by depositing white eggs on the caterpillar's body. When the egg hatches, a small maggot burrows into the caterpillar and begins feeding. Tent caterpillars are also subject to a virus disease called wilt. While such natural enemies will reduce the number of tent caterpillars eventually, this process is gradual and may take 2 or more years. During that time, the affected trees may suffer such severe damage, that they will not recover.
An effective way to control tent caterpillars is to remove their egg cases from trees. The egg masses are a brown or gray frothy material which has hardened to look rather like Styrofoam. Most egg cases are one-half-inch long bands that appear on twigs. While some are long bands found on twigs, others are flattened shapes found on tree trunks or buildings. In both cases, egg cases can be picked by hand or pruned out.
Once the caterpillars have hatched, the simplest way to control them is to remove and destroy the larvae and their nests by stripping or pruning them from branches. Burning the nests, a traditional method of control, is no longer recommended because of the fire hazard.
Adult tent caterpillars are attracted to lights at night, and females may lay eggs on building surfaces near these lights. To reduce this nuisance, use yellow light bulbs temporarily as night lights outside wherever possible during the flight period (July). Yellow lights are far less attractive to night-flying insects.
Evening and early morning are the best times to prune or spray because tent caterpillars tend to congregate in their nests at night.
When tent caterpillars are numerous or hard to reach, chemical controls are recommended. Infested trees may be sprayed with a biological insecticide, Bacillus thuringiensis (B.t.). This control is selective; it kills only caterpillars and is relatively safe for other insects, fish, birds, and warm-blooded animals. Caterpillars must eat a moderate amount of treated leaf to get an effective dose. Thorough coverage of foliage is necessary, and spraying should not begin until early signs of leaf damage appear. The effects of B.t. are not immediately apparent. Caterpillars sicken and stop feeding right away, but they do not die for a few days. Younger larvae are more susceptible to B.t.; once caterpillars are migrating, they do not eat much, and other pesticides are more effective.
Other registered chemical controls can be found in the Pacific Northwest Insect Control Handbook. All County WSU Extension offices have a reference copy at their disposal. Additional information is available on line at WSU Hort Sense.
Many chemical controls are very toxic to foraging bees and should not be used on bloom or around flowering plants. Be sure to check the potential for bee hazard on the material label you choose. B.t. is nontoxic to bees and most other beneficial insects.
CAUTION: When spraying fruit trees or ornamentals near the vegetable garden, be sure to use only those products registered for use on edible crops, or protect vegetable beds with plastic or newspaper. This information will appear on the insecticide label. Always read and follow label directions carefully. Spraying is most effective when the caterpillars are newly hatched and most susceptible. Spraying will not destroy caterpillar nests. Remove nests by hand or allow them to weather away.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
For more information contact your local WSU Extension Office.