Tent caterpillars—Malacosoma spp.
The larvae (caterpillars) of these moths (Lasiocampidae) feed on various broadleaf trees and shrubs. Hosts include ash, birch, fruit and nut trees, madrone, oak, poplar, redbud, toyon, and willow.
Adults are hairy moths, usually dull brown, orange, or yellowish, with a wingspan of 1 to 1-1/2 inches. Eggs occur in a group of 100 or more on bark. Larvae are covered with hairs, feed in a group at least when they are young, and grow up to 2 inches long.
Western tent caterpillar, Malacosoma californicum, larvae are reddish brown with some blue spots and covered with tufts of orange to white hairs. Larvae spin large, silken webs on leaves and twigs and feed on foliage mostly within tents.
The Pacific tent caterpillar, M. constrictum, resembles M. californicum, except more blue is visible and the larvae usually feed only on oaks. Pacific tent caterpillars produce small tents a few inches wide. Larvae feed in groups exposed on foliage, and enter tents mostly to molt to the next instar.
Forest tent caterpillar, M. disstria, larvae are mostly dark blue with wavy, reddish-brown lines. They have distinct, white, keyhole-shaped markings along the back. Larvae feed in groups on foliage, not within a webbed nest.
Tent caterpillars overwinter in dark-brown to pale-gray eggs in a group encircling small twigs, or in a flat mass on bark. They hatch and begin feeding in the spring, and some species form silken webs on foliage. Mature tent caterpillars spin silken cocoons in folded leaves, on bark, or in litter. Adults emerge in midsummer. Tent caterpillars have one generation per year.
Larvae chew leaves, causing ragged foliage. Some species cover infested parts with obvious silk webbing. Otherwise healthy plants tolerate the loss of some leaves, and tent caterpillars generally do not warrant insecticide application.
Tent caterpillars are often controlled by natural enemies. Eggs, larvae, and pupae are killed by over 100 species of parasitic tachinid flies and wasps. Larvae are consumed by various predaceous beetles and bugs. Eggs, larvae, and moths are eaten by birds. High populations of larvae are often quickly reduced by a caterpillar-specific nucleopolyhedrosis virus.
Where tent caterpillars have been a problem, inspect plants regularly for them during spring. When larvae are young, prune out tents or clip and dispose of branches infested with group-feeding larvae, if this can be done without cutting large limbs.
Bacillus thuringiensis (Bt) controls larvae if high-pressure spray equipment is used so that insecticide penetrates webbing and also thoroughly sprays foliage around webbing. Bacillus thuringiensis kills only caterpillars (butterfly and moth larvae) that feed on sprayed foliage. A second application of Bt about 7 to 10 days after the first is recommended because of its short persistence.
The somewhat-selective spinosad is also effective, but can adversely affect bees and certain natural enemies. Because it is toxic to bees for several hours after the spray has dried, do not apply spinosad to plants that are flowering.
See the California Forest Insect and Disease Training Manual (PDF) and Western Forest Insects (PDF) for more information.
Western tent caterpillar on tent
tent caterpillar larva
Tent caterpillar adult moth
Tent caterpillar eggs