Western tent caterpillar (Malacosoma californicum) is a significant defoliator of broadleaved trees and shrubs. It is one of the most frequently observed and widely variable tent caterpillars in North America. Six subspecies and several unclassified forms have been recognized. When outbreaks occur, infested plants may be completely defoliation. Outbreaks may persist for three to five years.
Distribution & Habitat
Western tent caterpillar is prevalent across the western United States, southern Canada, and northern Mexico. Various subspecies of western tent caterpillar are localized to specific areas. M. californicum californicum, M. californicum ambisimile, and M. californicum recenseo are abundant in California. M. californicum pluviaele is common in the Pacific Northwest and Canada, where it ranges from southern British Columbia to western Quebec. M. californicum lutescens is found across the Southern United States, the Great Plains, and the Prairie Provinces of Canada. M. californicum fragile is distributed across the Southwestern United States. M. californicum has been reported from the Western United States to the Southwestern United States. Outliers of western tent caterpillar have been discovered in Minnesota, New Hampshire, and New York.
Western tent caterpillar and its subspecies collectively infest a broad range of plants. Quaking aspen is the preferred host across the Rocky Mountains, as well as in Northern Mexico. In the Pacific Northwest, western tent caterpillar favors cottonwoods, crabapple, oaks, poplars, red alder, and numerous fruit trees. In California, western tent caterpillar infests oaks, willows, and fruit trees. Other hosts that are frequently infested by western tent caterpillar include ash, birch, bitterbrush, Ceanothus, chokecherry, hazel, hawthorn, mountain mahogany, ninebark, Pacific madrone, serviceberry, three leaf sumac, skunkbush sumac, wild currant, and wild rose.
Eggs are laid in masses that partially encircle twigs and branches. Each egg mass is comprised of 150 to 250 eggs. Egg masses are encased in spumaline, a frothy material that varies from dark brown to pale gray in color. When newly hatched, the larvae measure around 3 mm long. Mature larvae extend between 1 ¾ and 2 inches in length. The larvae are variable in coloration and markings. Most feature a pale blue head and body, with mid-dorsal stripes that are banded black or a mixture of yellow and orange. Each abdominal segment is laden with small orange to orange-brown hairs with white tips.
The cocoons that the larvae construct are composed of white silk, and dusted with a white to yellow powder. The pupae are ¾ of an inch to 1 inch in diameter. They are reddish brown to black. The adults are heavy-bodied moths, with a wingspan of 1 to 2 inches. The coloration of the adults varies from dark brown to yellow, tan, or gray. The forewings of the adults feature a pair of lines that appear darker or lighter than the wing color.
Western tent caterpillar produces one generation per year. The insect passes through four stages: an egg stage, a larval stage, a pupal stage, and an adult stage. Masses of larvae emerge from the eggs in spring, during leaf expansion. The larvae are gregarious. They establish colonies, which feed together for much of the larval stage. Once they have hatched, the larvae commute to nearby foliage, where they commence feeding.
The larvae construct silken tents on the branches of infested plants. They gradually enlarge the tent to accommodate their increasing size. Often, the larvae from two or more egg masses will combine to form a large communal tent. In between feeding periods, the larvae remain in the tent, which conceals them from predators and inclement weather. The tents also serve as molting sites for the larvae as they develop. The larvae progress through five or six instars prior to maturation. Late stage larvae vacate the tent and become solitary feeders. In four to five weeks, the larvae mature.
The mature larvae weave silken cocoons, which they attach to nearby branches or leaves with a few thick strands of silk. The cocoons are often clustered together during outbreaks. The larvae envelop themselves in the cocoons to pupate. After 12 to 18 days, they emerge as adults. The adults are adept fliers; swarms of moths can often be observed fluttering around infested plants following pupation. The adults come together to copulate. Upon mating, the females navigate to nearby plants, where they lay flat, oval-shaped egg masses on a bevy of twigs and branches. The egg masses are covered with spumaline, which hardens, protecting the eggs from freezing conditions and desiccation. Within 3 to 4 weeks, pharate larvae form in the eggs. The pharate larvae overwinter inside of the eggs until the following spring.
Symptoms of Infestation
The silken tents produced by the larvae can be observed on the branches and twigs of host plants in spring. The tents gradually increase in size as the larvae develop. As the larvae feed, the defoliation of individual branches becomes apparent. During outbreaks, entire plants may be defoliated by late spring. Successive years of defoliation can result in extensive foliar dieback. On fruit trees, the persistent feeding of the larvae can reduce fruit yield. Severely weakened plants may succumb to infestation. In late fall and winter, the cocoons that the larvae weave are conspicuous on defoliated plants. Sometimes the cocoons may be discovered on the branches of alternate hosts, and in leaf litter.
- A multitude of natural parasites, predators, and disease organisms contribute to declines in western tent caterpillar populations. Thirty six species of parasites and predators attack the insect. Some of the most important parasitoids are wasps. Tetrastichus malacosoma parasitizes the eggs, while Habrobracon xanthonotus is a parasite of the larvae. The larvae of the parasitoids feed on the exterior of the western tent caterpillar larvae. Eventually, the feeding kills the western tent caterpillar larvae, resulting in a decline in populations. A nuclear polyderosis virus (NPV) can infect the larvae. Infections of the NPV can spread rapidly, and often cause populations to collapse. Larvae affected by the NPV will typically suspend themselves from branches or nests with their heads down, or fixed in a “v” shape.
- Foliage depletion can cause larvae to starve, limiting population growth.
- When populations are low, egg masses on small plants may be stripped or pruned from branches in winter.
- Infested branches containing tents can be pruned off and destroyed. The destruction of the occupied tents will reduce or eradicate the larval populations.
- Infested plant material can be immersed in soapy water to dislodge the insects.
- Several chemical and biological insecticides are registered for use in controlling western tent caterpillar. Applications should begin in late spring, as the larvae emerge. Subsequent applications can be performed at 7 to 14 day intervals, as required.
Photo courtesy of Franco Folini