Tent Caterpillars - July 30, 2003
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Tent caterpillars are a common summer occurrence across Northern Arizona. People often become excited about them because they appear to cause extensive damage. In fact, they are native insects that are controlling/pruning/fertilizing wild grown trees and shrubs. However, in landscapes and orchards, their presence is less welcome and a home gardener or farmer may choose to aggressively control them.

Tent caterpillars colonize a wide range of host trees including: alder, apple, ash, birch, cherry, currant, cottonwood, willow, poplar, hawthorn, fruit trees, and roses. Of the species present here, the western tent caterpillar (Malacosoma californicum) is the most noticeable because they create a web-like tent on branch tips of trees and shrubs. These tents provide the young larvae with protective cover. Mature western tent caterpillars are hairy and dull yellow brown with a row of blue spots adjacent to orange spots on top of the body.

The forest tent caterpillars (Malacosoma disstria) are less conspicuous because they do not build a true tent. Instead, they spin silken mats on tree branches or trunks where they rest and seek protection. Its body is blue with black spatters. Both caterpillars have one generation per year and mature caterpillars of both species are about 2 inches long.

Tent caterpillar eggs hatch in early spring just as the new leaf and flower buds open. The larvae molt (shed their skins) four times during their 5- to 6-week growing period. Early in their development, tent caterpillars tend to feed gregariously, eating all of the leaves on one branch before moving on to the next. As the caterpillars mature, they begin to feed in small groups or singly. Just before they spin their cocoons in mid-summer, they crawl about looking for a protected place in plants or on structures to attach their cocoons.

Adult moths emerge from their cocoons 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 have numerous natural enemies including flies, wasps, birds, viruses, and fungi. Pupae provide nutritious meals for small mammals, while birds and bats eat moths. Natural enemies are one of the factors that keep wild tent caterpillar populations in balance. However, natural enemies cannot usually reproduce fast enough to manage damaging populations affecting landscapes and orchards.

Mechanical removal of egg cases is an easy strategy if you can reach the plant. If you missed them, wait for the young, gregarious caterpillars to return to their tent (early morning or evening), prune out the branch tip, and destroy it.

Bacillus thuringiensis (Bt) also can provide excellent control of tent caterpillars. Bt is a naturally occurring bacterium that will cause paralysis within the gut of immature insects in the family Lepidoptera (moths and butterflies); no other group of organisms is affected. Due to the specific nature of this pesticide, it is very safe to apply.

Chemical or synthetic pesticides are not recommended for control of tent caterpillars in home and garden situations. If it is a big, old tree, with a tent caterpillar colony way up high, just rest assured that the green leaves from that tree are being recycled into a time-release fertilize that is being spread evenly on the ground below. Tent caterpillars are primarily a nuisance, so relax, don't worry.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: February 5, 2003
Content Questions/Comments: jschalau@ag.arizona.edu
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