Western Tent Caterpillar

Order: Lepidoptera (moths & butterflies)
Family: Lasiocampidae (tent caterpillars & lappet moths)
Genre: Malacosoma californicum pluviale

Visit the Tent Caterpillar Update Site to see what happening now!

Biology & Life History:

The western tent caterpillar has one generation a year and over winter in the egg stage. In early spring, when deciduous trees begin to leaf out, larvae hatch from the eggs and feed on the emerging leaves. Western tent caterpillars find safety in numbers; the larvae aggregate together in communal silken tents during the night. Tents are usually formed in the crotches of smaller branches. Young larvae do not venture far from the tent and usually feed on the foliage of a single branch. The caterpillar’s body is dark with spots of white, orange and blue. White and orange-yellow tufts of hair poke out from each segment.

As larvae mature, the caterpillars begin to become more solitary, feeding by themselves or in smaller groups. In June, the mature larvae wander away from the host tree and pupate by forming a white, puffy cocoon. After about ten days, adult moths emerge from the cocoon. Shortly after emerging, the adults mate. Adult moths are hairy, reddish-brown, stout moths. Often you will find these moths head-butting your porch light bulbs in June and July. Females seek new host trees to lay eggs on. Eggs are laid in frothy masses in batches of 100 to 350 on host tree branches. The eggs will stay glued on the tree until the larvae hatch next spring.

Damage:

Western tent caterpillars feed on most species of deciduous trees and shrubs. If you see tents present on conifers, in spring, chances are good that those are the caterpillars of the silver-spotted tiger moth. Western tent caterpillars feed on most species of deciduous trees and shrubs including: alder, apple, ash, birch, cherry, cottonwood, willow, roses and other fruit trees.

Like the silver-spotted tiger moth, the western tent caterpillar usually causes aesthetic damage to trees. The tent and denuded branch can appear unsightly to many people. Rarely is there more than one tent per established tree. However, on small trees, a single tent can result in 20% defoliation. Larger trees can tolerate this damage. Every now and then, we do have tent caterpillar outbreaks. These outbreaks are generally regulated by weather and temperature.

Management:

Recently, a Seattle news station covered a story about western tent caterpillars infesting Seattle. The reported stated, “Some people use a little gas and a match [to control the caterpillars].” NEVER recommend fire as a control method for tent caterpillars (or any other pest problem). Burning the tents can result in more damage to the tree. Additionally, this practice may result in personal injury and property damage. A simpler solution is found in pruning off the tented branch. It is best to do this at night or early morning, when the caterpillars are congregated in the tent. Dispose of the branch by placing into a yard bag or trashcan so the larvae can’t escape and continue feeding. Other mechanical control methods can be implemented to manage the egg stage. Egg masses are easily seen on small trees and will simply peel off from the bark. You can do this in fall and winter.

Weather and temperature are important natural regulators of tent caterpillars. Many animals will also regulate these caterpillars such as birds, parasitic wasps and flies, and diseases (virus and bacteria). Conserve these good guys by reducing broad-spectrum insecticides and non-target applications of insecticides.

Chemical management of tent caterpillars is an extreme response to a serious tent caterpillar problem. Most times, mechanical controls are effective. However, during severe infestations insecticides may be warranted. If a biological insecticide is used, be sure to spray the entire foliage that is being affected. Caterpillars must eat a good amount of the treated foliage to be sickened. If a contact insecticide is used, treat the larvae that congregated in the early morning. This way, you do not have to spray the entire tree, just the tents. In doing this, be sure that your pesticide is penetrating the tent. Making tears into the tents may be worthwhile before you spray. As with all our chemical recommendations, be sure to read and follow the label. Visit WSU Hortsense (http://pep.wsu.edu/hortsense/) for current recommendations.

Visit the Tent Caterpillar Update Site to see what happening now!