Raise Your Awareness to the Western Tent Caterpillar

by: Karen Ripley, DNR Forest Health Program Manager

larvae on tent
Photo: Mike Johnson, WDNR

Landowners in Western Washington should be alert to the possibility of upcoming defoliation caused by the western tent caterpillar (Malacosoma californicum). The native insect can be an alarming nuisance as voracious hordes consume the foliage of alder, willow, cottonwood and many other types of broad-leaved ornamental and orchard trees. The 1 ½ inch long skinny, smooth, orange and black caterpillars can be abundant on trees, roads, houses, mailboxes and just about everywhere, especially when the time comes for them to complete their feeding and they start wandering about trying to find protected sites to construct cocoons. Many landowners express alarm at what’s perceived as a devastating invasion. They are disgusted by the tremendous numbers of crawling critters and seek a way to protect their precious trees.

It’s useful for landowners to recognize some features of these outbreaks.

First, the trees are not being devastated. Healthy trees are unlikely to be damaged, even if all their leaves are consumed. Deciduous trees are “used to” replacing their foliage each year and will even produce a second crop of foliage in the next few weeks. The plants growing beneath the caterpillar laden trees are getting extra exposure to sunlight and a shower of nutrients in the feces and leaf fragments that drop from above.

Secondly, the event will not last forever and human intervention is not necessary to bring this episode to an end. Although defoliation may re-occur for several springs, there are also populations of predatory wasps, flies and viruses building up, consuming the caterpillars. Many birds, spiders and bats eat the adult moths. Even the trees reduce the digestibility and nutrient content of their leaves in response to caterpillar activity. Caterpillars with a white dot (an egg) on their heads are being eaten by a fly. Dead caterpillars that droop or smear easily were killed by a virus. Human intervention is not necessary to bring this episode to an end.

Thirdly, many people may want to protect the appearance and productivity of favorite plants or simply reduce caterpillar activity on their property. There are many ways to kill tent caterpillars such as squishing them, trapping moths in soapy water, disposing of the nests, and manually removing egg masses from twigs. There are many pesticides available to kill caterpillars. Read pesticide labels carefully, follow all instructions exactly, and make sure that the application method will be effective. For example, one of the reasons the biological insecticide B.t.k. is so safe to use around people or other animals is that it becomes activated after it is eaten by a caterpillar. So, it must be applied to leaves that the young caterpillars will eat. It won’t kill older caterpillars that have finished feeding. It won’t kill the moths either.

Tent caterpillar outbreaks are a natural, cyclical event in northwest forests. About every nine years tent caterpillars rise to conspicuous levels and the populations remain high for about three years. The actual caterpillars are only present for about six weeks each spring, eating the new leaves. They construct conspicuous, persistent, dark, silken nests for protection from adverse weather, and as they grow and move about, the caterpillars and defoliated trees become more apparent.

It’s useful to raise your awareness of the western tent caterpillars as they will start to make their appearance this May. This year, there is likely to be heavy defoliation in parts of Whatcom, Island, Skagit, Lewis, and maybe Snohomish and King Counties.

If you want to learn more on the western tent caterpillar, consult Washington State University’s Bulletin or the US Forest Service’s Forest Insect and Disease Leaflet number 119.