Tent Caterpillar Activity Reported


Native insect affecting local deciduous forests

by Karen Ripley, DNR Forest Health Program Manager
Photo: Elliott Menashe

Forest and suburban landowners in Island and Whatcom counties are reporting increased activity of the western tent caterpillar Malacosoma  californicum). This 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 are abundant on trees, roads, houses, mailboxes and just about everywhere, especially as they complete their feeding and are 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.

In fact, 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. In July and August, there may be clouds of brown tent caterpillar moths flying about, mating and laying silvery masses of eggs on tree twigs. As the moths die, all is quiet until the following spring when a new generation of hungry caterpillars hatches.

It’s useful to recognize three 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. 
  • Second, this event will not last forever. 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.
  • Third, 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’s 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.

Even if you aren’t seeing tent caterpillars in your neighborhood lately, it’s useful to raise your awareness of them. In the last outbreak that affected the Puget Sound area from about 2000 to 2004, the defoliation was first observed in the northern counties (San Juan, Island, Whatcom), then started in the central Sound (Kitsap, Snohomish and King counties) a year later. The expansion may be faster this time. Wandering caterpillars (but not nests or defoliation) have already been observed in Thurston and north Lewis counties. So, it’s likely that populations are on the rise throughout western Washington and there will be much more widespread activity to see in 2013. 

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

DNR’s   Forest Health Program
In Washington State, forest landowners are encouraged to maintain their forest lands in a healthy condition to meet ownership objectives, protect public resources, and avoid contributing to forest insect or disease outbreaks or increasing the risk of uncharacteristic fire. In cooperation with the US Forest Service, the DNR Forest Health Program provides forest insect and disease monitoring, technical assistance, and education to forest landowners across Washington.

Karen Ripley, DNR Forest Health Program Manager, can be contacted at 360-902-1691, or karen.ripley@dnr.wa.gov