They’re baack… Tent caterpillars spotted in Island and Whatcom counties

Western tent caterpillars
Western tent caterpillars basking in sunlight on the trunk of an apple tree. Vashon Island, Washington, 2003. Photo: Karen Ripley/DNR.

Yes, your deciduous trees might have campers! It’s simply the pesky western tent caterpillar
(Malacosoma californicum) ganging up and consuming the leaves. Their favorite trees are alder, willow, cottonwood, and many other types of broad-leaved ornamental and orchard trees.

The caterpillars are currently seen in Island and Whatcom counties. But don’t worry. These creatures are native to Washington and most likely will not kill your trees if they’re healthy.

Tent caterpillar outbreaks are a natural, cyclical event in northwest forests. About every nine years, they rise to noticeable 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.

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.

The 1½-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. They construct noticeable, dark, silky nests for protection from bad weather.

It’s useful to recognize these features of outbreaks: 

  • Healthy trees are unlikely to be damaged, even if all their leaves are consumed. Deciduous trees naturally replace their foliage each year and will even produce a second crop of foliage in the next few weeks.
  • This event will not last forever. Although defoliation may recur for several springs, there are also populations of predatory wasps, flies and viruses building up, eating 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.
  • If people want to protect the appearance or reduce activity on their property, there are many ways to kill tent caterpillars, such as squashing 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 and follow all instructions exactly.

In the last outbreak that affected the Puget Sound area between 2000 and 2004, the defoliation was first observed in the northern counties (San Juan, Island, Whatcom), then started in the central Sound (Kitsap, Snohomish, 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.

Learn more about the western tent caterpillar from Washington State University’s Bulletin or the U.S. Forest Service’s Forest Insect and Disease Leaflet Number 119.

In cooperation with the US Forest Service, DNR’s Forest Health Program provides forest insect and disease monitoring, technical assistance, and education to forest landowners across Washington.

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