Western Tent Caterpillars

Going back through my photos from this summer, I was reminded of the local tent caterpillar infestation and thought it might be useful to answer some of the questions I heard at the time.  The infestation was so large that it even made it into the local news:
Caterpillars setting up camp in Columbia County
Caterpillars invading Washington side of the Columbia too
During the summer, I rode my bike about once a week in an area west of Longview/Kelso, which was in the area infested with tent caterpillars, and on June 18th I noticed caterpillars covering the road and bushes, and then realized many of the alder trees along the forest road were nearly defoliated!  When I stood quietly and listened, it sounded like it was raining even though there were no clouds in the sky...it was the sound of all the droppings of millions of caterpillars falling through the trees and bushes, gross but fascinating!

Western Tent Caterpillars during an outbreak west of Longview/Kelso

There are many moth species with larvae that feed in communal groups and spin large webs or "tents" for protection, and are therefore known as tent caterpillars or webworms.  The most common species in our area is the Western Tent Caterpillar, Malacosoma californicum (see here for photo of adult and more information).  The Western Tent Caterpillar usually goes unnoticed, with only a few tents scattered high up in deciduous trees, but occasionally goes through a period of epidemic outbreaks, such as we saw this June.  Another large outbreak occurred in the Mt. St. Helens area in 2011 and 2012, where they completely defoliated alder trees.  These outbreaks usually end after one or two years because of viruses and/or lack of food.  Even if the trees are completely defoliated, they are rarely killed because of the relatively short period of the outbreak.  In the area west of town that I mentioned, the alder trees were already leafing out again less than two months after the caterpillars stopped feeding and pupated.

Mostly-defoliated alder trees west of Longview/Kelso on June 18th

Although alder trees are the favored food source of Western Tent Caterpillars in this area, they will feed on nearly any deciduous trees and shrubs, often switching to maples, poplars, fruit trees and berry bushes when they've consumed all the alder in the area.  By the time this occurs however, the caterpillars are usually in the final stage of development and only have around one week before they pupate and turn into adult moths.  At 1 1/2 inches, the caterpillars are also more noticeable in the final stage, and seem to appear out of nowhere because up until this point they have spent most of their time high up in the trees and out of sight.

Defoliated (left) and mostly-untouched (right) alder trees west of Longview/Kelso on June 18th