By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties

It is shaping up to be another exciting year in forest health here in northwest Oregon. Fortunately, neither of the two defoliating insects currently on the scene are serious threats to forest or human health, but they are certainly causing a stir.

Right now, Columbia County is in the midst of the largest documented western tent caterpillar outbreak that Oregon has seen in two decades, according to the Oregon Department of Forestry. I first noticed a few tent caterpillar clusters on one site in the area two years ago. Last summer, our Extension office received many calls as the caterpillar population built up.  Aerial surveys done a few weeks ago show that at least 13,000 acres are affected in the county this year.

Map and aerial view showing extent of western tent caterpillar defoliation, early June. Affected areas are brown in the photo. Source: Oregon Department of Forestry
Map and aerial view showing extent of western tent caterpillar defoliation, early June. Affected areas are brown in the photo. Source: Oregon Department of Forestry

The caterpillars are everywhere in the most heavily affected areas. It’s impossible to move without stepping on them, and looking up through a stand of alder, it looks like early spring as there are no leaves left on the trees.

Western tent caterpillars infected by a virus hang in an upside-down V.
Western tent caterpillars infected by a virus hang in an upside-down V. Photo: Amy Grotta

When the population gets to this level, natural parasites and diseases set in. Upon closer inspection, one can see that some of the caterpillars are hanging limply from their midsections: a symptom that these diseases are beginning to take hold, signaling the end of the boom years and the beginning of the bust.

Other defoliating insects follow similar boom-and-bust cycles, in concert with their respective natural enemies. The western oak looper, which made its appearance in 2012 and 2013 in the mid-Willamette Valley, and the pine butterfly, which affected over 250,000 acres in eastern Oregon in 2011-12, are two examples. Reports are beginning to trickle in that the oak looper is still on the scene in places this year, but the pine butterfly outbreak is over. In 2013, researchers in eastern Oregon observed abundant “boom” populations of two insects that are predators of the pine butterfly larvae.

In Columbia County, 2014 will go down in the books as another “year of the caterpillar”. Longtime residents can recall the years marked by previous outbreaks of these insects, just as with big floods and wind storms. One Rainier old-timer recalls another big tent caterpillar year in the 1950’s.

The interactions between these forest insects and their natural enemies are an example of how biodiversity at the smallest scale within a forest system leads to patterns that we can observe. Sadly, in our coastal ecosystem, a pathogen is killing off sea stars in unprecedented numbers. It remains to be seen whether the sea stars will be able to rebound, or if they will be busted for good.

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