Parts of Columbia County are inundated with one of the largest Western tent caterpillar populations in the last 20 years. The Rainier/Apiary/ Alston Mayger areas are especially hard hit. These very hungry caterpillars have consumed almost all the leaves from many alder, willow, poplar, and apple family trees and are now looking for other less favorite food. In affected areas, they are dropping by the millions onto lawns, houses, and cars in search of leftover leaves. One caller described her lawn as a wriggling mass of these caterpillars. She was not amused. Some weddings had to be moved inside. One caller described taking five-gallon buckets full from their gutters. Soon the caterpillars will stop eating, spin a cocoon, and in about three weeks, emerge as the adult moths. These moths fly around (watch your evening lights), mate, lay eggs on twigs, and then die. It’s a rather short adulthood but that’s the way it is with the Western tent caterpillar. The eggs stay glued to twigs until next spring when they hatch and the caterpillars emerge, determined to feed.   For most trees that are defoliated by their feeding, there will be little permanent damage. Deciduous trees have latent buds where each leaf joins the stem. When the caterpillar chomps down the leaf, a signal is sent for the bud to start growing into a new leaf.  It has been my experience that a completely defoliated alder forest may not show any signs of the earlier feeding frenzy 5-6 weeks after it stops. There probably is a little tree growth reduction but that wouldn’t be perceptible to most people. Trees that are already stressed and weak could die from the heavy feeding. But this is a natural part of the thinning/wedding out process. The caterpillar poop is a food fertilizer for the forest floor and some trees or shrubs suppressed by the tree canopy may grow a little more in heavy tent caterpillar outbreaks, assuming that they weren’t also on the menu.  

Home orchards need to be protected at the first sign of tent caterpillars
by spraying the leaves that they will feed on with an approved insecticide. As always, read and follow all label instruction when using any pesticide.  Orchards seriously affected this year will leaf back out again but many people have lost this year’s fruit crop.  

The caterpillar population explosion slows down of its own accord. There are some birds that can tolerate their hairy exteriors and they do well in these years. A wasp likes to lay it eggs into the body of the unsuspecting caterpillars. The population also contains the seeds of its own destruction in the mix of disease causing organisms (bacteria and viruses) that amplify during heavy caterpillar years.   Bottom line is that generally, no intervention in forest settings is necessary. And it is sort of cool to see nature so over the top. There is another tent caterpillar, the fall webworm, which will show up in about six-eight weeks.  Usually, they are not as abundant.    Chip Bubl Agricultural Extension Agent and  Oregon State University Extension Service Columbia County 505 N. Columbia River Highway St. Helens, OR 97051 Phone:<personal data hidden> Fax: <personal data hidden>