The Extension Service is here to serve the residents of Columbia County, providing knowledge from universities nationwide. We provide research-based information to families, youths, schools, agricultural producers, home gardeners, foresters and governments.
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We publish three newsletters from our office: Country Living (gardening and farming topics), Tall Timber Topics (forestry/wood lots), and Columbia Columns (for 4-H members and leaders). If you would like to receive these newsletters by mail, please give the office a call.
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.
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 fly 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 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.
The caterpillars have slowed and so have the phone calls. The largest Western Tent Caterpillar outbreak in 20+ years is winding down as the caterpillars cease eating and spin their cocoons. The moths will fly in about 45 days to mate and lay eggs on tree twigs and branches for next year’s party. Given the huge numbers, I would be foolish to predict next year’s crop. But our Forestry Extension agent, Amy Grotta, noticed signs of disease on several caterpillars. So we shall see. Most of the county was largely spared this infestation (see map of the 14,000+ acres that Oregon Department of Forestry aerial surveys showed to be the most affected). Moth flight patterns, predation and parasitism, and probably wind direction and other weather patterns will determine where and how many show up next year. Last year, there were lots of moths in the late summer in the Warren/Scappoose area but the population didn’t materialize.
There were a few surprises. I had never seen an apple tree completely stripped of leaves and fruit. I did this year. I also saw them on blueberry plants, normally not high in their food preferences. Next year, I would probably encourage more aggressive control on fruit trees than I did this year to protect the fruit crop. Leaves are starting to re-sprout on some of the earliest defoliated trees and most other affected trees and shrubs should soon be showing signs of revival.
Hours of Operation
8 a.m. to 5 p.m.
Closed Fridays from Noon to 1 p.m.
OSU Columbia County Extension
505 N. Columbia River Hwy
St. Helens, OR 97051
For information on public transportation, visit the Columbia County Rider website.
We offer classes, workshops, opportunities for youth and adult volunteer activities:
Nutrition education events
Home Garden educational events