Diggin’ in the Dirt: Western Tent Caterpillar

Western Tent Caterpillar

As this is being written, 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 early June weddings had to be moved inside. Soon the caterpillars will stop eating, spin a cocoon, and in about three weeks, emerge as the adult moths. These moth 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 its 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, that will show up in about six weeks.


In praise of Cascara

Cascara trees are finely getting their due as a desirable native species in the landscape trade. I have always admired these nicely shaped, smallish (25-50 feet) deciduous trees. Their bark is a subtly mottled gray with brown and white accents. Cascara leaves (on most sites) are deep green with deeply incised veins. The flowers aren’t much to speak of but the berries they produce are gobbled by birds and a number of insects make passing use of both the fruit and leaves while doing no real harm. A real virtue is that beaver don’t like this tree and will leave it alone. The tree almost disappeared by overharvesting the bark that was made into a laxative. Check it out.

Plant a few extra rows of vegetables for your non-gardening friends and neighbors and for the food bank.


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Free newsletter

The Oregon State University Extension office in Columbia County publishes a monthly newsletter on gardening and farming topics (called County Living) written/edited by yours truly. All you need to do is ask for it and it will be mailed to you. Call 503 397-3462 to be put on the list. Alternatively, you can find it on the web at

http://extension.oregonstate.edu/columbia/ and click on newsletters.


Contact information for the Extension office

Oregon State University Extension Service – Columbia County

505 N. Columbia River Highway (across from the Legacy clinic)

St. Helens, OR 97051

503 397-3462

Email: chip.bubl@oregonstate.edu