Poulsbo, WA – Last June, we arrived back in Washington and began to notice June’s unique collection of nature treasures.  One of the most noticeable features was the presence of caterpillars and their silky nests everywhere. We soon learned that they were Western Tent Caterpillars building sturdy tents lashed to the limbs of trees and bushes, particularly the alder trees. These orange and black caterpillars spend their days eating leaves, but like good little campers, young larvae return to their snug tents at night. Later stage larvae strike out on their own and become solitary feeders in the vast forest wilderness far from the safety of the tent.

Western tent caterpillar nest
Western tent caterpillar nest


Tent caterpillars are actually part of the design of the forest.  They chew their way through leaf after leaf, but rarely kill the main tree or plant.  In fact, the removal of the leaves, mostly alder leaves, defoliate much of the forest, allowing precious sunlight to reach the slower growing trees and ground cover plants below. Caterpillar waste also fertilizes the forest floor as they grow and molt four times over five to six weeks before stopping their feeding frenzy in order to spin cocoons in which to the lay their eggs. It only takes about two weeks for adult moths to emerge, who immediately mate, lay eggs and die a few days later. So much for a long life span.


Western tent caterpillar
Western tent caterpillar

Some years, the caterpillars seem more plentiful than others and I wondered what conditions cause the little creatures to flourish. Apparently, the Western Tent Caterpillar has an arch enemy; the parasitic tachinid fly, which lays its eggs on this caterpillar species, as well as others such as the Monarch butterfly. Worse than that, the resulting fly larvae then burrow into the poor caterpillars, eventually killing them, reducing their vast numbers. The two populations increase and decrease in direct relation to each other. This gruesome cycle repeats itself every six to ten years.


Western Tent Caterpillars (CC BY-SA 2.0)
Western Tent Caterpillars by Franco Folini (CC BY-SA 2.0)

As you walk through stands of alder during the winter months, watch for the grayish brown egg masses which are a few centimeters along a smaller branch. In April and May, notice small caterpillars munching leaves and pitching their white tents in woodland branches. When you start to see the golden rays of sun nourishing the beautiful undergrowth, look up with wonder and think of the creatures who may be of some benefit to our beautiful forests.


Abandoned nest
Abandoned nest

Note: The Western Tent Caterpillar might be seen as beneficial to the forest, but they are a nuisance in your yard and garden. The best way to get rid of them is to don heavy gloves, cut or scrape overwintering egg masses off foliage where you can before they hatch. Once caterpillars are present, wait until early morning or evening when caterpillars have returned, then cut the tent out, enclose in plastic bag and dispose. For more discussion, check Planet Natural.

WT LogoCultivate Wonder… Discover Design

When and Where – 
Forests and woodlands, especially places where there are many maples and alder trees.  They are also commonly found in fruit trees.

Wonder Triggers for Young Trackers:

  • Can you find a large tree near where you live?  Watch the tree year round to see not only the leaf cycle, but also creatures who live in its branches.
  • During the winter, see if you can find Tent Caterpillar egg masses on tree bark, especially alder trees.  Use a magnifying glass to study them.  What do they look like?  How large are they?
  • In the spring, the trachinid fly lays eggs on the caterpillars.  The eggs look like a white bead, often on the head of the caterpillar. Can you find caterpillars with an egg or eggs on it? Does it look like the caterpillar can do anything to rid itself of the egg?
  • Find a caterpillar tent. Observe it at the end of the day when caterpillars are returning.  Approximately how many caterpillars do you estimate live in the tent?

References and Resources

Kitsap Sun

Washington State University

US Department of Agriculture

Image: SPegany ©2018-19

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