The new tents appearing on trees in western Washington are not the return of western tent caterpillars typically seen in spring, according to Washington State University Snohomish County Extension educator and entomologist Sharon Collman. They are the product of the fall webworm caterpillar.
This insect forms much larger tents than the more familiar spring tent caterpillars, said Collman. As they run out of food, they spin more webbing to incorporate new leaves.
“I wondered why caterpillars make such large tents in fall when spring caterpillars only need small tents,” said Collman. “So I spent some time watching them.”
What she noticed is that yellow jackets, wasps, mud daubers, and other predators kept trying to get into the webbing to feed on the caterpillars. Fall webworm eggs hatch in late July. Because there are so many more predators at this time of year, these caterpillars protect themselves by making large tents that are expanded to include new areas for feeding.
“This suggests that one natural way to control the webworms would be to use a stick or forceful stream of water to break open the tent to allow access by predators and parasitoids,” Collman says.
Because the tents are so large, pruning often isn’t practical because it can require removing entire branches, according to Collman. Otherwise healthy branches will develop new leaves once the caterpillar larvae are gone.
A single nest can be ignored, but if they become numerous they can be sprayed with a commercial form of the naturally occurring bacterium, Baccilus thuringiensis (Bt), that is fatal to moth caterpillars especially when small. Because the caterpillars don’t leave nests to feed, only the area around visible nests needs to be treated, Collman said.