What should I do about webworms in my birch trees?

Asked June 18, 2015, 10:41 AM EDT

Caterpillars have infested my birch trees. Someone has identified them as webworms. Is there any danger in ignoring this infestation, and if not what treatment can I employ to eliminate this problem?

Clark County Washington

1 Response

Webworms are found in the Pacific Northwest in the fall. We've seen a lot of western tent caterpillars this spring. Please see this article from Washington State University's Hortsense site, which gives management options: Tent Caterpillars

People have seen the caterpillars traveling in grass to get to trees and shrubs. They are voracious. Others have been in woody infested areas and can hear hundreds of them chewing leaves. Because we had a bumper crop of tent caterpillars last year, we will continue to deal with them this summer, too. This overabundance will probably continue for another one to two years, until parasites, scarcity of food, weather, or a combination of these factors control their population. Meanwhile, Hortsense advises these actions:

  • Pick out and destroy the foamy-looking, grayish, 1/2" egg cases during the winter. These may be found in bands around twigs or in flattened masses on trunks.
  • Several natural parasites and predators help control tent caterpillar populations. Avoid use of broad-spectrum insecticides which kill beneficial insects. Bacillus thuringiensis (BT) is a biological caterpillar killer which can be used, if necessary. However, it also kills the butterfly caterpillars.
  • Strip or prune out and destroy nests and caterpillars as soon as noticed. This is best done in early morning or evening, when caterpillars are gathered in the nests.
For more information and a list of chemical agents, see Hortsense.