Oregon typically has one generation a year of the fall webworm. This moth overwinters in the pupa stage and emerges in late spring through summer. Adults mate and females lay 100-400 or more eggs in clusters on over 100 different kinds of deciduous trees and shrubs.
The feeding preferences of fall webworms vary from one place to another. Madrones are often attacked, but fruit and nut trees are also favorites. Fall webworms have lately been seen feeding on rose bushes, smoke, gum, alder, apple, pear, cherry, walnut, redbud, willow, cottonwood and oak trees, to name a few. Vineyards have also seen damage by the fall webworm this year. These pests don’t usually attack grapes, but this year grape leaves seem to be one of their new favorites.
An effective approach to controlling webworms is to simply cut away the nest of webworms and discard it in the trash. A strong stream of water from the hose will also dislodge worms from their nest. Sprays of Bt. k. (Bacillus thuringiensis kurstaki) or spinosad will control the worms organically, and are most effective if applied when the worms are small. Chemical controls such as sevin or malathion are effective, but are more toxic and can kill beneficial predatory insects.
Many trees are very large and difficult to spray for the homeowner, but controlling these pests isn’t usually necessary as damage doesn’t typically kill trees and shrubs and the webworms most likely won’t show up next year. High populations are usually cyclical and their numbers will decrease and rise through the years as weather patterns change and beneficial insect numbers increase or wane. Our long, cool, wet spring might have killed off many beneficial insects that typically parasitize or feed on fall webworms. That is why their numbers seem to be high for two years in a row now.