WASHINGTON AG NETWORK
Driving home from Olympia the other day, I noticed a lot of “tents” on deciduous trees along Highway 12. These are caused by fall webworms. The tents are formed in August, while another pest, the tent caterpillar, will be found in the spring.
This insect is native to North America and can be found throughout the United States Canada and Mexico. They have a wide host range that includes more than 100 deciduous trees and shrubs. While inflicting the most damage on shade trees, it can also be found in apple and pear orchards, and nut trees seem to be a preferred host. They sometimes feed on the apple fruit, as well as the branch. The larvae spin large webs in the trees, which may enclose a whole branch.
Conspicuously hairy, the insect appears rusty tan with a dark, almost black body color. Some populations have a lighter-tan color phase.
The insect causes minor defoliation in most forested situations. Large infestations can cause substantial defoliation, which may weaken the tree’s defenses against other opportunistic pests. In ornamental gardens, the webs can cause loss of visual quality.
Generally, population control is not needed, as the biological impact from defoliation is not enough to hurt the tree. Fall webworm is food for many natural enemies, such as birds, whose feeding decreases the population nicely. Paper wasps and parasitic wasps, as well as yellow jackets, also prey on the insect.
Stressed or recently planted trees can be substantially weakened, making them more susceptible to other pests and or diseases.
Direct control when the larvae and colonies are small, such as hand picking them off the trees or cutting out the affected branches and smashing or burning the webs is best. Use of pesticides is only recommended as an alternative in the spring with young populations.
Bacillus thuringiensis var. kurstaki, (“Btk”), is effective for young caterpillars. Use high spray pressure to break up the web to get the insecticide inside to the caterpillars and the leaves. If the caterpillars are not there, then insecticide sprays are useless. If an insecticide is used, make sure that both the tree impacted and fall webworm are listed on the pesticide label. Follow all label directions. Home owners should not attempt to treat trees over 10 feet tall.
Linda Lee was a WSU Master Gardener in Grays Harbor County. She now lives closer to her family in Orofino, Idaho.