Fall webworms are back
August 10, 2007 - Author: Dave Smitley, Michigan State University Extension, Department of Entomology
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
If you see lots of webbing covering the branch tips of some of the trees in your neighborhood, it's probably fall webworm. Walnut, hickory, ash and crabapples are some of the favorite targets, but many other types of trees can also serve as an adequate food source. The small yellowish-gray webworm caterpillars hide in the webs during the day.
Some trees have actually been defoliated from having a tent on nearly every branch. But in most cases, each tree has three to four tents. They look bad, but cause little harm to the trees. Tree health is not affected until more than 50 percent of the foliage is consumed, and even then, the trees usually come back just fine the following spring.
Pruning-out infested branches is a good strategy if you can reach the branches. Spraying the foliage around tents with B.t., Sevin, Orthene or a pyrethroid insecticide will prevent further feeding injury, but the old tent will remain visible. We have many natural parasites and pathogens of fall webworm that will bring these infestations under control. Places with lots of fall webworm this year may not have much next year. Outbreaks usually last two to three years.
Dr. Smitley's work is funded in part by MSU's AgBioResearch.