Jeffrey Hahn, Asst. Extension Entomologist
Fortunately, fall webworm normally has little impact on the health of large, vigorously growing, well-established trees (it is possible that small trees or shrubs can be completely defoliated in one season and could be injured). Fall webworms are usually no worse than an eyesore because of the webs they construct, making management unnecessary. This is especially during late summer as this feeding has little impact on plant health. There are also natural enemies that help keep fall webworms in check and prevent serious outbreaks.
If you want to try to improve the tree's appearance, you can try to pull the webbing and caterpillars off the branches (assuming you can reach them). Although it may be difficult to remove the entire web, you may be able to damage it enough to eliminate the fall webworms. You can prune out branches containing webs as along as removal is not excessive or the tree or shrub is left unsightly. Do not attempt to burn webs; this is more harmful to the tree than any control that is achieved.
If there are circumstances where it is necessary to treat fall webworms, they are vulnerable to insecticides if they are applied soon after the caterpillars start to construct their webs. There are a variety of residual products that can be effective, including permethrin and bifenthrin. If you wish to use a low impact product, try Bacillus thuringiensis, a bacterial insecticide. It is specific to butterfly and moth caterpillars and has no impact on other insects as well as people and animals. Once webs are larger, direct sprays do not penetrate through the webbing very well. Another option is to use the dinotefuran, a type of systemic insecticide. Another, systemic insecticide, imidacloprid, however, is not very effective against caterpillars.