While looking in your yard, you discover that your crab apple has several branches with webbing on the ends of them. Inside this webbing, there are lots of worm-like insects feeding on the leaves.
1. What are these insects?
They are caterpillars known as fall webworms. They are first active in July and feed into September. Fall webworms can be recognized by their yellowish or greenish bodies, long, fine white hairs and two rows of black spots down their backs. However, it may be easier to recognize fall webworms by the webbing they produce in trees.
2. Are there other trees they may attack?
These caterpillars feed on the leaves of over 100 different species of hardwood trees, including black walnut, fruit trees, American elm, hickory, and maple. On the positive side, however, they do not feed on evergreens, like pine or spruce.
3. Will they kill my tree?
Although the webbing of fall webworms may make trees look unattractive, their feeding has little impact on plant health, especially if the trees are vigorous and well-established. The webbing is typically concentrated in several spots but does not cover the entire tree (unless it is a small tree). Natural enemies, such as parasitic wasps, help keep fall webworms in check, helping to prevent serious outbreaks.
4. What is the best management for these insects?
If you don't like the webbing, try to pull it off the branches. Although it may be difficult to remove the entire web, you may be able to damage it enough to eliminate most of the insects. If the webbing is on a small branch, just prune out the limb. Do not attempt to burn webs while they are in the tree. Fire is more harmful to the tree than any control that you can achieve.
Fall webworms are vulnerable to insecticides for just a short time when young larvae are first constructing webs. Bacillus thuringiensis, a bacterial insecticide, is a selective product that is effective against fall webworm but is a very low risk to people, pets, and nontarget insects. Once webs are larger, sprays do not penetrate through the webbing very well.
