For most folks, the sight of a webby mass clinging to the branches of a deciduous tree brings on thoughts of tent caterpillars. I learned early on in my youth that if you see this mass in the spring, it is tent caterpillars, but if it is late summer or fall, you are looking at fall webworm, an entirely different insect. If you’re still not sure, you can verify which insect it is be monitoring the movements of the caterpillars. Fall webworms stay within their webby nest all the time, eating and sleeping there. They do not leave until they are ready to pupate. Tent caterpillars, on the other hand, only use their webby nest for shelter at night and during inclement weather. When they get hungry, they must leave the nest to forage on nearby leaves.
Fall webworm is actually a native insect, native to North America and Mexico. In the 1940s it was introduced to Yugoslavia and Japan (presumably unintentionally), and now it is considered an holarctic pest (it’s everywhere). In its adult form, fall webworm is actually a rather pretty white moth with some yellow or orange patches on its front legs. It is very hairy, and quite small (wingspan less than one and a half inches).
Is fall webworm a major forest pest? Not really. While the webby nest is dirty and messy, the insects rarely kill their host tree. The larvae stay within their web, eating the leaves contained therein. When they are very small, the larvae feed only on the upper surface of the leaves, but as they get older, they begin to consume the entire leaf. The web is usually situated at the tip of the branch, and although the larvae enlarge the web as their own sizes and appetites increase, it rarely gets larger than a foot in length.
Once the leaves are all gone (in about four to six weeks), the larvae ditch the nest and drop to the ground. Here they burrow into the leaf litter and spin a thin cocoon in which they pupate and over-winter. The following year, the adults emerge and live only long enough to reproduce. The female seeks a satisfactory tree and lays her eggs in ‘hair’-covered clusters of about 100 on the undersides of leaves. Within a week the eggs hatch and the cycle begins again.
If in your perambulations you come across a gossamer nest of wiggly caterpillars, you have two options. One, you can say “Ah…fall webworm,” and move on, or two, you can remove the infestation and destroy it. I opted for number two and cut off the offending branch. It is now submerged in about 50 gallons of water, where hopefully the larvae will expire.