Extension News

Yard & Garden Column: Web-Making Caterpillars of Autumn

fall webworm

Note to media editors: This is the Garden Column for the week of Sept. 1, 2006.


By Donald R. Lewis, Extension Entomologist

Most insects can go about their buggy little lives completely unnoticed until they do that one thing that calls attention to themselves. Some chirp or buzz, some scurry when the lights come on and some eat themselves out of house and home, or at least eat enough foliage that you notice something is wrong with one of your plants. Tomato hornworms, for example, can be on the plants for weeks, camouflaged by their leaf-green color, and the gardener doesn’t suspect a thing until the hornworms are grown and the leaves are gone from a major portion of the vine.

On the other hand, a few insects announce their presence from the very beginning. Some of the most conspicuous create a noticeable structure or shelter. We may not see the insects but we certainly notice the webs they make.

One such insect is the fall webworm. From mid-July through the end of the summer the fall webworm is easily noticed on walnut and other hardwood trees. Clusters of fall webworm caterpillars construct loose, gray, silken webs on the ends of the branches. We always have a few fall webworms, but reports and observations indicate there are more than usual this year. I don’t know why the number is up around the state. There is always some variation from year to year and from place to place. I suspect it’s related to the weather.

Fall webworm tents start small, but the inch-long, hairy, yellow caterpillars enlarge the web every few days as they grow and consume the leaves within the tent. By the end of the summer webs may be 2 to 3 feet long and enclose the entire end of a branch. 

Although the fall webworm has been recorded feeding on more than 200 species of deciduous trees and shrubs, the favored host in Iowa is the black walnut tree. Tents are particularly common on walnut trees growing in the open or on trees at the edge of the woodland.

Because it is so easily-noticed, the fall webworm gets a great deal of attention. It also comes with a great deal of misinformation. For example, many people call this web full of caterpillars a bag worm (a bag-full-of-worms; get it?). Unfortunately, this nickname is incorrect because another insect already has the name “bagworm.” Similarly, some well-meaning but misinformed people claim these bags full of worms are the dreaded gypsy moth of New England fame. Wrong. The gypsy moth does not occur in Iowa, it does not make any kind of web, and it feeds in June, not August.

Damage caused by the fall webworm is not significant to well-established, otherwise healthy trees. Damage is more unsightly than serious because of the limited amount of foliage consumed and the time of the year. Only the leaves inside the webs are eaten. The rest remain alive, functional and unaffected. And by mid-August, the webs you see are what you get. The caterpillars do not spread or multiply; only the leaves inside the existing webs have been lost.

Trees are not killed by this pest and control is not essential. Applications of insecticide are of little or no benefit and should be avoided in order to preserve predators, parasites and other biological control organisms. “Revenge sprays” in very late summer or early fall are especially harmful to the environment and are the least likely to make a difference to the tree. One further reason not to spray: To be effective sprays must be applied with sufficient pressure to penetrate the silk web and reach the leaves within. Insecticides applied to the outside of the web or to the foliage will not reach the caterpillars inside.

The only practical control is to cut off webs when they are first noticed. Prune and discard the webs you can reach and ignore the rest. Because caterpillars remain in the tent, time of day is not important.  

The fall webworm caterpillars do not harm the tree, but they do really, really annoy the tree-owner. People uniformly hate this small, insignificant leaf-eater. They hate it so much that they go to great lengths (some would say they go to great stupidity, but that sounds unkind toward my family and friends) to make sure they are dead. Suffice it to say that doing anything with a flaming, kerosene-soaked towel on the end of a pitch fork is not a smart idea (especially if you are standing on a ladder at the time). Similarly, if you’re tempted to hold a propane torch in one hand while clinging upside down on a tree branch, please call 9-1-1 before you climb the tree. In seriousness, remember the caterpillars are a temporary nuisance to the tree. If you scorch the tree bark with a too-hot flame, that will be a permanent injury. As will be the dent in your head when you fall off the ladder. Let’s be safe out there. The webworms are not the enemy.


Contacts :
Donald R. Lewis, Extension Entomology, 515-294-1102, drlewis@iastate.edu Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu Photo is by Laura Jesse, ISU Extension. High resolution photo of fall webworm