University of Illinois Extension
Bruce Spangeberg

These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.

Stateline Yard & Garden

 

Fall Webworm Visible in Trees

July 23, 1998

Conspicuous light brown webs are visible in shade trees. These are not Eastern tent caterpillars –a pest of very early spring. These are not gypsy moth, which does not make webs. This is the work of the fall webworm.

Inside these visible webs are found many caterpillars. When young, they are feeding primarily on the bottom layer and the inside of the leaf, but not the upper leaf surface; often called "window feeding." This allows a brown leaf to remain. Older caterpillars will eat everything but a few major veins, damage known as "skeletonizing." When the caterpillars need more to eat, they will enlarge the web to cover more foliage.

When they are done feeding, the mature larva will leave the web and drop to the ground to make a cocoon. The following year, the small adult moths will emerge and lay eggs on host trees, starting the cycle over again. Many kinds of trees may be host to fall webworm. Some of the more common species include crabapple, maple, hickory, and walnut.

While unsightly, timing of damage to tree foliage typically makes this insect little threat to tree health. As we advance later into summer, the leaves on trees become less and less productive as trees start to prepare for dormancy. Thus losing some leaves is not likely to cause major damage. Defoliation on a smaller tree or a recent transplant may cause more concern, however, and make control necessary.

If reachable, either clip the off the web or open the web and remove the caterpillars. Insecticides could also be used, but they need to be applied inside the webs. Bacillus thuringiensis (Dipel, Thuricide, Caterpillar Attack) is a microbial insecticide that is very effective specifically on caterpillars.

If the web is located high up in a shade tree, control is not critical because the insect is not likely to be causing much actual damage to overall tree health. Also keep this in mind as other caterpillars may show up in late summer. Some, such as the giant silk moth caterpillars, may be quite large. But control is rarely needed since the actual amount of leaves being consumed is low and leaves are getting less and less productive as fall gets closer.

 

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