Timberlines

Fall Webworm

By Shelly Van Landingham

Colorado State Forest Service

September 3, 2004

Fall webworms have already started to show off their "tents" in the cottonwoods of Eads this year. Although these tents look unsightly and they skeletonize the leaves, these worms occur late enough in the season to cause little damage to the trees. These tents always appear in the fall, when trees are already beginning to go into dormancy and loose their leaves, and thus the worms usually have very little effect on the health of the trees. Only in areas where heavy, repeated defoliation occurs, and if a tree is already diseased or failing, is control warranted.

Fall webworm occurs mostly on cottonwood and chokecherry, but over 100 species of hardwood trees may also be hosts. They are the most common "tent-making" caterpillars found in the region. Fall webworms spend winter in the litter or soil as a pupa and the adults may be found in June or July. The eggs are laid in masses of 200 or 300 on the underside of leaves. In the course of their gregarious feeding, young larvae skeletonize the upper surface of leaves and produce a small web over them. As the larvae develop the web grows and, where populations are large, whole trees may be enshrouded. Although larvae may be found from late June to October, most are present in August. Ultimately, the larvae leave the web and drop to the ground, where they change to pupae, completing the life cycle.

Natural predators, including a host of parasites as well as disease organisms, appear to be the controlling factor and lead to the eventual collapse of outbreaks of fall webworm

As stated above, controlling fall webworm should really only be carried out when heavy, repeated defoliation occurs, and if a tree is already severely stressed from disease or other factors. On small ornamental trees, the detection and destruction by hand of small webs containing larvae is effective. Spraying with Malathion is most effective shortly after tent construction begins. Spraying the tents after they become large doesn't provide adequate control unless the spray can penetrate the webbing. Burning the tents usually results in damage to the tree: more damage than the worms cause. When pruning off infested limbs, you must keep in mind that particular limb will probably leaf out in the spring and be perfectly healthy. So you are taking off living limbs that the caterpillars are only defoliating for a short amount of time during the fall.

For further reading, refer to Colorado State University Cooperative Extension Bulletins 506A (Insects and Diseases of Woody Plants of the Central Rockies) and XCM-38 (Insect Management Recommendation Guide), or go to the CSU Cooperative Extension website at www.ext.colostate.edu. Search under publications and then fact sheets. Under fact sheets, search under insects for #5.586 "Tent-Making Caterpillars".