Controlling Fall Webworms
By Pat Curran, Horticulture Educator, Tompkins County Cooperative Extension
QUESTION: Some of the roadside trees have large webs on them, but I thought tent caterpillars appeared in the spring. What's going on?
Eastern tent caterpillars are a native spring pest. They make their webs in branch crotches and especially favor fruit trees and ornamental crabapples. If the egg masses are spotted in winter, they can be destroyed then. Otherwise, wait for the caterpillars to return to the web and then use a gloved hand to gather up the web and caterpillars so that they can be crushed on the ground. It is also possible to apply Bacillus thuringiensis (Bt), a bacterial disease of caterpillars that can be used to control tent caterpillars when they are small. Bt is generally less effective on larger caterpillars. Other pesticides are labeled for tent caterpillar control if necessary. Tent caterpillars can weaken stressed trees by forcing them to send out another set of leaves in late spring. Unfortunately, trees stressed by drought may have fewer reserves and not be able to do so.
Fall webworms form webs in late summer and fall. These gregarious native caterpillars tend to construct their webs on the ends of the branches. The web enlarges to cover more leaves as they get larger and need to eat more. They feed on many deciduous trees, including walnuts. Fall webworms are very conspicuous, but they are mostly a cosmetic problem, rather than a real threat to the health of the tree. The reason for this is that the growing season is mostly over, and normally trees have had enough time to produce the food reserves they will store over winter and use to produce foliage the following spring. Other pests or diseases that attack late in the growing season are also usually a cosmetic rather than a health issue. It is also often the case that by the time a leaf disease is noticed, it is too late to try and control it.
Healthy trees in a normal year may have up to double the leaf growing area they actually need to produce food, which gives them a margin of safety when leaf loss occurs. In a drought year like this one, the leaves of some trees are showing stress symptoms already. Leaves are showing off colors or dropping prematurely. Hopefully, these trees have made enough reserves already. At this point in the growing season, we do not want these trees to send out another set of leaves. It is better for them to go dormant early than to use up their reserves on leaves that may be functional for only a few weeks.
Last updated October 27, 2014