Come September, Fall Webworms are found munching on a large variety of deciduous trees and shrubs. Preferred host trees include Hickory, Ash, Birch, Black Walnut, Crabapple, Elm, Maple, Oak and Pecan, but their feeding is not limited to these species. Fall Webworms are often confused with the Eastern Tent Caterpillar, a destructive plant pest in Illinois. The main difference between these two caterpillars is their feeding habit. The Fall Webworm in Northern Illinois feeds primarily in fall, while the Eastern Tent Caterpillar is a spring feeder. This is much more harmful for the tree, because the spring leaves have not had a chance to nourish the growth of the tree. Because the Fall Webworm feeds so late in the season, the damage done is primarily cosmetic. The leaves the Fall Webworm feeds on have already served their purpose for the growing season and will soon drop from the tree. But because of its unattractive appearance, homeowners may want the caterpillars eliminated from the landscape.
Like many insects, the Fall Webworm has several appearances, depending on the time in its lifecycle. By the time we have reached late summer, the female moth has deposited eggs on the undersides of tree leaves. Caterpillars hatch from the eggs and begin feeding on the leaves of their host tree. There are two types of Fall Webworm caterpillars, each with a slightly different look. Both are pale green or yellow and covered with long white hairs, with either black spots and black heads or red heads. The caterpillars build the silken web nests around themselves as protection from predator insects and birds. These nests expand as they continue feeding on leaves. This same nest creates an unsightly mess of brown leaves, insect excrement and writhing caterpillars. Typically, the nests stay active with live caterpillars for about six weeks. The caterpillars then drop to the ground and overwinter as pupae.
Because the damage done is primarily aesthetic, the homeowner may choose to ignore the Fall Webworm nests without many repercussions. There are a few methods for controlling the pest, if the insect population is large or becomes a nuisance. Oftentimes, simply removing the webbing will remove the caterpillars at the same time. Also, removing the infested branch will eliminate the nest. Use care when removing a tree branch, as this may weaken the tree structure. There are also insecticides that effectively control Fall Webworms. It’s important that the spray pressure breaks the nest. Otherwise, the insecticide will not reach the caterpillars inside. Keep in mind, the Fall Webworm is a native insect that ebbs and flows over time. Some years will have heavier infestations than others. Naturally, there may be several years of high insect numbers, followed by quite a few years of a lower insect population. Treating the infested trees or shrubs won’t necessarily affect the insect population the following year.
Contact the Plant Health Care Department at KTS for advice on treating a variety of plant insects and diseases. In addition, Certified Arborists are available to personally inspect a landscape, determine causes for the problems the homeowner is experiencing and propose a “plan of action” to help solve those problems.