Don’t Panic: Fall Defoliating Insects Are Not Likely to Kill Your Trees.

Every fall, Extension Centers get numerous calls from homeowners who are concerned about defoliation of their trees by various insects. Often, the callers report that leaves disappear overnight, with no insects seen in the vicinity. In other cases, the defoliation is more gradual, and the culprits are readily observable. There is a multitude of insects capable of stripping the leaves from trees in late summer and fall, including yellownecked caterpillar, walnut caterpillar, June beetle, Japanese beetle, blister beetle and fall webworm. Some of the most common of these are described in two excellent MU guides written by Dr. Bruce Barrett. See http://muextension.missouri.edu/xplor/agguides/pests/g07271.htm and http://muextension.missouri.edu/xplor/agguides/pests/g07270.htm

Alarming as it may be, the damage done by late summer or fall defoliating insects is usually not life- threatening to the host tree. Often, affected trees will produce a second set of leaves and will then go normally into dormancy as the fall progresses. In most cases, once the damage has been done, the insects move on and are not likely to be affected by insecticide sprays applied to the plant.

Fall webworm is a good example of an insect pest that causes a lot more excitement than it merits. Every fall, a wide range of trees, but especially those of walnut, pecan, hickory and persimmon have multiple tents on their branch tips, some up to 2 feet long. While these look dramatic, they are generally of little consequence to tree health. Taking a philosophical view, the best way to deal with this insect may be to learn to sit back and admire its web-making productivity. Insect populations tend to go in cycles, building to impressive numbers and then declining as natural enemies such as birds, predatory and parasitic insects and diseases take their tolls. Often, when chemical control measures are used, insecticides reduce the populations of predators and parasites as much as that of the target insect. Then, the pest manager gets locked into a chemically-oriented approach.

For those somewhat less philosophical, there are some control measures for fall webworm that go easy on the natural enemies. Of course, for small populations, pruning may be an option. Another predator-friendly management practice is to disrupt the webs mechanically or with a hard stream of water to allow birds easy access to the larvae. Given that the caterpillars are the larvae of a moth (Hyphantria cunea), Bt (Bacillus thurengiensis) can be fairly effective when the larvae are just beginning to make webs in late July or early August. It should be noted that, by Labor Day, the larvae have finished their feeding, have left their tents behind and have dropped to the ground to spin pupation cocoons in hiding places near the soil surface. Pesticides applied to the abandoned tents are definitely a waste of time, money and natural enemies.

Fall Webworm

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630