Insects can be very efficient defoliators, but a University of Missouri specialist said fall leaf-eating insects rarely cause lasting damage to the trees.
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“Every fall, extension centers get numerous calls from homeowners concerned about defoliation of their trees by various insects,” said Chris Starbuck, MU associate professor of horticulture. “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.”
In either case, “don’t panic,” he said. “Fall defoliating insects are not likely to kill your trees.” And insecticides can do more long-term harm than the leaf-chomping insects. “Once the damage has been done, the insects move on and are not likely to be affected by insecticide sprays applied to the plant.”
A multitude of insect species are capable of stripping the leaves from trees in late summer and fall, Starbuck said, including yellow-necked caterpillar, walnut caterpillar, June beetle, Japanese beetle, blister beetle and fall webworm.
For example, fall webworm, which is the larvae of the Hyphantria cunea moth, “causes a lot more excitement than it merits,” Starbuck said. Every fall, a wide range of trees — especially walnut, pecan, hickory and persimmon — have multiple tents on their branch tips.
“While these look dramatic, they are generally of little consequence to tree health,” he said. “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 usually build to impressive numbers, then decline as natural enemies such as birds, predatory and parasitic insects, and diseases take their toll. Chemical control measures often reduce the populations of predators and parasites as much as that of the target insect, Starbuck said. “Then, you’ll be locked into chemical pesticide management forever. Pesticides applied to the abandoned tents are definitely a waste of time, money and natural enemies.”
Bt (Bacillus thurengiensis) can be fairly effective when the larvae are just beginning to make webs in late July or early August, he said. “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.”
There are other control measures for fall webworm that “go easy on the natural enemies,” he said. Pruning is an option for small populations, and another predator-friendly practice is to disrupt the webs with a stick or a hard stream of water, allowing birds easy access to the tasty webworms.
“Alarming as it may be, the damage done by late summer or fall defoliating insects is usually not life threatening to the host tree,” he said. “Often, affected trees will produce a second set of leaves and will then go normally into dormancy as the fall progresses.”
For more information on fall defoliating insects, check out two extension guide sheets written by MU entomologist Bruce Barrett: http://muextension.missouri.edu/xplor/agguides/pests/g07271.htm and http://muextension.missouri.edu/xplor/agguides/pests/g07270.htm.