Michigan is experiencing a heavy outbreak of forest tent caterpillars (FTC) this year, particularly in areas around Gaylord, Cadillac, Traverse City and Petoskey in the northern Lower Peninsula, according to the Department of Natural Resources.

Widespread outbreaks of FTC occur at intervals of 10 to 15 years. These outbreaks last for two to five years, with most running their course in two to three years. FTC epidemics commonly begin over large areas simultaneously. This is caused by favorable weather conditions preceding an outbreak. Population buildups often follow periods of unusually warm, dry springs. Fortunately, FTC outbreaks eventually subside as caterpillars succumb to parasites and other insect natural enemies.

Defoliation begins in early May in the northern Lower Peninsula and late May in the Upper Peninsula. Defoliation can be dramatic and becomes noticeable by early to mid-June. However, cool weather slows development and feeding, extending the duration of outbreaks.

"Defoliation from FTC normally does little damage to the tree," said Roger Mech, forest health specialist with the DNR. "FTC infestation will reduce the vigor of the tree, but the tree usually recovers within a few years, after FTC infestation dies down."

Mech noted that most trees will develop a second set of smaller leaves around mid-summer, after the initial loss of leaves from FTC. He added that trees rarely die from FTC infestation alone.

Native flies play an important role in natural control of FTC, but fly populations tend to increase as a result, and can create another nuisance for the public. The DNR can provide technical advice to landowners and landowner groups experiencing FTC infestation. Landowners interested in technical advice should contact their nearest DNR office.

Aerial applications of pesticides may help reduce caterpillar nuisance during an FTC outbreak. Applications must be made at the appropriate time and by licensed experienced applicators. Once caterpillars are full grown and defoliation is nearly complete, pesticides are not effective. The DNR can help homeowners determine whether aerial spraying will be effective.

When spraying is warranted, the DNR recommends the use of a biological insecticide called Bacillus thuringiensis (also known as Bt). The DNR recommends the use of Bt because of its environmental safety. Bt is a naturally occurring bacterium that is effective only against caterpillars and is registered as an insecticide by several companies. Only caterpillars feeding on leaves sprayed with Bt are affected. Spraying Bt on caterpillars will not harm them. It has no effect on other insects, birds, people, and other animals.