2010-05-27 / Outdoors

Warm weather brings forest tent caterpillar outbreaks

With the recent mild weather, forest tent caterpillars are beginning to hatch from egg masses where they spent the past winter. Populations of forest tent caterpillars build up during favorable weather conditions like this unusually warm, dry spring. Outbreaks commonly occur over large areas simultaneously.

Forest tent caterpillars feed on the leaves of maple, aspen and oak trees, stripping them of their green color. This is called defoliation.

Defoliation begins in early May in the Northern Lower Peninsula and late May in the Upper Peninsula. Most Michigan residents will notice it around mid-June when it becomes more dramatic.

“Defoliation from forest tent caterpillars may look bad, but normally does little damage to the tree,” said Roger Mech, forest health specialist with the Department of Natural Resources and Environment. “Forest tent caterpillars will reduce the vigor of the tree, but the tree usually recovers within a few years after 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. He added that trees rarely die from defoliation alone.

Forest tent caterpillar outbreaks typically last two to five years, and occur in 10- to 15-year intervals. Based on previous infestation cycles, it is likely that caterpillar numbers will decrease in some areas this spring. Outbreaks eventually subside as caterpillars succumb to parasites and insect predators.

Aerial applications of pesticides may help reduce forest tent caterpillar nuisance. 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.

When spraying is warranted, the DNRE recommends the use of a biological insecticide called Bacillus thuringiensis, also known as Bt. Bt is a naturally occurring bacterium, known for its environmental safety. Only caterpillars feeding on leaves treated with Bt are affected -- spraying Bt on caterpillars will not harm them. It has no effect on other insects, birds, people and other animals.

The DNRE can help homeowners determine whether aerial spraying will be effective. For information, call Roger Mech at (517) 335-4408 or Robert Heyd at (906) 226-1324.

The Department of Natural Resources and Environment is committed to the conservation, protection, management and accessible use and enjoyment of the state’s environment, natural resources and related economic interests for current and future generations. Learn more at www.michigan.gov/dnre.

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