Forest Tent Caterpillar Population Expected To Hit Peak 2014-2015

The defoliator has little impact on long-term tree health but it does slow growth of hardwoods.

Published on: May 20, 2013

Since 2007, forest tent caterpillar populations have been rising in some northern and west-central Minnesota counties and that trend is expected to intensify, according to the Minnesota Department of Natural Resources.

Data suggests forest tent caterpillar populations and the associated defoliation of trees could be building towards a 2014 or 2015 peak.

The forest tent caterpillar is a native defoliator of a wide variety of hardwood trees and shrubs. Its range in North America extends from coast to coast and from the tree line in Canada to the southern states.

Defoliation normally begins in mid-May in central Minnesota and late-May in northern areas and is usually completed by mid- to late-June. The insects feed on the leaves of aspen, birch, oak and basswood trees.

Jana Albers
Jana Albers

The heavy snowfall and late arrival of spring may delay the egg hatch, but will have little impact on the survival of eggs laid last year.

Defoliation has little long-term impact on healthy trees, but can result in temporarily slowed growth. However, if trees are under stress from prolonged drought or have root system damage, secondary infestations by other pests can further weaken or kill those trees – particularly oaks and birches.

Outbreaks can result in dramatic swaths of defoliation in areas with abundant aspen, birch, oak or basswood stands. They occur at intervals of 10 to 16 years and last three to five years. They begin over large areas simultaneously, often occurring in Ontario, Minnesota, Wisconsin and the upper peninsula of Michigan. Locally, outbreaks normally last two to three years. Widespread outbreaks peaked in Minnesota in 1922, 1937, 1952, 1967, 1978, 1989 and 2001.

Dealing with forest tent caterpillars can be frustrating.

"While the caterpillars don't cause a health risk to humans, the presence of hundreds (or thousands) of them can be a real headache," said Jana Albers, DNR forest health specialist in Grand Rapids. "The effects of defoliation on shade trees, ornamental plantings and gardens can also be of concern to homeowners."

While homeowners may want to use insecticides to protect trees and preserve their appearance, the DNR encourages consumers to first consider the type of insecticide and its effectiveness, and discourages the use of treatments that may pose any environmental concerns.

Insecticides containing Bacillus thuringiensis var kurstaki can be effective against forest tent caterpillar defoliation when applied while the caterpillars are small. The DNR strongly recommends it over other insecticides because of its environmental and human safety.

More information about the biology and management of forest tent caterpillars can be found online here.