Weather Forecast


Forest tent caterpillar numbers increasing

Forest tent caterpillar populations have been rising in some northern and west-central Minnesota counties since 2007, and that trend is expected to intensify, the Minnesota Department of Natural Resources (DNR) said.

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, Malacosoma disstria, 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.

“These insects feed primarily on the leaves of aspen, birch, oak and basswood trees,” said Jana Albers, DNR forest health specialist in Grand Rapids. “The only hardwood not regularly fed upon is red maple.”

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 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.

Since it is a native insect, lack of food supply, as well as native parasites and predators ultimately push an outbreak to a crashing halt, Albers said. After a few years of population buildup, the large numbers of caterpillars need more foliage than is available. Up to 95 percent will die from starvation. A native, parasitic fly kills most of the remaining pupae in their cocoons, ending the outbreak.

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,” Albers said. “The effects of defoliation on shade trees, ornamental plantings and gardens can also be of concern to homeowners.”

The DNR website offers tips for managing the nuisance of large numbers of forest tent caterpillars.

Although homeowners may want to use insecticides to protect trees and preserve their appearance, the DNR encourages people 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 at The DNR also provides technical advice on this website to homeowners and land owners interested in treating their vegetation.