Weather Forecast


Forest tent caterpillars increasing in northern Minnesota

This mass of tent caterpillars was found in Itasca State Park on Saturday, June 1 / 4
Another group of tent caterpillars found in Itasca State Park on Saturday, June 2 / 4
The forest tent caterpillar, Malacosome disstria, is a native defoliator of a wi3 / 4
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The Minnesota Department of Natural Resources (DNR) will soon be flying an aerial detection survey across the state to pinpoint where defoliation from forest tent caterpillars has occurred this spring and to what severity.

"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 the homeowner."

Homeowners and foresters are noting an uptick in the forest tent caterpillar population in Minnesota's northern counties. In west-central counties, the insect often mistakenly called the armyworm, have been in outbreak mode for several years and caused 61,000 acres of defoliation in 2011. The caterpillars are back again this year and are moving northward.

The forest tent caterpillar, Malacosome 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 aspen and birch trees in northern Minnesota and on basswood and oaks in central and southern Minnesota," said Albers. "The only hardwood not regularly fed upon is red maple."

Defoliation starts in mid May in central Minnesota and late May in northern areas and is normally finished by mid to late June.

Outbreaks occur up north at intervals of 10 to 20 years, and are three to five years in duration. They begin over large areas simultaneously, often occurring in Ontario, Minnesota, Wisconsin and the upper peninsula of Michigan.

Locally, outbreaks normally last for two to three years. Widespread outbreaks existed in Minnesota in 1922, 1937, 1952, 1967, 1978, 1989 and 2001. Outbreaks in the west-central counties of Minnesota are not simultaneous, cover just a fraction of the acres and are quicker to collapse as caterpillars focus feeding on basswoods and oaks.

Since it is a native insect, native parasites and predators ultimately push an outbreak to a crashing halt, said Albers. Normally there is little effect on the defoliated trees; reduced growth is the main effect. However, if trees are under stress from prolonged drought or have root system damage, defoliation can cause other pests to weaken or kill those trees.

Dealing with forest tent caterpillars can be very frustrating. During May and June, they can be a downright nuisance, dropping from trees and landing on anything that sits below.

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 such as Bacillus thuringiensis var kurstaki (Btk) can be effective against defoliation when applied while the caterpillars are small and the DNR strongly recommends its use over other insecticides because of its environmental safety. Btk is a natural occurring bacterium that has no effect on birds, people, other animals and most insects.

The DNR provides technical advice to homeowners and land owners interested in treating their vegetation. More information about the biology and management tips for forest tent caterpillars can be found at

Denton (Denny) Newman Jr.
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