Weather Forecast


Forest tent caterpillar peak; too soon to predict

Forest Tent Caterpillars in Minnesota.

Forest tent caterpillar populations have been on the rise in some northern and west-central Minnesota counties since 2007, but this year’s spotty hatching shows some sign that populations may be declining early in some areas, said the Minnesota Department of Natural Resources.

Data suggested forest tent caterpillar populations and the associated defoliation of trees were building toward a 2014 or 2015 peak. Acres of defoliated trees reached 1.1 million in 2013.

“Based on the data, we would’ve expected the numbers to continue growing in 2014 and 2015, but this year’s hatch, so far, appears to be more sporadic,” said Mike Albers, DNR forest health specialist.

A small sampling in five northeastern Minnesota counties last July revealed the cocoons in some areas were killed by a native flesh fly known as the “friendly fly.” According to Albers, “Those high rates of parasitism, up to 90 percent in some areas, were much higher than expected at this point in the outbreak, but it’s too soon to know if it signals a downturn in the caterpillar population cycle, or was a fluke in the small sample size.”

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. The insects feed primarily on the leaves of aspen, birch, oak and basswood trees. 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 egg hatch, but has little impact on the survival of eggs laid last year. Most forest tent caterpillar egg masses survive temperatures of -40°F.

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 are 3 to 5 years in duration. They occur over large areas simultaneously, often involving large portions of Ontario, Minnesota, Wisconsin and the upper peninsula of Michigan. Locally, outbreaks normally last for 2-to 3-years. Widespread outbreaks peaked in Minnesota in 1922, 1937, 1952, 1967, 1978, 1989 and 2001.

“Since it is a native insect, 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 and 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 or harm to pollinators. A product containing Bacillus thuringiensis var kurstaki (Btk) can be effective against FTC defoliation when applied while the caterpillars are small. The DNR strongly recommends its use over other insecticides because of its human and environmental 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 landowners interested in treating their trees and shrubs.