Caterpillars are on the march

Cloquet’s Duane Buytaert remembers the forest tent caterpillar invasions of his childhood with a combination of horror and amazement. The school board member was in school then, but his memories of the millions of marching, hairy, striped caterpillars are vivid.

“They covered everything,” said Buytaert, who was 9 or 10 years old when there was a particularly massive outbreak in northern Minnesota. “Riding your bike, all you would hear is them squishing underneath you. Riding down a residential street [with a tree canopy overhead], they would come down off the trees and the guts would come flying up off your bike tires. It was fun and gross at the same time. You would look at the trees and they would just be moving, there were so many caterpillars.”

Now, it seems, his three children can enjoy making similar memories, because the forest tent caterpillar is back.

Mostly blue and black, with a row of whitish-yellow, footprint-shaped markings on their backs, homeowners and foresters are noting an uptick in the population of the caterpillars in Minnesota’s northern counties, according to a recent press release from the Minnesota Department of Natural Resources.

Known incorrectly to most people as “armyworms,” the forest tent caterpillar – scientific name 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.

Jana Albers, DNR forest health specialist in Grand Rapids, explained that true armyworms are an insect found primarily in the western United States, which feed on wheat fields. Also known as cutworms, they are not the same thing as forest tent caterpillars (FTC).

“If you look up armyworms on the web, you will end up getting information for the crop pest, not the caterpillars we have here,” she explained, admitting that there is no short-and-sweet correct nickname for the forest tent caterpillar.

Albers reckons this is the first year of a three-year outbreak cycle for the bugs.

“This is only a prelude,” Albers said.

During outbreaks, forest tent caterpillars can number from 1 to 4 million caterpillars per acre. They create an extreme nuisance to people living or vacationing in forested areas. Young caterpillars spin threads and fall from the trees onto picnic tables, patios, and people, causing serious annoyance. Large, mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters of thousands of caterpillars on buildings, tree stems, cars, campers, and other stationary objects. Caterpillars often emit a greenish-black fluid when disturbed that stains paint and cloth. During the height of defoliation, insect frass (excrement) becomes a serious nuisance as it rains down from insects feeding in the tree crown.

“I remember my sister wouldn’t even go outside to play,” Buytaert said. “And you definitely didn’t want to go barefoot.”

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.

“These insects feed primarily on aspen and birch trees in northern Minnesota and on basswood and oaks in central and southern Minnesota,” said Albers, adding that the only hardwood not regularly fed upon is red maple. “They are voracious eaters though, and they also like what we like – garden plants, berries, apple trees – in addition to their normal food.”

For homeowners dealing with the caterpillars now, Albers suggested one way of getting rid of small numbers of the pests is to hand pick them and drop them into a pail of soapy water, where they drown.

Buytaert said he remembers people covering their trees with tinfoil and Crisco vegetable shortening to deter the slow-crawling caterpillars.

“I don’t know if it worked or not,” he said. “The idea was that they couldn’t crawl past that.”

Defoliation starts in mid-May in central Minnesota and late-May in northern areas and is normally finished by mid- to late-June, when the full-grown caterpillars spin silky cocoons to pupate and emerge as tan-colored moths. The moths are nocturnal and are attracted to lights at night. Adults live for about five days. During this time they deposit 100 to 350 eggs in gray cylindrical masses surrounding small twigs. The eggs overwinter and larvae hatch from them during the next spring. There is only one generation per year.

To help prevent larger FTC outbreaks near homes next year, Albers suggested turning out any nightlights or other lights near the home when the moths hatch (sometime after June 23, she estimated).

Although the end of the caterpillars is welcome, the moth stage comes with its own set of problems. Mass flights of FTC moths are common during outbreaks. These flights can move millions of moths hundreds of miles creating a nuisance where the flight ends. Mass flights can trigger new outbreaks suddenly where the insect had not been a problem before. These flights are often associated with the passage of a cold front.

Still, there is hope for those disgusted by nature’s excesses.

Since it is a native insect, native parasites and predators ultimately push a FTC 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.

The DNR will soon fly an aerial detection survey across the state to pinpoint where defoliation from forest tent caterpillars has occurred this spring and to what severity. Flown in six-mile flight lines, the aerial survey will take most of the summer so results won’t be available until autumn.

In west-central counties, the insects have been in outbreak mode for several years and caused 61,000 acres of defoliation in 2011.

Tags: blueprint, budgeteer, environment