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Tent caterpillar season looks mild in Minnesota, DNR forest expert says

Tent caterpillars chew their way through the leaves on a tree near Detroit Lakes, Minn., in 2017. (Photo/ Mike Parisio, Minnesota Department of Natural Resources)

If just the thought of forest tent caterpillars is enough to make your skin crawl, there's favorable news in the woods this spring as trees begin leafing out across the region.

This year's tent caterpillar season looks pretty meek across Minnesota, forest experts say.

"It's hard to know exactly, as it is every year, but we're not poised for a major outbreak or anything like that," said Mike Parisio, forest health specialist for the Minnesota Department of Natural Resources in Bemidji. "I'd say we're in for a pretty typical year."

The leaf-munching caterpillars, a native species that eventually turns into a moth, are especially drawn to trees such as aspen, birch, basswood and oak. Major outbreaks generally occur every 10 to 15 years, Parisio said.

Often mistakenly called armyworms, tent caterpillars gather in wriggling masses—sometimes by the thousands—on tree trunks and branches as they chew away on the leaves.

Tent caterpillars in the flesh—or whatever the squishy substance they're made of is—typically hatch in early to mid May as aspen leaves begin to emerge.

Right about now, in other words. And when they're out in force, being outside can be downright unpleasant.

Tent caterpillars might be gross, but they're harmless enough to humans.

"Unfortunately, they like a lot of the same habitats where people like to spend their free time," Parisio said. "Lakeshores are, for whatever reason, a more favorable microclimate, and they really thrive there where people are recreating."

Minnesota appeared to be poised for a major tent caterpillar outbreak in 2012, when aerial surveys in key forested areas tallied more than 80,000 acres of trees defoliated by tent caterpillars, Parisio said.

The timing would have been about right, given the occurrence of severe outbreaks every 10 years or so. According to DNR statistics, tent caterpillars in 2001 chewed their way through 7.5 million acres of foliage across Minnesota and damaged 7.37 million acres of forest lands in 2002.

Then, for whatever reason, numbers dropped in 2013, Parisio said. There were minor jumps in 2014 and 2015, but tent caterpillar outbreaks since then have held steady, he said. The DNR estimated damage last year at 40,433 acres, and severe weather in 2016 prevented the DNR from conducting its aerial survey early enough to get an accurate count on foliage damage.

Last year, Parisio said the only serious tent caterpillar damage he saw in the DNR's Northwest Region was in northern Cass County near Cass Lake, Minn. Defoliation generally isn't fatal, but it can weaken trees and make them more vulnerable to diseases and other insect pests.

"From a forest health standpoint, it's not something we really worry about unless we're in a period of extreme drought for multiple years," Parisio said. "The trees recover. It's something that frightens people because they're worried about having their enjoyment time ruined by a horrible caterpillar mess."

For property owners, simply dealing with the tent caterpillars until they run their course is the easiest option if the scale of the outbreak isn't too severe, Parisio said. Hosing off porches and driveways also can keep numbers in check, if only temporarily.

People willing to use insecticides should look to products containing the active ingredient BTK, he said. BTK, short for Bacillus thuringiensis kurstaki, is a bacterium fatal to caterpillars that ingest the substance but doesn't kill on contact.

"You have to spray it on the foliage," Parisio said. "It has to be physically ingested."

Eventually, Mother Nature intervenes to keep tent caterpillar outbreaks in check. A large, non-biting native fly called the "friendly fly" traditionally increases with tent caterpillar numbers, and their larvae eat the pupating caterpillars inside their cocoons.

Cold, wet springs and diseases also can curb tent caterpillar outbreaks.

• On the Web: mndnr.gov.

Here's a look at acreage defoliated by tent caterpillars in select northwest and northern Minnesota counties during a severe outbreak in 2001, when more than 7.75 million acres statewide were defoliated:

• Clearwater: 150,923 acres.

• Kittson: 35,881 acres.

• Koochiching: 803,210 acres.

• Lake of the Woods: 220,685 acres.

• Marshall: 180,936 acres.

• Roseau: 155,618 acres.

• Becker: 860,006 acres.

• Beltrami: 654,797 acres.

• Itasca: 865,564 acres.

• St. Louis: 2.65 million acres.

-- Minnesota DNR

Brad Dokken

Brad Dokken is a reporter and editor of the Herald's Sunday Northland Outdoors pages. Dokken joined the Herald company in November 1985 as a copy editor for Agweek magazine and joined the Herald staff in 1989. He worked as a copy editor in the features and news departments before becoming outdoors editor in 1998. He also writes a blog called Compass Points. A Roseau, Minn., native, Dokken is a graduate of Bemidji State University. 

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