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Dokken: Those nasty caterpillar tents in trees or shrubs aren't forest tent caterpillars

Eastern tent caterpillars should not to be confused with the forest tent caterpillars that can chew away millions of acres of foliage during springs with severe outbreaks.

Eastern tent caterpillar.jpg
The tents made by eastern tent caterpillars might be gross, but they shouldn't be confused with forest tent caterpillars, which cause more damage but don't actually form tents. (Photo/ Brad Dokken, Grand Forks Herald)
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The sight of dozens of caterpillars wriggling and squirming inside of what looks like a spider web tent in a crotch of the branches is gross, no doubt, but the critters are more unsightly than harmful, a Minnesota Department of Natural Resources forest expert says.

They are eastern tent caterpillars, not to be confused with the forest tent caterpillars that can chew away millions of acres of foliage during springs with severe outbreaks, says Val Cervenka, forest health program consultant for the DNR’s Division of Forestry in St. Paul.

Video: Eastern tent caterpillar
Despite the name, forest tent caterpillars don’t actually gather in tents like their less harmful eastern tent caterpillar relatives, Cervenka says.

And unlike forest tent caterpillars, she says, eastern tent caterpillars don’t occur in outbreaks or boom-and-bust cycles.

“Eastern tent caterpillars are just there every year,” she said. “They make a tent, they kind of look ugly, but other than that, there’s no lasting damage.”

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Typically, the nests of eastern tent caterpillars are found in trees or shrubs that are members of the apple and plum family, Cervenka says.

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“They rarely cause any kind of damage at all,” she said. “Unless you have a tree, and you’re hoping for apples, and that tree is being consumed by the caterpillars, you don’t have anything to worry about.”
I began noticing caterpillar tents in the branches a couple of weeks ago in northwest Minnesota when returning from Lake of the Woods after the state's fishing opener.

Mistakenly thinking they were forest tent caterpillars, I feared the worst. There hasn’t been a severe forest tent caterpillar outbreak in northwest Minnesota since 2001, so I’m glad I was mistaken.

In 2001, forest tent caterpillars defoliated more than 7.5 million acres of hardwood trees and shrubs in Minnesota, DNR statistics show.

“That’s the danger of common names,” Cervenka said. “(Forest tent caterpillars) do belong to the family of caterpillars that are called tent caterpillars, but they actually don’t make a tent.”

Forest tent caterpillar outbreaks are one of Mother Nature’s cruel jokes in my world. The leaves they strip usually grow back later in the summer, but when forest tent caterpillars occur in numbers like they did in the spring of 2001, they’ll wriggle and crawl onto anything that’s in their way. Their droppings falling from the branches sound like rain, and highways can become slimy and slippery in areas where the forest tent caterpillars cross en masse.

As for eastern tent caterpillars, homeowners or others with the unsightly tents in their trees either can leave them be or simply destroy the nest with a rake or similar implement, Cervenka says; chemical control isn’t necessary.

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“The eastern tent caterpillar doesn’t have near the numbers – at all,” Cervenka said “You don’t see them on trunks of trees lining up like you do with forest tent caterpillars.”

That behavior of gathering en masse also has led people to mistakenly refer to forest tent caterpillars as army worms, which are an agricultural pest, Cervenka says.

“There is a caterpillar called an army worm, but it is a cutworm,” she said. “And unless you are a farmer, you probably don’t know what these cutworms are.”

Forest tent caterpillar outbreaks typically occur every 10 to 16 years, according to the DNR website . Based on history, the DNR was predicting another forest tent caterpillar outbreak in the 2013 to 2015 timeframe, Cervenka says; it didn’t happen .

“It just fizzled out, and not only in Minnesota, but in Wisconsin, too,” she said. “We don’t have any idea why forest tent caterpillars are not increasing. It could be climate change, it could just be that spring weather is cold and wet and the little caterpillars are killed before they get a chance to get a foothold – we don’t know.”

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Another forest tent caterpillar outbreak could occur sometime between 2023 and 2029, the DNR says, but time will tell on that. In the meantime, those tents filled with squirming eastern tent caterpillars shouldn’t spark fear that a repeat of 2001 is on the horizon.

To that, I say “whew!”

“They’re only noticeable at this time of year when you see the tent,” Cervenka said. “And that’s it.”

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Brad Dokken column sig.jpg
Brad Dokken

Brad Dokken joined the Herald company in November 1985 as a copy editor for Agweek magazine and has been the Grand Forks Herald's outdoors editor since 1998.

Besides his role as an outdoors writer, Dokken has an extensive background in northwest Minnesota and Canadian border issues and provides occasional coverage on those topics.

Reach him at bdokken@gfherald.com, by phone at (701) 780-1148 or on Twitter at @gfhoutdoor.
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