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Forest tent caterpillars creating a gooey mess in city

Forest tent caterpillars can be seen in large masses in Hubbard County, above, where they weave a large silk tent to emerge from. (Shannon Geisen / Enterprise)1 / 2
Tent caterpillars are crawling up the wall of the Park Rapids Area Library. Spraying can eradicate them. (Sarah Smith / Enterprise)2 / 2

An invasion of forest tent caterpillars from Hubbard County and around the state has now found its way to city trees.

The one-inch black worms are crawling up the walls of the Park Rapids Area Library, among other sun-dappled buildings.

They are defoliating trees throughout the city, having devoured many in the county, likely the result of a warm winter and ideal spring conditions.

A once-in-a-decade phenomenon, forest tent caterpillars are about right on time, according to the Minnesota DNR. They peaked in 2001-02, when millions of acres of trees throughout the state were defoliated.

But this year's invasion, aided by a balmy winter, likely won't be much more than a nuisance, predicted NDSU Extension entomologist Jan Knodel.

Unless they're left to proliferate. Very young trees and those ravaged by other factors are usually hit harder, she said.

And tree owners can get ahead of the invasion before they see the worms crawling. Generally eggs are visible on tree branches, or look for a milky-colored web, a sure sign they've settled in.

Hence the name "tent caterpillars."

"We recommend a bacteria natural control called Bacillus thuringiensis, or BT," she said.

"It works when they eat it and it causes their stomachs to essentially rupture, killing the insect."


BT, which has numerous trade names, is found in most garden centers or greenhouses, she said.

"It's environmentally friendly," she added, a bit too late to erase the yuck factor.

You can also use insecticides such as Sevin and Malathion, she said, but watch for drift. Malathion can take the paint off a car.

"They're fairly common and they do pretty well after a warmer winter than we normally see," she said. "We'll see a lot more when the environmental conditions are favorable."

Multiple sprays are available, she assured tree lovers.

Lavonne Edelman called to say Lake George is peppered with the tents and worms emerging from them.

"They're so bad in some places," she said. "I don't think people take care of them," she added of the webs. By care-taking, she meant elimination.

"You want to examine the foliage," Knodel suggested. "Usually 10 days after the eggs hatch, you'd want to spray. You don't want to wait too long."

Their preferred diet is fruit trees, but they diversify their food triangle by munching on maples, ash and nearby plants.

"A lot of times they're kept in check naturally by biological control," Knodel said. "There are some viruses that attack the caterpillars and then there's predators like birds or parasitic wasps that attack them, so they can be kept in check naturally."

But once the population gets very high, it's "hard for the biological control to keep them" in check.

"Most trees can tolerate the defoliation, but if they're stressed from drought or real young trees that don't have as many branches, they're more susceptible," she said.

And once attacked, they can be susceptible to other tree diseases."

Sarah Smith

Sarah Smith is the outdoors editor. She covers courts, business and breaking news in addition to outdoors events.

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