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Clumps of forest tent caterpillars eating their way through Twin Cities

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Tent caterpillars
Tent caterpillars seen on a crab apple tree in Kellogg Mall Regional Park on Wednesday, May 26, 2010.
MPR Photo/Stephanie Hemphill

They don't really make tents, but forest tent caterpillars can be a nuisance.  

If they're not in your yard -- yet -- you can look at some close-up at one of the city parks, like Kellogg Mall Regional Park in downtown St. Paul.  

Adam Robbins with the St. Paul Parks and Recreation Department examines a clump of the soft, squishy creatures, wriggling their way up a crab apple tree.

"The forest tent caterpillar has keyhole-shaped or footprint-shaped white dots along its back, with long hairs.  You also see a greenish-blue stripe along the sides of the caterpillar," he says.

That doesn't sound so bad.  But wait til you see them in a writhing mass the size of your hand, worming their way up your favorite shade tree or even up the side of your house.

Up north, when you walk through an aspen forest, you can hear them eating.

"Right now they're about an inch-and-a-half to 2 inches long, which means they're nearing the end of their life cycle, ready to pupate and turn into moths," Robbins says.

The moths are tan, and they only live for about a week.  They fly around at night and lay eggs in cylindrical masses around twigs.  The next generation of caterpillars will emerge from the eggs next spring.

This year the caterpillars are stalking trees in the Twin Cities in much greater numbers than usual.  Chris Boche, St. Paul's arborist, said he hasn't seen this kind of attack in his 35 years with the city.

"I remember the canker worm wars we used to have, but the tent caterpillar is real sporadic, and we don't have trees they like here," Boche said.  "They love aspen, that's why they have the outbreak up in Duluth.  They'll eat for two or three years, there'll be a big population, and then they'll just disappear."

Nearly everything about this caterpillar is ishy.  Adam Robbins points to some brown scum on the paving at Kellogg Park.

"That's actually the caterpillar excrement, called frass. It'll fall on our park benches, it'll fall on areas where people like to eat lunch," he says. "It's not a health concern, but it is an annoyance, and if more people knew what it was, they might be a little bit grossed out by the caterpillar excrement."

"Up north, when you walk through an aspen forest, you can hear them eating.  You can hear the frass hitting the leaves too, it sounds like a light rain, but it's a hard rain because it bounces off."

Another thing about tent caterpillars in the north woods:  For about a month it can feel like midsummer, but the woods have a strange winter look to them because all the branches are bare.   

Adam Robbins says healthy trees here in the metro should recover.

"Even though they're mostly defoliated in some cases, they will send out another set of leaves, so in about a month's time, if we take good care of them, they should recover fully," Robbins says. "We shouldn't see any ill-effects of the caterpillar this season."

Next year could be another story.  Robbins says trees can begin to suffer if they're defoliated too often.  So the city might go out in the early spring with shop vacs to suck up caterpillars as they huddle in clumps on tree trunks.  

Yet another way for this unloveable creature to gross us out.