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Published June 05, 2012, 09:49 AM

Forest tent caterpillars busting out in Minnesota

MINNEAPOLIS (AP) — Forest tent caterpillars are expected to take a bigger bite out of Minnesota's leafy trees this summer, but experts say the impending defoliation is just an appetizer building up to the main course in two or three years.

MINNEAPOLIS (AP) — Forest tent caterpillars are expected to take a bigger bite out of Minnesota's leafy trees this summer, but experts say the impending defoliation is just an appetizer building up to the main course in two or three years.

The caterpillars develop in cycles over the course of decades or more. They set a record in 2002-03, stripping 7.5 million acres of hardwood trees of their leaves in central and northern Minnesota.

The Department of Natural Resources is seeing an uptick in the forest tent caterpillar population in Minnesota's northern counties this year. In west-central counties, the insects have been in outbreak mode for several years.

This year's feast is "not the big one, but the one leading up to a big outbreak in a few years," DNR forest health specialist Jana Albers told the Star Tribune for a story published Tuesday (http://bit.ly/JSK0tO).

Forest tent caterpillars are native to Minnesota. They eat aspen leaves from the Mille Lacs Lake area north into the Arrowhead. In a different set of infestations, they eat basswood and oak leaves from Mille Lacs westward across west-central Minnesota. They defoliated about 61,000 acres in that area last year, Albers said.

Defoliation can slow aspen growth but, fortunately, affected trees almost always recover. They sometimes can grow new leaves as soon as July 4, Albers said.

When foresters detect small infestations for several years running in the Mille Lacs area, as they have for the past two or three years, that indicates a major spread northward is likely in the next two to three years, Albers said. Such outbreaks can cover 2 million to 7 million acres.

As part of their life cycle, caterpillars morph into pupae, which attract a species of fly that feeds on them. Ultimately, the caterpillars either wipe out so many leaves they starve, or their pupae are killed off by predator flies, or both, and the cycle starts over.

While peak infestations can make a stinky mess of the outdoors, Albers said the process has some benefits. By defoliating aspen, the caterpillars allow other tree species to gain a foothold in the forest, while their waste serves as fertilizer.

The DNR will begin aerial surveys next week to track the caterpillars and other pests.

While the caterpillars aren't a health risk to humans, Albers said the defoliation of shade trees, ornamental plantings and gardens can worry homeowners. The DNR strongly recommends the use of Bacillus thuringiensis var kurstaki (Btk) over other insecticides against the caterpillars. Btk is a natural occurring bacterium that can be effective when the caterpillars are small, and has no effect on birds, people, other animals and most other insects.

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Online:

DNR information about the biology and management of forest tent caterpillars: http://www.dnr.state.mn.us/treecare/forest_health/ftc

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Information from: Star Tribune, http://www.startribune.com

Copyright 2012 The Associated Press.

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