During peak infestations, forest tent caterpillars, which are native to Minnesota, can defoliate millions of acres. Affected trees almost always recover, and new leaf growth can sometimes be seen within weeks.

Joel Koyama, Star Tribune

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Forest tent caterpillars rev up for 'the big one' in northern Minnesota

  • June 5, 2012 - 11:11 AM

Forest tent caterpillars are once again eating their way through the forests of northern Minnesota, but this year's defoliation is "not the big one, but the one leading up to a big outbreak in a few years," said DNR forest health specialist Jana Albers.

The caterpillars, which during peak infestations can make a stinky mess of the outdoors, develop in cycles over the course of decades or more. They set a record in 2002-03, clearing 7.5 million acres of hardwood trees of their leaves in central and northern Minnesota.

The Department of Natural Resources will begin aerial surveys of Minnesota forests next week to track the caterpillars and other pests. It will be the 58th summer for the survey, which involves a plane flying a grid pattern at 1,500 feet over wooded areas.

Forest tent caterpillars are native to Minnesota. From about the Mille Lacs area north into the Arrowhead, they eat the leaves of aspen. 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.

When foresters detect small infestations several years running in the Mille Lacs areas, 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.

The caterpillar infestation usually occurs in early summer, and defoliated trees sometimes can grow new leaves as soon as July 4, Albers said. Defoliation can slow aspen growth, but trees almost always recover.

Caterpillars transform 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 reduced by predator flies, or both, and the cycle starts all over.

Albers said the process has some benefits. By defoliating the aspen, the caterpillars allow other species of trees to gain a foothold in the forest, while their waste, although messy, serves as fertilizer.


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