Published June 22, 2009, 12:00 AM

Experts: Peak forest tent caterpillar invasion in 2011 or 2012

They crawl, they eat, they poop and then they turn into moths that fill the air — but probably not at crazy-big levels for a few more years. It’s not a new horror movie but the cyclical invasion of forest tent caterpillars, the little worms that defoliate Northland trees and rise to an unstoppable, some say intolerable, infestation once every decade or so.

By: John Myers, Duluth News Tribune

They crawl, they eat, they poop and then they turn into moths that fill the air — but probably not at crazy-big levels for a few more years.

It’s not a new horror movie but the cyclical invasion of forest tent caterpillars, the little worms that defoliate Northland trees and rise to an unstoppable, some say intolerable, infestation once every decade or so.

“We’re seeing a few of them in more places now,” said Jana Albers, forest health specialist for the Minnesota Department of Natural Resources in Grand Rapids. “We’re on the ramp-up for a big outbreak in another one to three years. I’m thinking the peak will be in 2011, maybe 2012.”

That means if you have outdoor events planned for this summer, it’s probably safe. But think twice about that outdoor wedding in June 2012.

The leaf-eaters set their all-time record in 2001 when about 7.7 million acres — 12,000 square miles — of northern Minnesota forest were defoliated, according to aerial surveys by the DNR. They nearly reached that with 7.4 million acres hit in 2002, the second-worst year on record.

By comparison, 2008 saw only 10,000 acres defoliated by the forest tent caterpillars, often wrongly called army worms, statewide.

But their numbers are about to grow exponentially again.

This year, forest insect experts are seeing building numbers in the Twin Cities and in central Minnesota near Lake Mille Lacs and at the Rum River State Forest, as well as in Meeker and Kandiyohi counties, where the caterpillars are in their second year of munching on oaks and basswood. That’s usually a precursor to a major outbreak in northern forests.

The bugs don’t actually move across the state. But their population increase seems to happen sooner in the south and central hardwoods.

Reports are filtering in from across the Northland of people seeing one or two of the hairy, blackish, speckled caterpillars — forebears of the next invasion.

The caterpillars, native to the region, are always around at low levels. But, for reasons not fully understood, their numbers build every decade or so to huge levels. About this time of year they hatch from egg clusters on trees and start eating every leaf they can reach. The region’s vast tracts of aspen are especially vulnerable, but almost every type of tree can be a target.

Some trees are completely denuded. And although most trees re-grow leaves and recover quickly, previously diseased or stressed trees can perish. Up to 10 percent of the trees that are defoliated die with each outbreak.

The sheer numbers of caterpillars marching by the millions, crawling across roads and driveways and up the sides of homes and garages in search of more leaves, can be repulsive for some people. They also drop little black specks of manure.

Some homeowners battle the beasts to keep them off apple and other special trees (soapy water seems to work), but little can be done at the forest level.

After eating their fill of leaves, the caterpillars metamorphose into moths that can fill the sky and build up in huge numbers near outdoor lights. The moths lay eggs in clusters that hatch next year.

Meanwhile, friendly flies — they don’t bite people — also explode in number to feast upon the caterpillars. They bring their own disgusting invasion into back yards and campsites in numbers so high they can drive even outdoor people indoors.

Then, as quickly as caterpillar and fly numbers build, they crash. Within a few years after the peak, caterpillar numbers drop back to nearly undetectable levels.

Albers thinks outbreaks in recent decades have been compounded by repeated periods of drought over the past 20 years, and she says climate change is to blame. The unusually cool spring this year could slow the buildup and delay the peak outbreak by a year, but won’t lessen it when it comes, she said.

“The peak FTC outbreaks coincide with periods of drought,” Albers said. “And we are having more of those.”

Brian Schwingle, forest health specialist for the Wisconsin DNR at Rhinelander, said he hasn’t seen a single forest tent caterpillar this season. Like Minnesota, Wisconsin saw a massive outbreak in 2001.

“We know we’re going to see them pretty soon, we just don’t know when,” he said.

Wisconsin insect experts are forecasting an outbreak of eastern tent caterpillars, a different and less numerous pest that makes “tents” in the crooks of tree branches, especially fruit trees. That outbreak is expected to be visible in several northern counties this summer, including Ashland, according to the DNR.

Tags: