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Forest tent caterpillar numbers should be minimal this year in Minnesota

An army of forest tent caterpillars swarm the yards of residents in Lakewood Township near Duluth in May 2000.1 / 2
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DULUTH -- Forest tent caterpillar numbers crashed in 2014, a year they should have multiplied, surprising forest pest experts but leaving residents and visitors of northern forests grateful and their trees intact.

Even better news is that the number of leaf-eating north woods caterpillars likely will be down again this year just when it was expected they would peak.

That's good for anyone planning outdoor reunions, weddings and camping trips in May and early June when billions of the caterpillars were expected to be writhing around the region.

"My prediction for this year is that most people and most places will not be seeing FTC," said Mike Albers, Grand Rapids-based forest health specialist for the Minnesota Department of Natural Resources and one of the state's leading authorities on the pest. "My guess right now is that it will be similar to last year."

Albers has been out in the woods in recent weeks looking for forest tent caterpillar egg masses in the crooks of trees, mostly aspens. Those caterpillars -- sometimes incorrectly called "army worms" -- will crawl out any day now, probably about May 1.

He cautioned that he checks only a tiny portion of the millions of acres of trees in the region. But in most of the places Albers looked for the caterpillars this spring he found none, or found them in very low numbers.

That means if you didn't see many caterpillars last May, or many little brown moths in July,  you probably won't see many this year.

Paul Cigan, forest health specialist for the Wisconsin DNR in Spooner, echoed Albers' findings.

"I'm just not seeing them in any large amount," Cigan said via cell phone as he was checking for egg masses. "I just don't think people are going to see many of them this year. It's certainly not going to be a big population."

Albers did find a few hot spots, including one southwest of Tower, were residents may notice a lot of caterpillars hatching soon after aspen leaves sprout.

"This area has had significant levels of defoliation for at least three years already," he noted.

Forest tent caterpillars last peaked in 2001 and 2002 when they defoliated more than 7.3 million acres of forest each year, the largest outbreaks in recorded history. They rendered trees bare, ruined orchards and wiggled up the sides of homes and across roads and driveways, eating and defecating before they wrapped themselves in cocoons to metamorphose into little brown moths that lay eggs and start the cycle again.

At their peak infestation, forest tent caterpillars can number up to 4 million per acre.

There were so many caterpillars in 2001 and 2002 that people canceled vacations and events, and people spent weeks trying to keep the caterpillars from defoliating their favorite fruit and shade trees and off their houses, decks and patios. Some people reported leaving, so totally grossed out by the marching army of caterpillars that they chose to retreat.

As expected, the caterpillar numbers eventually crashed after they overpopulated, eating themselves out of food and starving. They also succumbed to disease, especially a parasitic fly that emerges on the heels of every forest tent caterpillar outbreak.

By 2006, only 1,900 acres were defoliated statewide, the low point of the most recent cycle.

Then slowly, as they have every decade since people paid attention, the native caterpillars began to expand in number and range across northern Minnesota. By 2012, they defoliated 274,000 acres and in 2013 that number skyrocketed to 1.1 million acres.

By then, because large-scale outbreaks have occurred every 10 to 15 years over the past century, Albers and other experts assumed that the next peak caterpillar outbreak was near, likely between 2014 and 2016.

But then something happened to reverse the caterpillars' course. Last spring, very few if any caterpillars emerged from their egg masses to munch aspen leaves, their favorite food.

Albers has data from last summer's aerial surveys that showed forest tent caterpillars defoliated only 156,000 acres across northern Minnesota in 2014, a year when experts expected millions of acres of defoliation.

"Most of that was a very low level of defoliation to the point that on a lot of locations mapped it was hard to see to any defoliation by a person standing on the ground, but was visible from an airplane," he said.

Albers, whose wife, Jana Albers, also is a DNR forest health specialist,  was perplexed. Now it's unclear if 2013 was a peak year -- it would be the lowest peak on record -- or if something else is at play.

"I still have no way to predict whether the outbreak will continue to decline or if the population will rebound," he said.

Cigan said Wisconsin never saw the big 2013 increase in forest tent caterpillars and so far sees no sign of a peak outbreak anytime soon.

"At this point, it's not likely that's going to happen," said Cigan, who has checked trees form Polk County to Ashland County. The one place he found a few egg masses was in Douglas County "but even there, it's going to be spotty. Most people probably won't notice them unless they are looking for them."

Albers has a theory why forest tent caterpillars never reached their expected peak and appear to be declining. Those parasitic flies -- often called friendly flies because they like to land on people, but can't bite people -- increased far sooner than usual during an FTC cycle.

"I think the parasitic fly population built up very fast," Albers said, before describing in gory detail how the fly does its job. "The fly lays a maggot on the cocoon of the caterpillar. The maggot burrows into the pupal case into the insect and eats it. So we had lots of caterpillars and defoliation in 2013 (and then) lots of the FTC were killed by the fly while in the pupal stage."

Albers found high levels of pupal parasitism in July 2013. Thus many caterpillars never made it to the moth stage and fewer eggs got laid to start the cycle over -- resulting in far fewer caterpillars in 2014.

"It seems strange that the fly population built up so fast so early in an outbreak," he said. "I haven't seen that happen before."

Albers said speculation that cold winters and springs in 2013 and 2014 killed forest tent caterpillars probably were off-base. The caterpillars are a native species and have adapted to withstand extreme cold winters and late springs. The hatch just waits for the aspen leaves to emerge.

FTC facts

In the past 125 years, forest tent caterpillar outbreaks in Minnesota have peaked in 1891, 1898, 1912, 1922, 1937, 1952, 1969, 1978, 1990, 2001 and -- maybe -- 2013.

Young caterpillars spin threads and fall from trees onto picnic tables, patios and people. Mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters on buildings, tree trunks, cars, campers and other stationary objects.

If they run out of aspen, oak, birch and basswood -- their favorites -- they will gladly munch on apple tree leaves and even garden vegetables.

Forest tent caterpillars (sometimes wrongly called army worms, which are a different species) often emit a greenish-black fluid when disturbed that stains paint and clothing. During the height of defoliation their excrement often rains down from tree branches above.

Most healthy defoliated trees can easily withstand losing their leaves for a few weeks in May and grow a new set by summer, and most are unaffected in the long run. But the stress of losing leaves at a key point of summer photosynthesis can kill trees already stressed by drought or another disturbance. With each major outbreak, about 10 percent of defoliated aspen trees die.

There are some benefits from FTC outbreaks: A few weeks with less shade in the woods can help some other species on the forest floor get enough sunlight to grow, experts say, and all of that caterpillar poop actually helps fertilize the forest soil.

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