The better plague? Flesh flies may take down tent caterpillars

Parasitic flesh flies are swarming across northern Minnesota — and you should be happy about it.

Early results from Minnesota Department of Natural Resources studies suggest the insect known as the "friendly fly" may have saved Minnesotans this year from a plague-sized hatch of tent caterpillars.

The threat of a massive tent caterpillar hatch has been building since the infestation of 2001, which left 8 million acres of forest across the state without leaves, DNR forest health coordinator Val Cervenka said.

• More: Tent caterpillars expected to invade Minnesota

The caterpillars hatch from cocoons every year in May and June to eat leaves and fall from trees onto passing pedestrians and vacationers. Most years the defoliation causes only minor annoyance, but every decade or so their numbers start to grow.

For two or three years hatches build, then there's a really big one. Cervenka had predicted this would be such a year.

In 2012, tent caterpillars defoliated 275,000 acres of trees in Minnesota. Last year they got 1.1 million acres. The forests seemed primed for an explosion of caterpillars, but so far the hatch has been spotty and light.

We may be able to thank the friendly flies for that.

Those flies lay their eggs in the cocoons of tent caterpillars, Cervenka said. The young fly eats the young caterpillar over the winter. While it's too early to know for sure, friendly flies have already taken flight from tent cocoons, suggesting the tent caterpillar hatch has likely already peaked, Cervenka said.

So this year at least, Minnesota likely dodged a bullet, or at least exchanged one pest for a less annoying pest.

Tent caterpillars don't really cause much harm. The two-inch insects cluster in squirming green and brown patches. They eat some leaves, but unless a tree is already stressed by drought or illness, it'll come back.

The real damage is inflicted on weddings and graduation parties — Cervenka called it the "nuisance factor."

"These tent caterpillars crawl all over everything," she said. "There are shed skins all over the place. Their droppings fall from trees. It's nasty and it happens right when people are getting married, graduating, and going on vacation."

Friendly flies, which resemble common houseflies, don't bite, but they won't leave.

"You brush them off and they come right back and land on you," Cervenka said.

So far, DNR scientists aren't sure if this year's light tent caterpillar hatch is just a one-season reprieve or the worst of a 10-year cycle. By the end of the summer, aerial forest surveys should give a better idea of the future.

When tent caterpillars do eventually emerge with a vengeance, Cervenka has only one piece of advice.

"Just go inside," she said. "There is no deterrent."