The Free Press, Mankato, MN

State, national news

June 13, 2014

Flies vs caterpillars

ST. PAUL (AP) — "Parasitic flesh flies" sound pretty unpleasant, but Minnesotans may have them to thank for stopping a big outbreak of tent caterpillars this year.

Minnesota Public Radio News reports the threat of a massive tent caterpillar hatch has been building since 2001, when 8 million acres of forest were defoliated statewide.

The tent caterpillars hatch from cocoons in May and June to eat leaves. Most years they cause only an annoyance, but every decade or so their numbers start to grow. In 2012, tent caterpillars defoliated 275,000 acres of trees in Minnesota. Last year they got 1.1 million acres.

This year's hatch was expected to be worse, said Minnesota Department of Natural Resources forest health coordinator Val Cervenka, but so far, the hatch has been spotty and light.

Meanwhile, the parasitic flesh flies are swarming northern Minnesota.

The flies lay their eggs in the caterpillars' cocoons, then the young flies eat the caterpillars over the winter. Cervenka said it's too early to know for sure, but the flies have already left the tent cocoons — suggesting the tent caterpillar hatch has peaked.

Tent caterpillars don't typically cause long-term damage to trees, but they are annoying.

"These tent caterpillars crawl all over everything," Cervenka 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 parasitic flesh flies, which resemble common houseflies, don't bite, but can also be a nuisance.

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

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