Parts of Minnesota are seeing the worst armyworm infestation in decades.
The worms are damaging corn and other grain fields from the southeastern part of the state through west-central Minnesota and into North Dakota.
Most fields do not have any armyworm damage, said Bruce Potter, an entomologist with the University of Minnesota Extension Service in Lamberton, adding that it depends on where the moths laid their eggs.
"The damage happens pretty quick as most of the feeding happens during the last week of life," Potter said. "Probably about 80 percent of the plant material is consumed at that time, and they can take a cornfield and completely defoliate it. They'll check the leaves down to the mid-rib of the corn -- small grains they can defoliate."
The worms are the larvae of moths that ride the jet stream up from the South and lay their eggs in grassy fields. Cool, wet weather this spring created favorable conditions for them.
It's somewhat unusual for armyworms to hatch this far north, said Geir Friisoe, director of the plant provision division of the Department of Agriculture.
"But when they do, their name is well-deserved because they show up sort of in an army and they basically ... just sort of mow things down like a big army," Friisoe said.
Ag inspectors will verify the damage and alert growers about signs of armyworm so they can treat fields with insecticide.
Armyworm activity is winding down across the state, Potter said.