Hastings called out plows after a particularly robust hatch. One accident was blamed on the mess.
Mayfly mating is creating more than its usual mess in Minnesota again, sending cars skidding into each other, rousting snowplows out in July and prompting some cities and bridges to go dark.
The annual affair always culminates with piles of dead insects, but this year also turned into a major mess in Hastings on Sunday.
A slick mat of bug carcasses on the Hwy. 61 bridge over the Mississippi River caused a driver to lose control and hit another vehicle head-on, causing minor injuries.
Earlier in the evening, traffic authorities had shut off the lights that threatened to lure a similar swarm to the Wakota Bridge.
While the annual mayfly ritual is usually just a messy nuisance for river towns, it's the beginning of the end for the short-lived bugs.
Mayflies spend most of their lives -- about a year -- in the rivers, said Jeff Hahn, an entomologist with the University of Minnesota Extension.
When they emerge from the river as adults, a fast and furious mating party begins.
"It's not usually one or two at a time," Hahn said. "They try to be in sync with one another. So when you see big clouds of them, that's a mating swarm. "
"It's like a big speed dating party," he said.
"Their life is so short -- and their only goal is to mate -- it's in their best interest to have everyone coming out at the same time."
But the party is short-lived and, before they die, the females lay their eggs in the water.
While the hatch happens every year, the mayhem does not.
"But this one [in Hastings] seems to be unusual in that it was particularly synchronized," Hahn said. "It makes me wish I had been down there," Hahn said. "It's not something you see every day. It must have been a tremendous sight."
So was the morning after.
Snowplows were pulled out of summer storage to clear the dead bugs and spread sand for traction. Dawn's light brought out city crews armed with leaf blowers and street sweepers to tidy up the town for its summer River Town Days festival, which begins this week.
The mayflies' emergence usually happens about this time every year, Mayor Paul Hicks said. A light hatch came Saturday night, so the city was prepared for the subsequent big hatch.
The swarming bugs are often drawn to bright lights, so officials turned off street lights Sunday night around downtown, including the dome lights on the historic City Hall, Hicks said.
Undimmed business signs attracted so many bugs that they couldn't be read, he said.
The multitude of mayflies is good news about the Mississippi River, Hahn said. "It means the water quality is good."
And that means Nick Sampson, manager of the American Legion in Hastings, likely will be revving up the leaf blower again on Tuesday, cleaning up the remains of the mayfly mating madness.
"We had a good inch covering up our patio," he said. "You just clean it up, and then life goes on as usual,"