HASTINGS, Minn. (WCCO) — Millions of dead mayflies created an incredible mess on several streets in Hastings Monday morning.

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The bugs lay their eggs in the Mississippi River. Each summer they hatch and then swarm to mate with each other.
Pete Svihel was in the midst of the annual clean up Monday. As he swept the outside of his Hastings home, he removed the remains of some unwanted guests.

It was around 10:30 p.m. Sunday when the mayflies made their appearance.

“It happens at least once a year,” Svihel said. “It’s not too bad until it gets dark and then they collect around the lights.”

Millions soon swarmed businesses and roads near the river. The blanket of bugs even posed a risk similar to snowy streets. Two vehicles crashed on the 61 bridge over the Mississippi. The mayflies made for slippery driving and caused one car to lose control.

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“This was one of those times when they all, literally, came out all at the same time,” said Jeff Hahn, a University of Minnesota extension professor.

For Hahn, an entomologist (one who studies insects), the size of this year’s mayfly swarm isn’t unusual. But predicting when it will happen is the challenge.

“It’s not predictable,” he said. “When they’ll emerge is not predictable, if there’s going to be a large hatch or not.”

Their lifespan isn’t long, from a few minutes to a few days. Often times, the weather will disperse the eggs so the swarms aren’t as large.

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On the bright side, entomologists say a mayfly swarm is an indication that the river is healthy.