The first swarm of mayflies has hit the Midwest, showing up on radar and reportedly causing a three-vehicle car crash in Wisconsin.

The flies live for a year or two as larval nymphs in the sediment of the Mississippi River before emerging for about 48 hours of life where they mate and lay eggs before dying.

Sunday marked the first swarm of this year, an unusually large one.

"What made this unique was the massive number of insects that were involved," Zack Taylor, a meteorologist with the National Weather Service in La Crosse, Wis., told the Minneapolis Star-Tribune. "The signature on the radar was pretty impressive."

Mayflies can make roads slick when cars crush the insects and release liquid. The first swarm of the summer was blamed for a three-vehicle collision in Wisconsin that put one person in the hospital, the Star-Tribune reported. Four similar mayfly swarms were documented last summer in June and July.

"Almost every night in the summer, there's some sense on the radar that there's something coming off the river," Dan Baumgardt, science and operations officer for the National Weather Service in La Crosse, Wis., told the Associated Press. "We don't know what kind of bug it is ... until we have people calling or saying, 'Oh my gosh, there's mayflies all in the La Crosse area.'"

Their habit of clustering in piles 2 feet high on roads can be dangerous, but the flies are also a positive indicator for the local environment due to their status as "sentinels" for water quality. Sensitive to pollution, mayflies function as filter feeders, living on decaying organic matter on the bottom of the river, according to the Fish & Wildlife Service.

In the 1920s, mayflies disappeared from a 70-mile area south of Minneapolis and St. Paul, not returning until 1978 after wastewater treatments and other conservation efforts had taken effect. 

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