By M.L. JOHNSON, Associated Press

MILWAUKEE (AP) — Mayflies have begun emerging from the Mississippi River in swarms that show up on radar like thunderstorms, coat roads and leave behind slimy messes. They've already been blamed for at least one car crash this week in Wisconsin.

The flies hatch and then spend a year burrowed into the sediment on the bottom of the river that serves as a border between Wisconsin and Minnesota. They emerge the next summer to mate, lay eggs and die, all in less than 48 hours.

Mayflies, sensitive to oxygen levels and pollutants in the river, serve as "sentinels" for scientists and others concerned about water quality, said Mark Steingraeber, a U.S. Fish and Wildlife Service biologist. Mayflies disappeared from a 70-mile area south of the Twin Cities in the 1920s and didn't reappear in significant numbers again until 1978, when wastewater treatment and others actions taken under the Clean Water Act began to have an impact.

The National Weather Service captured a massive swarm on radar Sunday night as the flies came out of the river and drifted north on the wind. The radar system picks up energy reflected off the flies, with the image's intensity reflecting the density of the bugs.

A loop recorded Sunday shows yellow patches directly over the river that morph into a green band as the flies drift north. The bugs become blue dots as they disperse.

A second, smaller swarm recorded Thursday night starts as a green band before exploding like fireworks into blue dots.

"Almost every night in the summer, there's some sense on the radar that there's something coming off the river," said Dan Baumgardt, science and operations officer for the National Weather Service in La Crosse. "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.' "

The weather service typically records several swarms each year from June through August. Air and water temperatures have been usually cool this year, helping explain why Sunday's emergence was the first big one this summer, Steingraeber said.

Attracted by light, mayflies congregate on roads, bridges and other surfaces in piles that can be nearly 2 feet high. Cars crush the flies, releasing liquid in the females' eggs and making roads slick. Two people were injured Sunday in Trenton Township in western Wisconsin when a car slid on a road covered by flies, crashed into a second car and then struck a van.

Flies that aren't crushed spend 24 hours shedding exoskeletons in a final stage of development before taking to the air again to mate. The females then seek water to lay their eggs.

"If they happen to come across a cup of beer, they will deposit them in a cup of beer," said Steingraeber, who needs volunteers to help track the flies' appearance for his research.

Adult flies die after laying their eggs. Within hours, their offspring hatch in the river, and the cycle begins again.

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Online:

National Weather Service video: http://www.crh.noaa.gov/arx/?n=mayflygeneral#July242014

Record your mayfly sightings: http://www.fws.gov/midwest/lacrossefisheries/mayfly.html