Mayflies are rising from the Mississippi River and swarming over vast areas, including large portions of Wisconsin. There, the insects are flocking over people, and have even been implicated in at least one traffic accident.

The insects are so numerous, swarms are appearing on radar screens, and their bodies are coating roadways in slime. On 20 July, the National Weather Service (NWS) in La Crosse, WI saw a massive collection of the insects coming off the Upper Mississippi.

"The radar detected the flies about 8:45 p.m., emanating from the river... with echo values similar to that of light-moderate rain (35-40 dBZ)... [T]he mayflies quickly moved north once in the air. As the flies dispersed moving north-northeast, they also gained altitude with some of the echo being detected... as high as 2500 feet above ground," National Weather Service officials wrote on their website.

Radar transmits low-energy electromagnetic radiation, and measures the strength of waves returning to the system. Population densities of the insects in the swarm were visible in the images created by the system.

Mayflies hatch from eggs near the Mississippi, and spend their first year of life burrowed in mud. At the end of this phase, the insects rise from the mud during summer, mate, lay eggs and die, all in a 48-hour period. Different species of mayfly hatch at different times of year. The animals that left the mud on 20 July are Bilineata, which are a fairly large, dark-colored variety.

After this latest emergence, the insects quickly invaded the towns of La Crosse, Stoddard and La Crescent. There, the creatures are attracted to light, and can pile up in roads up to two feet thick. As they are crushed by cars, females release a slick substance from their eggs that makes roads treacherous.

"Almost every night in the summer, there's some sense on the radar that there's something coming off the river. 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,'" Dan Baumgardt, from the National Weather Service in La Crosse, told the press.

The insects will soon shed their exoskeletons, take to the air once more, and copulate. Females will head back to the river to deposit their eggs, and the adults will die off.

During the 1920's, the insects vanished from areas south of the Twin Cities, and were not seen there again in large numbers until 1978. Conservationists believe recovery of the insects in the region was due, at least in part, to the Clean Water Act. Mayflies are highly-sensitive to pollution and low oxygen levels in water, and can serve as harbingers of environmental distress.

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