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Clouds of Mayflies Are Terrorizing Wisconsin

But as long as you don't mind slime-covered roads, that's actually a good thing.
July 29, 2014, 9:20amUpdated on July 29, 2014, 12:36pm

Ah, Wisconsin. Land of beer, cheese, and mayfly hatchings of biblical proportions.

If it doesn't sound like Exodus to you, maybe it sounds more like the beginning of a sci-fi or horror film. It's a clear summer night, and in the La Crosse National Weather Service office, a bored science officer is passing the time when all of a sudden, on the radar screen, a mysterious and unidentifiable cloud rises from the Mississippi River and spreads across the region, unleashing havoc as it expands ever outward.

Not only did this really happen last Sunday night, but apparently it happens all the time.

"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, 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.' "

So many mayflies. As they rose from the river banks to mate and lay eggs, they not only formed a cloud big enough to appear on the weather radar, they died in piles as deep as two feet. There were enough dead flies to cause a three-vehicle accident on the highway, as the bugs, attracted by streetlights, were crushed into a winged slime. Two people were injured when a car slid on the fly slime into another car, and then hit a van.

Massive amount of mayflies emerge in Wisconsin

— FOX 32 News (@fox32news) July 23, 2014

Wisconsinites, being a rather hearty people, dug out the next day and provided some pictures of life after the swarm. After the females lay their eggs, either back in the river, or apparently, in any glass of beer they come across, they die. The males live slightly longer, before returning to the water to be eaten by fish.

As this summer has so far been a cool one, this was the "best" swarm so far in 2014. According to the Star Tribune, there were four big hatchings by this time last year. Two years ago one of the swarms was bad enough to warrant the off-season use of snowplows.

Still, the general consensus is that the big hatching is a good thing. Not only do the swarms of mayflies generally end up feeding fish in the river, Mark Steingraeber, a biologist with the US Fish and Wildlife Service, called the flies a "sentinel" species. Sensitive to chemical pollutants, mayflies are one indication of the health of a given body of water. In the 1920s, mayflies disappeared from a 70-mile area south of the Twin Cities. After the implementation of the Clean Water Act reduced much of the point-source pollution in the Mississippi River, from industry and water treatment plants along the river, the mayflies showed back up again in the late 1970s. Now they're back in positively repulsive numbers.

The Mississippi River still has pollution issues stemming from urban runoff and agriculture. But judging from the mayfly swarms, the chemical composition on the section of the river that separates Wisconsin and Minnesota, at least, is doing really well.