Massive Swarms Of Mayflies Hatch Along Mississippi River

Sunday Night Marked Largest Emergence Of Mayflies Seen In Years

Mayflies clinging to a window after their emergence from the Mississippi River near La Crosse in 2011. Photo: Sleepy Cat 15 (CC-BY-NC-SA).

Swarms of mayflies hatched Sunday night along the Mississippi River, in the largest emergence residents along the river have seen in years.

When Bevin Sandstrom got to work at 4:45 on Monday morning, he immediately strapped on a backpack blower. Sandstrom — a maintenance worker at a few corporate office buildings along the river in downtown La Crosse — cleared the sidewalks so other employees wouldn’t have to walk through piles of dead mayflies as they arrived to work.

He said he’d be sweeping mayflies all day, and would throw them in dumpsters or back into the river before they start to stink.

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“The fish love them,” he said. “The fish are eating like kings now. But this is the most I’ve seen in a long time. They’re kind of really a nuisance. They stick to the window, then they’re all slimy, then the poor window cleaners got a heck of a job.”

The brown mayflies that hatched this weekend will only live a day or two. While some have sporadically appeared throughout the summer, the swarm that emerged last night is the largest in a couple years.

Dan Baumgardt is the science and operations officer with the National Weather Service in La Crosse. The emergence showed up on their radars, and Baumgardt said the winds carried the mayflies far to the north.

“A lot of the communities to the north of the river, like Galesville, saw an influx of these mayflies come off the river and up and northward,” said Baumgardt. “We detected them all the way above Black River Falls at about 2,500 feet above the ground.”

Mayfly hatches are generally an indicator that the river is in good health.