The bane and benefit of mayflies

LAKE CITY, Minn. -- It's summer on the Mississippi River -- and the mayflies are back. Once a year, these inch-long insects emerge from rivers and lakes en masse.

"They're like a tornado," said 7-year-old Max.

And like a tornado, the swarms are so large they can be tracked on radar. Though they don't bite or sting, mayflies can still elicit terror. Some river towns even turn off their streetlamps to keep the light-loving bugs at bay.

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Deceased mayflies blanket a road
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That's because mayflies drop dead after an exhausting 24-hour mating frenzy, leaving piles that can make driving treacherous.

As overwhelming as these mayflies can be, they actually bring some welcome news in a messy package.

"It's kind of a celebration for biologists," said Kent Johnson, who supervises environmental quality for the greater Minneapolis-St. Paul area.

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Swarms of mayflies as seen on radar
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Johnson says the flies spend most of their lives at the river bottom and need clean water to survive.

"They're indicators of excellent water quality," he explained. "As scientists we can only spot check it once in a while and make some assumptions based on that."

Put another way -- the mayflies act as a sort of environmental cheat sheet.

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Jerry Castor sucks up mayflies to bring back to Green Bay, Wisconsin
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With his shop-vac, professor Jerry Kaster is collecting mayflies to re-introduce them to Green Bay, Wisconsin, where they have disappeared due to polluted waters. He says the tourism and sports fishing business would flourish.

"Possibly $250 million a year could be added to the economy because of these mayflies," said Kaster.

The professor believes Green Bay's water is now clean enough, and hopes a mayfly hatch there next summer will prove it.