A mayfly clings to the bark of a tree.

Jim Gehrz, Star Tribune

BWO(blue wing olive) Mayfly


Mayfly's adult life is fleeting but very eventful

  • Article by: Jim Anderson
  • Star Tribune
  • July 24, 2014 - 5:52 PM


On the five-point Johnson Scale, Sunday night’s spectacularly freakish mayfly emergence and hatch along much of the Upper Mississippi was a doozy, rating a 4.5.

The unofficial scale, named for Kent Johnson, the Metropolitan Council’s manager of environmental monitoring and assessment, was developed by Mark Steingraeber, a fishery biologist with the U.S. Fish and Wildlife Service. With Dan Baumgardt, science officer with the National Weather Service in La Crosse, Wis., the three are part of a network of professional and amateur observers who monitor mayflies intently.

They’re not the only ones.

Film crews from as far away as Brazil and from the National Geographic Society have also been on hand to capture the event.

The insects, which often emerge from the water en masse into adulthood to mate and lay eggs before a quick death, have much to tell us about our environment — and ourselves and how we interact with the river, Steingraeber said.

After mayflies returned to the river in the late 1980s — thanks mostly to the Clean Water Act — he developed a formula for predicting major mayfly events, based on variables like water temperature and degree-days. The matrix has been 95 percent reliable in forecasting mayfly emergence within two or three days, but the variability has been slipping in the past few years.

He wants to figure out why. When mayflies are under stress, he explained, they can revert to a two-year life cycle from their normal one-year. It’s possible that this year’s event, in which billions of insects arose in a buzzing blizzard visible on Weather Service radar, was the result of eggs hatched in 2012.

There were several emergences last year, but they rated only a 3 on the scale. That’s another topic of study.

Steingraeber is always looking for more contributors to expand this important work, especially from people who live close to the river. To learn more, go to

Steingraeber is not expecting another major mayfly emergence this summer but quickly adds: “Stranger things have happened.”

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