Lynn Nardinger recalls as a young adult hearing a story about a convertible that was parked under a street lamp on the Wisconsin side of the Mississippi River, just across from Red Wing.

“Apparently the top was down, and when the mayflies started to hatch and eventually died that night, they filled the car right up,” said Nardinger, deputy director of public services for the city of Red Wing. “Mayflies are attracted to light — they just home in on it. I can’t imagine what the owner must have thought when he came to get his car.”

Every July near dusk, massive swarms of mayflies hatch, mate and die along the banks of the Mississippi River, creating what some call a “synchronized aquatic blizzard” that can cause all sorts of mayhem. Roadways and bridges get covered with the big-eyed and slippery-when-crushed insects, occasionally causing vehicle and motorcycle accidents. Well-lit gas stations and parks along the river become mayfly magnets. City officials use shovels, snowplows and street sweepers to clean up the mess. Even river boat operators have a salty — and unprintable — phrase to describe the swarms. Last July 21, in the La Crosse, Wis., area, a prolific hatch created a bow echo on radar, akin to what National Weather Service officials say would be made in a significant rainstorm.

Yet there’s an upside to the memorable and annual summertime ritual. Scientists say the presence of mayflies is a sign of a healthy Mississippi River. The mayflies had all but disappeared from the watershed (as well as the Great Lakes) because of pollution before staging a comeback after the 1972 Clean Water Act was passed. The insects also provide food for many critters — from fish to frogs to birds to other insects. Fly anglers even take advantage of the hatch.

Pinpointing the hatch

Mark Steingraeber is a fisheries biologist with the U.S. Fish and Wildlife Service in Onalaska, Wis. He has studied the big river’s mayflies since 1988, a year after they returned to the Twin Cities area, he said. Steingraeber tries to pinpoint when the river’s primary annual emergence will occur. It’s a question he is routinely asked by cinematographers, who want to capture the dizzying swarms, and by brides planning their dream riverside weddings, who would rather not.

“You wouldn’t believe the requests I get,” said Steingraeber, chuckling. “I generally tell people around or little before July 4th, plus 10 days to two weeks. That’s when you’re likely to get the first major emergence. There are also smaller, sporadic and isolated emergences along the river into August.”

To better predict the primary (and typically heaviest) emergences along the river, Steingraeber uses a water temperature model developed at Tennessee’s Oak Ridge National Laboratory in the early 1980s. To validate the model, Steingraeber’s agency recruits “citizen scientists” to gather field data (when and where emergences occur) to better predict when the millions of mayflies hatch along the Mississippi River.

Volunteers are asked to gather and submit field data using a website and smartphone apps developed by the USA National Phenology Network, he said. The technology, dubbed Nature’s Notebook, allows volunteers to record their observations (from the Twin Cities to St. Louis), after which they will be uploaded to public databases available to scientists like Steingraeber.

“Using citizen scientists not only helps with data collection, it gets them out on and learning about the river, which is very important,” said Steingraeber. “There’s also a practical reason to develop an accurate model: public safety. Emergent mayflies pose a unique risk to motor vehicle operators, not unlike icy roads in winter.”

Fishing and food

The mayfly hatch can be a public nuisance, but it also can be the inspiration for some good, clean fun. Dan Dieterman, the longtime Mississippi River biologist for the Minnesota Department of Natural Resources in Lake City, said he has been fly-fishing the river’s major emergences for the last 20 years.

“I’m not a huge fly fisherman, but I love fishing the Hex [mayfly] hatch on the Mississippi,” he said. “When you hit it just right, it can be quite an ordeal, with fish slurping everywhere. I’ve fished many times with clouds of mayflies all around me and on the water. It’s a lot of fun.”

This year’s first significant emergence occurred July 4th, from roughly Hastings downstream to the Quad Cities, in northwest Illinois and southeastern Iowa. For three nights beginning July 5, Dieterman waded the Mississippi near Frontenac State Park, casting a dry-fly Hex pattern to numerous fish species that were gorging themselves on the real thing.

He caught plenty of fish, too, including 25 white bass, 30 mooneye, 12 freshwater drum, five bluegills and two smallmouth bass.

“The hatch is a smorgasbord for pretty much anything that swims,” he said. “Over the years I’ve also caught carp, walleye and red horse on the surface. They were all eating dead mayflies after they mated.”

Mayflies are extremely important to the river’s aquatic food web, Dieterman said. Mayfly nymphs are eaten by other invertebrates, amphibians, reptiles, birds and mammals. Birds and bats eat adult mayflies with gusto. “I’ve watched turkeys come out of the bluffs and chow the dead mayflies,” he said. “Ducks and geese eat them, too. It’s quite a happening to witness.”

As for Nardinger at the city of Red Wing, he is hoping someone — anyone — can develop a model to more accurately predict the annual inundation of mayflies. “It would certainly be a big help; now we’re just guessing,” he said.


Tori J. McCormick is a freelance writer from Prior Lake. Reach him at