Mayflies, a keystone species, are on the decline on the Mississippi River

On July 4, 2015, crowds gathered in downtown La Crosse, Wisconsin, to watch the fireworks. But it wasn't long before they were confronted with something else soaring through the sky that night.

Thousands of mayflies that had spent the past year buried in the sand at the bottom of the Mississippi River rose en masse from the water, drifting on the southbound wind and heading toward the unsuspecting public on shore. Before long, the winged insects had coated the edge of the city, clinging to cars, streetlamps and gas pumps — an almost Biblical scene.

"The whole downtown was overcome," said Dan Baumgardt, science and operations officer for the National Weather Service's office in La Crosse.

Though the timing was a surprise, the phenomenon was not. Each summer along the upper Mississippi River, burrowing mayflies emerge, mate and die within roughly 48 hours, a fleeting life cycle that somehow still leaves room for them to cause problems. Municipalities sometimes have to use snowplows to clear mayfly carcasses. In 2014, local authorities blamed a three-car crash near Red Wing, Minnesota, on a mayfly emergence that reduced visibility and made the roads slippery.

Despite the consequences, their presence is actually a good thing. The insects need clean water to survive, so a bumper crop means the river they came from is relatively healthy.

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That's why a recent study that found a significant decline in mayfly abundance over the past several years has raised concern about the river and its food chain, to which mayflies are essential.

Because they hold such critical insights into water quality — and because their surprise emergence can be messy and unpleasant — scientists are interested in predicting the timing of mayflies.

But they need the public's help in doing so. They're hoping to beef up participation in a citizen science campaign, which a Wisconsin researcher originated, to help find some answers and to encourage people to get out more and observe the natural world.

Mayflies are canaries in the ecological coal mine

Prior to the passage of the Clean Water Act in 1972, many of America's waterways were polluted, and mass mayfly emergences largely disappeared from the upper Mississippi. Agricultural and urban runoff into the river depleted oxygen that the insects needed to survive.

Gradually, cleanup and conservation efforts brought burrowing mayflies back to yet again descend annually onto river towns.

Now, some data shows that could be changing again.

Researchers from Oklahoma, Notre Dame and Virginia Tech universities used weather radar to track the scale of mayfly emergences over the upper Mississippi River and Lake Erie over the last decade. Their results, published in a 2020 paper, showed a 52 percent decline in burrowing mayfly abundance on the river between 2012 and 2019.

"If these population trends continue, persistent environmental changes could threaten to once more extirpate (burrowing) mayflies from North America's largest waterways, making this ephemeral spectacle — and its vital ecological functions — a thing of the past," the researchers wrote.

Since mayflies are the food source of many fish and birds on the river, declines also could spell trouble for the river's food chain.

Researchers seek in-person verification for emergences

In a 1961 article in the Winona Daily News, Calvin Fremling — a Winona State University professor well-known for his mayfly research — wrote that much was still unknown about the arrival of the "cussed and discussed" creatures.

"What force coordinates this mass activity which encompasses 500 miles of river?" Fremling wrote. "Is it water temperature, river stage or possibly phase of the moon?"

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Such questions still linger for Mark Steingraeber, who worked for decades in La Crosse as a fishery biologist at the U.S. Fish and Wildlife Service. In the late 1980s, Steingraeber began to study mayflies as a way to measure the amount of polychlorinated biphenyls, or PCBs, in the upper river. Shortly before he retired in 2016, he was drawn to the insects again, this time wondering if he could more accurately predict emergences based on water temperature.

To do that, though, he'd need to know when they were happening. The National Weather Service's La Crosse office has tracked mayfly emergences on radar since 2012, but radar alone can't prove that burrowing mayflies indeed had hatched — only in-person observations can confirm that.

So Steingraeber approached Erin Posthumus, outreach coordinator for the USA National Phenology Network, which tracks the seasonal cycles of plants and animals to understand the effects of environmental changes.

The network, based at the University of Arizona, uses an app called Nature's Notebook to allow volunteers from across the country to contribute data. People could observe a tree in their backyard, for example, recording when it grows leaves, when those leaves change color and when they start to fall. The data is available online for the public and for natural resource managers, like the Fish and Wildlife Service.

Today, Mayfly Watch is one of the network's campaigns. People can pick a location on the Mississippi River to visit regularly and document emergences, Posthumus said — recording what type of mayfly they see, whether the insects are dead or alive, and how many there are.

But only a few dozen people are logging burrowing mayfly emergences each year, compared to other campaigns that receive hundreds or even thousands of submissions.

If they had more observations, Posthumus said, they might be able to answer Steingraeber's questions about the relationship between water temperature and mayfly emergences. With that information, they could give municipalities a warning about when to take practical steps ahead of a swarm, like turning lights off and encouraging drivers to be cautious about road conditions.

That could also provide on-the-ground evidence of whether the species is declining.

How to participate in Mayfly Watch on the Mississippi

Those interested in joining the citizen science campaign should start by creating a Nature's Notebook account.

Select the mayfly campaign and choose the location where you'll be making observations.

For step-by-step instructions about how to make observations, and for additional information, visit fws.usanpn.org/Campaigns/MayflyWatch.

This story is a product of the Mississippi River Basin Ag & Water Desk, an editorially independent reporting network based at the University of Missouri School of Journalism in partnership with Report For America and the Society of Environmental Journalists, funded by the Walton Family Foundation.