Mayfly swarms are visible on weather radar. Their declines spell bad news for ecosystem health

Cover photo source: Wikimedia commons

Stepanian, P. M., Entrekin, S. A., Wainwright, C. E., Mirkovic, D., Tank, J. L., & Kelly, J. F. (2020). Declines in an abundant aquatic insect, the burrowing mayfly, across major North American waterways. Proceedings of the National Academy of Sciences.

Mayfly magesty

It’s hard to overstate the enormity of mayfly swarms. They black out the sky, coat buildings and roads, and knock over whole trees with their weight.

As one Green Bay (WI, USA) resident reminisced, “It was like your worst nightmare… You just couldn’t help be covered with them. And you had to walk on them, walking on squishy flies… You couldn’t escape them.”

Naturalist Nick Di Cresce has a more favorable perspective on the flies: “My wife took off (on seeing them), but I was out there thinking, man, this is great! I was walking right into them.”

Some people love ‘em, some people hate ‘em, but it is indisputable that mayflies play an important role in the ecosystem. Unfortunately, decreasing water quality may make the spectacle of a massive mayfly swarm a legend of the past.

The life of a mayfly

While mayflies are most visible during their episodic swarming events, they spend the vast majority of their life (typically around one year) in the water. There, they serve as an important food source for fish and other freshwater animals. When they are ready to reproduce, mayflies will emerge from the water and molt twice before taking off into the air as part of a mating swarm.

Swarms of flies may seem frightening, but the good news is that they won’t bite. In fact, they can’t. Adult mayflies don’t have a functional mouth or digestive tract. Without the ability to eat, their only purpose is to mate. They typically die within a day of taking their adult form.

As mayflies die, their carcasses fall onto the surrounding land and become a nutrient source for plants and animals on land. This also helps to remove nutrients from the lake, where the nutrients could otherwise contribute to algal blooms.


Mayflies used to be enormously abundant around North American rivers and lakes up until the middle of the 20th century. However, with the rise of commercial agriculture and industrial pollution in the mid-1900s, water quality began to deteriorate until it was no longer suitable for the environmentally-sensitive insect. By the 1970s, mayflies had largely disappeared from North American water bodies.

With the passage of the Clean Water Act in 1972 and other targeted conservation efforts, mayflies were starting to make a comeback by the turn of the century. Unfortunately, recent data indicate that this improvement seems to have reversed.

Recent Research

In a study published this January, Dr. Phillip Stepanian and a team of colleagues from across the country used U.S. weather radar to track mayfly swarms over Western Lake Erie Basin. Yes, that’s right: weather radar. These mayfly swarms are so large and dense that they can be picked up using night time weather surveillance. What they found is as mind-blowing as it is concerning.

A map of counties bordering the Mississippi river. There is a blue highlighted section of river with little dots spreading out around it. These are swarming mayflies picked up on the radar
Image of a mayfly swarm along the Mississippi river captured on weather radar. Source: Flickr
Did I mention these are (or used to be) big swarms?

By analyzing radar outputs from 2012 to 2019, Stepanian and colleagues were able to quantify just how big these swarms are. They found that up to 88 billion mayflies can emerge in the course of just one night. Combined, these flies weigh over 3000 tons. Over the course of one summer, they estimated that an average of 60 fly carcasses fall on each square meter of land (though many more flies die near the shore of the lake)!

Most importantly, the researchers found that the number of individuals emerging over the summer has declined dramatically. Between 2015 and 2019, radar surveys indicated an 84% decline in the abundance of these mayflies.

Insect Apocalypse; Ecological Armageddon

Mayflies aren’t the only insects showing dramatic declines. Previous studies have yielded astonishing figures, including a 75% decline in total insect biomass in protected areas in less than 30 years. The decline in insect population has been coined “ecological Armageddon” for the devastating impacts it will have on ecosystem functioning.

Near Lake Erie, the lake studied in this paper, people are especially concerned for the wellbeing of insect-eating birds. At peak mayfly density, mayflies provide up to 12 trillion calories of food resources to the surrounding ecosystem. This could support over 53 million birds. Unsurprisingly, reports are already showing a decline in birds that eat flying insects.

Finally, declining mayfly populations are concerning, because mayflies are an indicator of water quality. Mayflies are highly sensitive to algal blooms, increased nutrient concentrations, and pesticides. Currently, concentrations of neonicotinoid, an agricultural insecticide, can be up to 40 times greater than the U.S. Environmental Protection Agency (EPA) standard. As the United States continues to cut back federal protections for aquatic systems, these insects may be under more danger than ever.

If you want to learn more about mayflies and help track their abundance, consider participating in the Mayfly Watch campaign of the U.S. Fish and Wildlife Service. You can also make a difference by working within your community to decrease the use of pesticides and fertilizers, and control runoff into the lakes and rivers that house these magnificent creatures!

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Abigail Lewis

I am a Ph.D. student studying freshwater ecology and biogeochemistry at Virginia Tech. Whenever possible, I enjoy thinking and writing about the role of science in society, including community-based science, science communication, and science for the public good. In between rehearsals, hikes, and long dinners I am working to build a career that will address environmental issues and build a more inclusive scientific community. Twitter: @lewis_lakes

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