This story was originally published in Midstory, a regional news nonprofit based in Toledo.
If you’ve ever spent the summer near a freshwater lake or river, chances are you’ve noticed them. Clinging to windshields, storefront windows and streetlights, these little creatures are an iconic harbinger of a Midwestern summer.
Swarms of mayflies have been observed across Northwest Ohio this season, and experts say the abundance of mayflies in this region owes its thanks to the current health of Lake Erie.
While mayfly populations fluctuate drastically by location and year, more recent years show a notable resurgence, with several stations recording “Good,” “Excellent,” or even “Imperiled” peak densities between 2021 and 2023.
Despite their name, mayflies are not true “flies” like the kind that buzz around a fruit bowl. They are related to dragonflies and damselflies, who, in addition to sharing this confusing naming convention, also share the ancestral trait of wings that cannot fold flat and remain upright when at rest.
The mayfly life cycle relies heavily on freshwater, and in Northwest Ohio, that primarily means the Maumee River and Lake Erie. Mayflies spend the first two years of their life cycle out of sight, underwater. Immature mayflies, or nymphs, are very sensitive to water conditions.
“Mayflies are quite particular, they require a lot of oxygen … they also are not very tolerant of pollution, so they need pretty clean water to be able to survive,” said Autumn Moore, the Northwest Ohio stream quality monitoring coordinator for the Ohio Scenic Rivers Program, part of the Ohio Department of Natural Resources.
If the conditions aren’t right in one year, the following summer’s mayflies won’t be as abundant.
“So, when we’re seeing them, we have the indication that they emerged from a healthy ecosystem,” Moore said.
While the mayflies may be of a nuisance, they indicate that the western basin of Lake Erie, from which they emerged, was able to support them through their two-year development.
It's a good sign for the lake, which has contended with toxic algal blooms that led to the 2014 Toledo water crisis.
Lake Erie's degradation
Following World War II, an increase in American industrialization turned the lake into a convenient dumping ground. Industrial waste and agricultural runoff from cities like Toledo, Cleveland and Buffalo flowed largely unchecked into the watershed.
At the time, it was estimated that 28,000 tons of phosphorus washed into Lake Erie each year. In the 50 years that followed, intensifying runoff and warmer waters caused eutrophication, an oversaturation of nutrients that amplified toxic algal blooms.
When the algae die, they sink to the bottom of the lake and rot. The decomposition process uses up oxygen, creating pockets of hypoxia that are uninhabitable dead zones.
These dead zones spell disaster for mayfly nymphs at the bottom of the lake.
According to Dr. Christine Mayer, a professor of ecology at the University of Toledo, these small insects are uniquely vulnerable. When water conditions worsen, mobile aquatic animals like fish can simply swim away to more oxygenated areas of the lake. Mayfly nymphs do not have that luxury.
By 1953, mayfly populations were so low that they were considered extirpated, or locally extinct, from the western basin of Lake Erie. Yet, at the time, this loss went largely unrecorded. According to Chase Fleece, an environmental historian at the Bowling Green State University Center for Great Lakes and Watershed Studies, back then, the mayflies were merely “a footnote in the story.”
The resurgence of the mayflies
Toxic conditions in Lake Erie persisted through the midcentury, and eventually, the media began to take notice.
In the wake of the 1962 publication of Rachel Carson’s “Silent Spring” came the establishment of the U.S. Environmental Protection Agency and eventually the 1972 signing of the Great Lakes Water Quality Agreement between the U.S. and Canada. GLWQA was one of the first major international agreements explicitly designed to tackle complex ecosystem pollution, and heavily degraded Lake Erie was the first priority.
Slowly but surely, the lake began to recover.
The phosphorus loading to Lake Erie saw a significant long-term decline between 1967 to 2013. This trend is especially pronounced from the 1990s onward, where the baseline stabilized at much lower levels compared to the peaks in the late 1960s and 1970s.
By the 1990s, the lake’s conditions had improved enough that mayflies started to reappear. But an entire generation had grown up without the insects, and the sudden, massive swarms caught the public off guard.
“People didn’t really know what they were when these bay hatches started happening again, they had to ask the old timers, because they were the only ones that remembered it,” Moore said.
For some, the return of mayfly swarms feels less like an environmental victory and more like something out of a Hitchcock film. But environmentalists and mayfly lovers like Moore, Fleece and Mayer want the public to know that these creatures are completely harmless.
The scientific name for mayflies, Ephemeroptera, comes from the Greek for “brief” and “wing.” Embodying “live fast, die young,” mayflies only live for 12 to 48 hours, during which their only responsibility is to mate. They don’t even need to eat, so adult mayflies aren’t equipped with mouths.
“They can’t bite you, they don’t have a stinger, they can’t harm you,” Mayer said. “All they can do is pile up and be stinky in your driveway.”
While their swarming behavior can be unsettling, it is a necessary biological adaptation for these nocturnal insects. When mayfly nymphs are ready to emerge, they are drawn toward the light from the moon and stars, guiding them up and out of the water. This phenomenon, called phototaxis, unfortunately also tricks them into gathering on streetlamps and porch lights.
When the 48-hour party is over, the mayflies start to die all at once, and piles upon piles of them carpet the pavement.
A gruesome image? Yes. A critical food source for local birds and fish? Also yes.
These insects are crucial to transferring nutrients between aquatic and terrestrial ecosystems, and in 2003, mayflies were broadly adopted as an environmental indicator species in Ohio.
“We should be grateful to the mayflies. They’re more than simply a nuisance, but instead an ally in the struggle for cleaner water,” Fleece said.
To Fleece, this year’s mayfly swarms are a highly visible reminder of the progress made for the health of the Great Lakes. But Lake Erie still faces periodic algal blooms that pose threats to humans and mayflies alike.
“Today, mayfly populations are strong. But, if the environmental history of the Great Lakes tells us anything, it’s that we cannot be complacent in the notion that these improved conditions will prevail forever,” Fleece said.