If it's July, it's time for the annual mayfly hatch. I was thrilled to see this great article about mayflies in the July 2025 Frontenac (MN) State Park Association newsletter. I am pleased to welcome Pamela Miller, an FSPA volunteer naturalist, back as a guest contributor to this column. Pamela is a Minnesota Master Naturalist volunteer and member of the very active Frontenac State Park Association (FSPA) - among many talents. She is also a wickedly good fungi-finder (I am in awe of her skills ), a fabulous writer and the editor of the FSPA's wonderful monthly newsletters. This timely post, reprinted with permission from the newsletter, will tell you about all things Mayfly.
They descend on us after sudden, massive hatches – delicate insects that thicken the air and trees near Lake Pepin, black out the lights at Kwik Trips, slick up the bridges over the Mississippi, mess up your car, and get tangled in your window screens and your fancy hairdo.
Eek! Or, perhaps … hooray!
Mayflies, whose lyrical scientific name, Ephemeroptera or Ephemerida, reflects the fact that they don’t live long, are vital to the aquatic food web and a welcome indication of relatively clean water. (We’re not sure why they’re called mayflies, though, since we see them mostly in July and August.)
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| A July mayfly swarm a year or so ago. There goes your hairdo! Image - Pamela Miller |
- Reproduction: It’s sex, then death. Mating takes place in flight, as females fly through a swarm of males. They then lay eggs in the water and die.
- What mayflies eat: Algae, fungi and decaying material (more reason to appreciate them).
- What eats mayflies: Apparently, mayflies are delicious, though we’ll resist the mischievous urge to include mayfly recipes here. Amphibians, reptiles, fish, birds and mammals eat the nymphs. Birds and bats nab the fluttering adults. And when the mayflies die in huge swarms atop lakes and rivers, they present a bird and fish buffet.
- Habitat: Mayflies need fresh water with healthy oxygen levels. This is why we’re happy to see them – they reflect the fact that the water is clean. Back in the 1960s and 1970s, when your newsletter editor was growing up in Old Frontenac, there weren’t many mayflies.
"The mayfly is sensitive to chemical pollutants, increases in suspended solids (erosion) and decreased dissolved oxygen levels. The primary cause of the collapse of mayfly populations during the late 1960s, 1970s and early 1980s was sewage. Little or no treatment of sewage was occurring during this time. We were literally flushing our toilets into the river. There were reports from this period of ‘cakes of fecal matter’ floating on rivers.
"While this sounds bad enough, the problem for the mayflies wasn’t the poop itself, but what happened next. As bacteria began to break down this mass of organic sludge, they used up tremendous amounts of dissolved oxygen. They used up so much that mayflies, stoneflies, caddisflies and even fish disappeared from the Mississippi River from the Twin Cities all the way down to Lake Pepin.
"Today, because of modern sewage treatment facilities and regulations on the disposal of toxic chemicals, mayflies have returned to most of our waterways. New challenges revolve around chemicals that come from our homes and yards. Medications and other pharmaceuticals that leave our bodies go right through the sewage plants untreated and are affecting aquatic life that eat and breathe these compounds."
So, when you see mayflies, salute rather than swat them! And don’t worry – they don’t bite!
If you don't subscribe to the FSPA newsletter, I encourage you to do so. You'll learn a ton and find out first about amazing free programs at Frontenac (MN) State Park. You can read an additional column by Pamela on the blog here: Naturalist's Corner: The Glory Days of Bucks
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