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How to Survive a Massive Mayfly Swarm

By Leslie Mertz

Near Lake St. Clair in the southeastern corner of Michigan’s Lower Peninsula, your summer starts off with a crunch. You pick up your foot and see what’s left of an inch-long burrowing mayfly (Hexagenia limbata) on the sidewalk.

This is the beginning of a short-lived but massive mayfly invasion, when millions and millions of the fluttering insects rise up from the lake and into the air in thick clouds, and then land on anything and everything from houses, stores, and cars to arms, legs, hair, and faces. Within a matter of days, the brief en masse mating free-for-all is over for the insects, and other than the deep piles of their carcasses on sidewalks, lawns, and roads, the onslaught is but a memory. If you’ve never experienced a mayfly swarm for yourself, take a look at this National Geographic video:

How do the people live through it? Here are a dozen tips for surviving the swarm.

1. Either revel in the spectacle of nature … or run. On one fine June night about three years ago, Nick Di Cresce, a naturalist at Lake St. Clair Metropark, and his wife decided to take a walk to a lakeside park. “About 10 feet above the water, there were literally so many mayflies you couldn’t see clouds, sky, stars, nothing. It was just completely black. I’ve never seen it that thick,” he recalls. “My wife took off, but I was out there thinking, man, this is great! I was walking right into them.”

2. Know your terminology. On Lake St. Clair, mayflies are known as fishflies. OK, it is confusing because there is a whole different group of insects that are formally called fishflies. Around Lake St. Clair, however, the oh-so-pungent fishy smell of combined living and dead mayflies trumps proper entomological naming. This leads to tip number 3.

Mayflies have a double life. For most of it, they live inconspicuously on the water bottom. Then, in a spectacular display, they get wings, fly out of the water in what can be a colossal swarm (shown here on the Upper Mississippi River), molt one last time, mate, lay eggs, and die in a matter of days. Photo credits: Eric Cummings, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, and NOAA Great Lakes Environmental Research Lab (inset).

3. Rub a little Vicks VapoRub under your nose. This will help tame the insects’ captivating aroma, which probably goes back to their aquatic origin. Although most people aren’t aware of it, mayflies actually spend 99 percent of their lives as aquatic animals, wandering on lake and stream bottoms and living on algae and decomposing matter. “They’re eating the same kinds of things that many of the fish are eating, so that may explain their smell,” said Jo Latimore, an aquatic biologist who has spent more than her fair share of time in mayfly swarms, and is now a lake, stream, and watershed management outreach specialist at Michigan State University.

4. Skip the bare feet and flip flops. During the big swarm, it’s impossible to avoid stepping on mayflies. So if you don’t want to pick insect wings or guts from your heels or between your toes, wear full-coverage footwear.


During their on-land mating extravaganza, mayflies coat houses, cars (shown here in a photo taken in Ontario), buildings and roads. Depending on your point of view, the swarm is an awesome spectacle or a great time to head out of town. Photo by Adolson13 / CC BY.

5. Dancing is optional. If you’re a tap aficionado, close your eyes and pretend you’re at a recital instead of walking down the sidewalk. “Just like any insect, they have exoskeletons that protect their bodies, so they have that satisfying crunch when you step on them or run over them,” Latimore said.

6. Use windshield wipers sparingly. Smeared mayflies are just nasty.

7. Leave the motorcycle at home or wear a full-face helmet. Nuff said.

8. Go light on the brakes. Mayflies are attracted to light, so their numbers are especially high near streetlights. That means they accumulate on the roads at intersections, and combined with the squishing afforded by passing vehicles, they can form greasy slicks. Picture smears of gray lard dotted with little wings. Or perhaps it’s better not to picture that. “When people try to stop, there are so many mayflies on the road that cars have slid into one another,” Di Cresce said. Try explaining that fender bender to your insurance company.


An adult mayfly’s whole purpose is reproduction. They don’t have functional mouthparts, so they waste not even a single a moment looking for food. Talk about a one-track mind! A few members of this mayfly hatch, taken in May 2015 on the shore of Eagle Lake in Cass County, Michigan, cling to the sides of a building. Photo by Lori Mroczek.

9. Remember, it’s a love thing. Well, not love, it’s a sex thing.They emerge from the water at the same time with the sole purpose of finding a mate and laying some eggs,” according to Latimore. “It’s all about reproduction.” To get yourself in the mood of the season, perhaps plan a date at the Bay-Rama Fishfly Festival. That’s right, fishflies get their own festival in this part of the world.

10. Get ready for a feast. And no, that’s not a feast of mayflies (although in one of the all-too-rare instances of sweet justice, they have been known to fly into the mouths of overly talkative people). The mayfly invasion — or “hatch” as it is often called — is one of the best times to catch many sport fish. The fish enter a feeding frenzy during the hatch smorgasbord, snapping up the fishflies as well as anglers’ lures. So when the local weather radar picks up the cloud of mayflies, get your fishing pole and your frying pan ready.


When mayfly swarms are large, they show up on radar. This one in Wisconsin was in the month of July. Mayfly is a misnomer: They’re really not flies at all, and depending on the species and other factors, hatches often don’t occur in May. Regional colloquialisms include fishflies, shadflies, and a slew of other names that aren’t fit to print. Photo by NOAA.

11. Adopt the right mindset. Think of it this way. The hatch is a remarkable demonstration of nature in action. For instance, you may be able to watch a mayfly molt right before your eyes. “If you’re lucky, one will land on your arm, its back will split open, it will emerge from the old exoskeleton, and then it will fly away leaving its little skin behind,” Latimore said. Yes, lucky you.

12. Repeat this mantra: “Mayflies are important to the ecosystem.” “I know everyone says they’re terrible, but here’s the thing: They are an indicator of clean freshwater,” Di Cresce asserted. “And not only that, they’re an enormous food source for several different animals here. Birds, for instance, are nesting and they’ve got young at that time, so mayflies are a huge input of food for them at a critical time in the development of their young. We have frogs, toads, and other amphibians that eat them, fish eat them, so mayflies are what’s on the menu. The minute we don’t see mayflies? That’s the time to worry.”

Besides, Latimore remarked, the whole thing is over very quickly. “The hatch only lasts a couple of days. After that, all that’s left are the bodies to be swept away and cleaned up.”

Yes, it’s just another summer at the lake.

Leslie Mertz

Leslie Mertz

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, She resides in northern Michigan.


  1. I have lived in Wisconsin and experienced the Mayflies. Also along Lake Winnebago you have the Lake Fly swarms also. Both are nasty to drive in or be outside when they swarm. It is an awesome sight to see and experience for the first time, not so once you have experienced it. LOL

  2. I just heard from the Bay-Rama Fishfly Festival, which is held in New Baltimore, Michigan, on the shores of Lake St. Clair’s Anchor Bay. Here’s what Lisa Edwards of the festival committee had to say: “In the late ’70s during the Bay-Rama festival, the fishflies were so bad that some of the rides had to be closed down. The thick layer of fishflies was causing the gears on some of the rides to be slippery. So, (over) the next few years the committee tried to move the dates of the festival to avoid fishfly season. But this was tough, because there were other area festivals to compete with. The committee finally decided to just embrace the fishfly and brand it into our festival. We have done so ever since!” Thank you, Lisa!

    • I ate one once. I could tell you that it was disgusting, but it wasn’t. I actually liked it, but then some irrational fear of potentially contracting some mysterious illness materialized into my head from who knows where. So I didn’t keep snacking.

  3. Lived through them on Lake Erie all 60+ years of my life.
    I’m about 20 miles west of Cleveland.
    Strange, yours looks different from the ones we have here?

  4. Very informative. Seeing a hatch here at Hilton Head Island we think so we googled your article to read about them. Thanks!

  5. Fish fly season here in New Baltimore is about 6 weeks because
    there are several hatchings in one season.

  6. I live on Lake Huron and we have the fishfly invasion every year, usually by now they are gone. It’s the middle of August and they are still hatching.

    • They are called mayflies in Michigan, its only in Canada and around St. Clair, that they are usually called fish flies. mayflies, are food for fish, and birds and other animals. They eat them, in Africa, raw or cooked. Thus they are not poisonous.

  7. There used to be a Dairy Queen at 9 Mile and Jefferson. During fishfly season that place was just covered in fishflies making for a NOT very appetizing place to eat! Crunch, crunch, crunch as you entered- and they couldn’t keep them out of course. Street sweepers would come through the area all the time to try and prevent accidents, cars sliding on those things. It was just gross.

  8. Thanks for the info. I’m new to Michigan, and am living on Lake St Clair (actually Anchor Bay) so they are EVERYWHERE!!! They look like fairies, with their delicate wings. However, there are so many now, it’s difficult to get into my car or my apartment without at least a few “hitchhikers!”

  9. I’m from an area in toledo,ohio called point place. It sits right on the southwestern corner of the lake Erie shoreline.I’m 66 and can remember these damn things back to when I was 5. Only here we call them junebugs ’cause they don’t come in

  10. This my first season of the”swarm”. I just moved to Illinois, right on the Mississippi. We were driving to Walmart along River road and kind of freaked out when driving through the cloud of them. Then I looked up at the trees and couldn’t believe the thickness of the mayfly gathering. An all out bug orgy! Then I read this article and I have gained a new respect for the critter. I think I am going fishing tonight!

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