Huge mayfly emergence captured on radar

July 22, 2014 • 10:46 am

From the Lacrosse, Wisconsin office of the National Weather Service, via reader Gregory, we see Doppler radar capturing a massive mayfly emergence on June 23 (a description of how Doppler radar works is here). I believe the species is the giant mayfly, Hexagenia limbata, though I may be wrong.

On Saturday evening, June 23 2012, a massive mayfly emergence occurred along the Mississippi River beginning just after 9 pm. By late evening, mayflies were swarming in La Crosse, La Crescent, and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season, this particular event was one of the best seen on radar yet. In the radar time lapse loop from 9 pm to just after 1030 pm (below), the yellows and oranges indicate a large magnitude of airborne mayflies.


 Also very evident was the northward track to the mayfly radar ‘echo’. This movement was due to the south wind direction over the area at emergence time. The radar would indicate the bugs were carried north, off of the river, into Blair and Taylor Wisconsin. The radar beam over these locations are detecting mayflies at over 3000 feet above the ground! Their existence was confirmed on the ground north into Trempealeau county near Galesville.


There is more information at the site.

Mayflies are in the order Ephemeroptera, meaning, in effect “short-lived winged things.” (They aren’t really flies, which are in the order Diptera.)  The Freshwater Blog details their life cycle; here’s a short extract:

A mayfly’s life cycle starts with the males forming a swarm above the water and the females flying into the swarm to mate.  The male grabs a passing female with its elongated front legs and the pair mate in flight. After copulation, the male releases the female, which then descends to the surface of the water where she lays her eggs. Once mated she will fall, spent, onto the water surface to lie motionless, with her wings flat on the surface, where fish pick them off at their leisure. The male fly rarely returns to the water but instead he goes off to die on the nearby land.

Because they live only a day, they cannot feed, as their mouthparts are vestigial (explain that, creationists!). In the species Dolania americana, females live less than five minutes as adults, the shortest life span of any adult insect. What a life—if you can call it that!

Here are some pictures taken the next day in Wisconsin, also from the site:


More from the Freshwater Blog:

Some species exhibit great synchronicity in their hatching.  The North American species Hexagenia limbatahatches in huge numbers from the Mississippi every year.  The total number of mayflies in this hatch are estimated to be around 18 trillion – more than 3,000 times the number of people on earth.  The newly emerged insects are attracted to lights in riverside towns and villages and the local authorities deploy snow clearing vehicle to remove their rotting corpses.  Ironically, what is seen as a nuisance in America is seen as a gift in Africa.  Locals around Lake Victoria gather adults of the mayfly Povilla adusta together with Chironomid midges to make a type of patty called ‘Kungu’.  This protein rich food stuff is an important part of their diet.

IMG_5745 IMG_0719 IMG_5768

Station KARE from Minneapolis/St. Paul reports:

The La Crosse, Wis. office of the National Weather Service (NWS) says this year’s mayfly hatch on the Mississippi River was so prolific that it created a bow echo on radar, similar to one that would be made by a significant rain storm. A NWS employee went out after his shift and captured some amazing images of the short-lived pests covering street lights, gas pumps, buildings, stairs — nearly anything in their path.

The mayfly hatch was a problem up and down the Mississippi. Police in the town of Trenton say a large hatch of mayflies may have triggered a three-vehicle crash on a Wisconsin road.

and adds a picture of the insects at a gas station:

Mayflies at a Trempealeau, Wis. gas station (Photo: National Weather Service/La Crosse, Wis.)




44 thoughts on “Huge mayfly emergence captured on radar

  1. “Awesome” is an overused word but it certainly applies here! Truly amazing. And forget about Alfred Hitchcock’s birds. Yikes!

    1. Agreed. “Awesome” is the first word that came to my mind too, and this time it’s entirely appropriate.

  2. One of the consequences of a warming globe is a lot more insects. The planet we are leaving our grandchildren is going to be hot, muggy and buggy.

    1. Sounds good for the frogs and birds. I would like some photos of near-gut-exploded frogs who cannot handle any more fly flesh in their GI tracts.

  3. I always find mayflies to be among the great beauties of the insect kingdom. There is something about their form, when at rest, that just seems like an art decoration to me.

  4. Thanks mostly to spending the last few years at this website I have come to find all the little crawlies very cute and fascinating. I actually enjoy seeing spiders nest in the corners of my room and moths landing on my screen. I want to visit there now. Does this happen every year?

  5. Wow that radar was really cool! Where I am, there are a bunch that come out by a major river. They die off & it actually smells bad because of it – not in the standard rotting smell but a different yucky smell.

  6. I was on the Mississippi River in this area ten days after this hatching event. Didn’t see a single Mayfly. But I did see the river in flood which was rather impressive.

      1. LOL. 🙂

        I’m very primitive in that respect. If it’s dead, it’s a potential meal waiting to happen.

        It’s the same with expiration dates…I rely on my nose instead. Sometimes it’s a swing and a miss, though…

        1. My wife can’t distinguish between “display until”, “best before” and “use by” dates. Partly because she refuses to put her glasses on except when she’s in the office.
          I use my nose too.

    1. Rainfall irregularity and rising heat norms already impact animal protein supply, and overfishing/ocean acidification will shortly remove that source. Consumption of insects from time to time in all areas on the globe to meet human population protein demand — by all but the 1% and their favored/necessary keep-us-safe-and-in-control minions, at any rate (even if that global population stops increasing, or in fact decreases significantly due to unpleasant climate change outcomes) — is as sure a thing as betting on the house at the casino.

      1. There must be a huge potential market for bug candy in the west.

        A couple of generations of getting used to it and people wouldn’t think twice about munching down some protein rich sweets during the day.

        And if we can eat for dessert we sure as hell can eat it as a main dish.

  7. 18 TRILLION? That’s unfathomable to me but perhaps true given the size of the radar signature. I’m always amazed at the sheer number of genetic experiments being conducted in parallel in such events– think also of the cloud of cottonwood seeds floating on the breeze or even dandelion seeds in the back yard. Let’s say the spontaneous mutation rate is on the order of 1 x 10 -9/bp and assume an arbitrary (but reasonable) genome size of 500 megabases (500 x 10 6 bp). That works out to 500 x 10 -3 or 0.5 mutations/genome; now multiply that by 18 x 10 12 independent experimental trials and you get 9 trillion new genomes, whose relative fitness is tested by the environment in very short order.

    I have no doubt that readers at this site will correct my back-of-the envelope calculations and I welcome any such corrections. In any case, I think it is our common inability to fathom such vast numbers that leads many to doubt, at least in part, the likelihood of random genetic processes leading to the magnificent evolutionary outcomes we see in the natural world that surrounds us.

    1. Since they reproduce sexually, presumably all 18 trillion of them are unique new genomes, regardless of the mutation rate.

  8. quote in an insect lab when I was in graduate school: “Ephemeroptera: live fast; die young”. There might have been a sketch of a mayfly wearing a leather jacket and smoking a cigarette…

  9. In the species Dolania americana, females live less than five minutes as adults, the shortest life span of any adult insect.

    Perhaps most mind-boggling is the selection pressure that first could possibly have caused this in the first place, and then that must result. When you’ve only got a five-minute window to reproduce, there’s absolutely no margin for error.


    1. Well, remember, that five minutes isn’t necessarily a given, but can evolve, too. For examples, it’s possible that females could leave the water and therefore live longer. But I presume it’s more advantageous to stay near where you hatch, which means that when you’ve used up your resources for mating, you fall into the water and are eaten immediately. But I can imagine scenarios where it would be advantageous to stay alive for, say, an hour.

      1. That’s what I mean. It’s mind-boggling to think of the selection pressures that would cause that five-minute lifespan (in that particular species) to evolve. Considering what sorts of constraints it puts on successful reproduction, those pressures have to be monumental. If you’ve only got that window of a few minutes to do everything…well, now you’ve got to coordinate your lifespan with potential mates, but that also means interspecific competition is furious.

        There’s clearly some seriously powerful factors that give advantage to the short-lived…but I’m at a loss to think of what it would be. Some sort of metabolic efficiency, presumably…but isn’t it cutting things awfully close?


        1. I think perhaps you have it backwards, Ben. Adults all hatch out simultaneously, I’m guessing, for the same reason that fish school and bats all leave the cave together: safety in numbers.

          Once you have a giant swarm of adults emerging from the water hot to trot, getting the job done in five minutes is the easy part. And since there’s no selective advantage in hanging around afterward, the pressure now is on conserving resources by letting mouthparts and digestive tracts atrophy.

          So the five-minute window isn’t a constraint to be worked around; it’s just a consequence of the fact that everybody’s already in the right place at the right time, and there’s no reason not to get on with it.

          1. I agree. And “getting on with it” actually does not impress the honeybee male, who has to get on in less than a second by exploding his genitals into the female, then detaching from his genitals, then die.

            “No margin for error” there, particularly the detachment part, if some of you care to try it.

            Gives a new meaning to premature.

        2. How about this?
          Mayfly flight is weak and easily blown off course. After emerging, those females who mated and laid their eggs sooner than others would have more chance of their eggs hatching in water rather than on land.

          The dying bit:
          The weaker flyers would drop towards the water more quickly – less chance of drifting away before laying.

          Perhaps the short lifespan is more due to their drowning or being eaten than to their physiology. Interesting to see how long short-lifespan mayflies lived if ‘rescued’ before they touched the water after mating.

  10. You ought to see them at night around street lights. Looks like a real heavy snowfall. Was told that the old bridge at Burlington, IA over the Mississippi was an open metal gridwork to cut down on slipperiness during hatches. The new bridge is concrete so DK.

  11. Awesome post.
    I have seen the swarming invertebrates at Lake Victoria in Africa via a BBC natural world programme.
    Each swarm, and there were multiple swarms at once with trillions of animals, they looked like a fine rain suspended above the lake. What was surprising was the amount of life these swarms supported, equatic and terrestrial.
    Humbled by nature once again.

  12. Wow, a radar signature like that … I hope there aren’t any mad survivalist militia in the area who have got BUK surface-to-air missiles.

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