Mayflies: Friend or Foe?

Over the last few days it’s become abundantly apparent that our 2025 mayfly season is underway. Mayflies (Ephemeroptera), also known as Shadflies or fishflies, are widely considered a seasonal annoyance by homeowners because their synchronous emergence can be overwhelming. Drawn to light at night (and virtually every surface, from window screens to deck furniture) the sheer abundance of mayflies during the summer can be a bit startling. But is it more than an annoyance? Are mayflies friend or foe? Let’s take a look, starting with one of my favorite photographs of Pam with a proud mayfly hitchhiking her cap.

Two cool tidbits as we dive a bit deeper into Ephemeroptera: they’re cousins to dragonflies, and their lifespans are measured in hours! Both mayflies and dragonflies are in the order Palaeoptera, effectively making them modern bridges to the dinosaurs. This ancient group of “primitive” insects are flying time capsules, bridging millions of years, and that’s sort of wild considering these critters live only the most ephemeral existence. Maybe that’s the key to longevity? Born to breed… and die?!

The Ephemeroptera are a small Order of aquatic insects, often referred to as Mayflies. The name comes from a Greek word meaning ‘living a day’ due to the adults having very short lives, usually between one and four days only. (Source: Amateur Entomologists’ Society (AES))

Short lived is practically the definition of the mayfly. And, strangely enough, that’s really the root of the problem. Not so much for them, but for us.

The life cycle of a mayfly starts out as a larva that resides on river or lake bottoms.  After 3 months to 2 years, depending on the species, they emerge as winged adults and fly in swarms to mate. Once they emerge, they only live for a few minutes to a few days (again, species dependent).

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The adults are often attracted by lights and can fly several miles inland, commonly creating a slippery mess as they pile up and die on the pavement below the lights. At times they have been so thick on bridges that they had to be cleared with snowplows.

Mayflies are sensitive to gross organic pollution and their presence is good news ecologically because it means that pollution such as sewage is not present in large amounts.  (Source: National Weather Service)

Now this is the upside of the mass emergence of these slightly annoying insects. Their abundance, though untidy for a few days, is positive proof that pollution in our waterways is in pretty good shape. Hurrah for Lake Champlain! And, after all, they live but briefly, and then the fertilize our lawns and plants. Not so bad either.

Unless cleaning up after a mayfly “hatch”. And, yes, that’s a drag.

Two cheers tonight for the terrific team restoring transparency to Rosslyn’s windows…

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Lacy spiderwebs are gone, mayfly skeletons are dispatched, and spot free visibility is restored! Thank you. (Source: Window Washing)

I posted that last summer 2024. The current round of tidying up after the mayfly emergence peaks and then crests still lies ahead. Joy!

But I haven’t really narrowed down the question of  whether or not mayflies are friend or foe. Let’s consider a few criteria.

Do Mayflies Bite?

No, adult mayflies do not bite humans. During their brief adult stage, they don’t even have functional mouthparts or digestive systems. They live for a very short time as adults (hours or days) and their one and only priority is reproduction. Mayfly nymphs, on the other hand, are aquatic and feed on algae, decaying plant matter (detritus), and other small particles found in the water.

Does Anything Eat Mayflies?

Yes, fish love to eat mayflies. Ergo fishermen (and women) love mayflies. And, as it turns out, some people simply skip the fish step in the food chain and eat the mayflies themselves!

Mayflies are important to fishing communities around the world. Some have hypothesized that they contributed to the origins of fly fishing. A lot of people use bait that resembles adult mayflies because fish can’t resist them.

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As many as 10 countries are known to have communities that directly feed on mayflies. For instance, in East Africa, the mayflies of Lake Victoria are collected from along the shoreline to be used as food. They’re usually sun-dried and ground into flour that’s used to make cakes and bread. (Source: WebMD)

Friend or Foe?

So what’s the verdict? Perhaps not 100% consensus on whether the mayfly is our friend or foe simply because they are considered to be a major nuisance necessitating significant cleanup. But, mayflies are generally considered friendly parts of our ecosystem. Their role in the environment is generally beneficial as they serve an important food source, contribute to healthy aquatic ecosystems, serve as an effective litmus test for the wellbeing of waterways, and do not cause any meaningful harm. Besides, did I mention that they’re basically living dinosaurs?!?!