Field Notes: May And Mayflies

May 12, 2020

For this month’s Field Notes, Susan Knight talks about the elegant , but short-lived mayflies common in our lakes and streams.

It is May, and time for mayflowers, mayapples and mayflies! Despite their name, mayflies usually emerge in mid-summer, but just thinking about mayflies is a welcome celebration of spring.

Mayflies are elegant insects; there is a gentle swoop of their body and delicate and graceful wings. I think of them as the Fred Astaire of the insect world. They look as light as air itself.

Mayflies are in the ancient insect order Ephemeroptera, a name suggesting something fleeting and transient. This name is appropriate, since the adults only live a short time out of the water. Mayflies typically live at least a year at the bottom of a lake or stream where they eat and grow, but the adults emerge only to fly, mate and die, often in less than a day.  

Their life cycle is quite extraordinary. Mayfly eggs are laid in the water and usually sink to the bottom. The larvae, also known as nymphs, go through many stages called instars with the instars getting progressively larger with each molt. After several instars and molts, the last nymph stage emerges as a subadult known as a subimago. Mayflies are the only insects that have this subimago lifeform – a sub-adult stage that is not sexually mature but that can fly. The subimago goes through one final molt to become a full adult. This flying adult has no way to eat or drink and has only reproduction on its mind. In keeping with the name Ephemeroptera, the adults live just minutes to hours; they have sex, the females lay eggs and then they all die.

One of the most spectacular phenomena regarding mayflies is that in a lake or stream, all the mayflies of one species emerge as adults at the same time. This can lead to such massive emergence events that the swarm can be seen on weather radar. In Wisconsin, the city of La Crosse seems to be Mayfly Central, as enormous numbers of adults emerge from the Mississippi River. Unfortunately, from 2012 to 2019, there has been a troubling 50% decline in the number of mayflies in the large emergence events.

We also have mayfly emergences around here. PJ Leisch, the University of Wisconsin entomologist who answers Wisconsinites’ questions about insects, had a wonderful experience with mayflies on a local lake. PJ wrote:

“I've seen [mayflies] at my family's place on Lake Shishebogama (just west of Minocqua) several years ago. It was early evening and my wife and I were eating dinner. I noticed a few mayflies bobbing up and down in their typical mating flight and within 15 minutes it had erupted into a full-blown mating swarm down by the dock. Of course, I ran down our hill to experience it as closely as possible and it sounded like rain as the mayflies were emerging, mating, and landing on the water around us. The local panfish seemed to love it as well. The "magic" lasted for about half an hour or so, before things started to quiet down.”

There are more than 3000 species of mayflies found in freshwater all over the world and they are especially common in streams. In North America, mayflies are principally found around the Great Lakes and Mississippi River basin, so Wisconsin is right at the center of their distribution. Some scavenge the stream bottom for bits of food while others strain particles from the water and still others are predators, feeding on other aquatic organisms. They are themselves food for a wide diversity of animals under water, including other insect larvae, snails, amphibians and birds.

But mayflies are probably best known as food for fish. Many fish feed on mayfly nymphs but some fish, especially trout, go into a feeding frenzy as the mayflies emerge into their subimago stage, often known as a “dun” to anglers. Trout anglers often use flies that mimic the subimago as they try to entice trout to bite at their lure.

Mayflies are partial to clean, pollutant-free water and their presence is often an indicator of high-quality water. Mayflies can be excellent bio-indicators. They are common in many habitats and are relatively easy to identify to some taxonomical level. They can be used to test for environmental pollution from mining drainage, agricultural pollution, and many other environmental assaults. Of the many insects tested, they were the most sensitive to neonicotinoid pesticides, those chemicals thought to be responsible for honeybee colony collapse. Though these pesticides are spread on land, they can run off into streams, harming aquatic insects such as mayflies.

While canaries are the traditional sentry warning of poor air quality, mayflies are our aquatic sentinels. Let’s hope our waters remain safe for these elegant insects.