Come May each year, a special insect emerges near some of our lakes. And while you may be familiar with swarms of these summer insects, called mayflies, how much do you know about where they thrive and how long they live? Mayfly life cycles are fascinating to many—but the adult stage is fleeting. We’ll delve into when, where, and for how long you can expect to see adult mayflies in the summer.


Why are mayflies unique?

The annual mayfly hatch doesn’t just indicate that summer’s approaching—according to Michigan State University (“MSU”) Extension, mayflies confirm the quality of the water in which they hatch.

“[Mayflies] are rarely found in degraded bodies of water because their external gills in the nymph stage are very vulnerable to silting and pollution. Mayflies are, therefore, used as an indicator species when testing for environmental quality,” the Michigan State University research found.

Mayflies may be seen in large groups due to the synchronous emergence. Also known as “fishflies,” their swarms - which are common near clean bodies of water such as lakes and rivers - are so large they can sometimes be seen on Doppler radar. In the summer of 2019, Ohio residents saw swarms of mayflies everywhere due to their proximity to Lake Erie.

What is the mayfly life cycle?

Per the MSU Extension , the mayfly life cycle consists of four stages: egg, naiads, subimago, and adult.
  1. Egg: Depending on the species of mayfly, the female can produce anywhere from 4,000 to 8,000 eggs on the water’s surface. The eggs settle to the bottom (or, they may become attached to an object such as a stick or plant that sinks to the bottom of the river bed). The eggs will remain here from a day to even weeks until they hatch into naiads.
  2. Naiad: The naiad stage can range anywhere from two weeks to two years, however, most naiads develop within 1-2 years. Naiads will then rise to the surface of the water and molt into the subimago form.
  3. Subimago: The subimago stage is akin to an adolescent stage of life for the mayfly. After emerging from the naiad stage, the subimago flies from the water’s surface towards land. This stage can last from 24 to 48 hours. The subimago then sheds its skin a final time and the adult mayfly emerges. This is unique in the insect world, as all other flying insects change from wingless immatures to winged adults. Mayflies are the only insect with a subimago molt.
  4. Adult: The adult lifespan typically lasts from one to two days. This stage does not feed, and most species die shortly after mating and laying eggs.

When is mayfly season and what areas do they frequent?

The emergence of adult mayflies can vary based on location. According to Michigan State University Extension, emergence in Southern Michigan occurs in June or July.

Despite their name, adult mayflies frequent the skies near clean water in late spring or early summer. So, while you may wish to steer clear of insects while enjoying the summer sun, keep in mind that seeing mayflies near a body of water is a sign of healthy water