Once the mayfly’s imago stage is complete, the insects mate in comingling swarms, with males using their enlarged front legs to grab and hold females. Within a few hours of mating, females release their eggs near the water and eventually drop to the surface to die. Males often die in terrestrial habitat.
Point of clarity: We often hear the term “mayfly hatch” referring to the adults’ emergence, but the actual hatching would have occurred a year or more prior when the nymphs came out of the eggs laid by a previous generation. Not trying to change decades of fishing tradition — it’s just an FYI.
Making their move
As for daily timing, University of Florida fisheries professor Dr. Mike Allen said, “A lot of these animals have adapted to being in the water column in the evening or night because predation is lower and they try to take advantage of low-light conditions.
“A lot of times, the emergence starts at dusk and goes through the night to minimize predation. But, of course, fish use what visual time they do have to take advantage of that hatch.”
It’s important to note that mayfly habitat ranges from quiet inland streams and lakes with consistent water levels to tidal rivers subject to fluctuations in depth and current. To that point, Allen said these insects will time their emergence to coincide with meteorological conditions that offer them the best chance of completing their reproductive mission.
During his 32 years in the Fisheries Division of Oklahoma’s Department of Wildlife Conservation, B.A.S.S. National Conservation Director Gene Gilliland had plenty of time to observe mayflies and their impact on fisheries. Summarizing the insect’s timetable, he points to the Farmers’ Almanac hardwired into their DNA.
“Most critters have built-in calendars — like bass spawning is dictated by water temperature, but it’s also governed very strongly by day length,” Gilliland said. “I think the life cycle of a lot of insects is kind of the same way. You tend to have these mass mayfly emergences around the same time of year, give or take a little bit because of temperatures and cold fronts that can shut things down for a short period of time.”
In normal conditions, May is generally the first month these namesake insects emerge, but subsequent hatches can follow well into summer. The difference, of course, is geography. Case in point: I witnessed a significant emergence in Lacrosse, Wis., last August during the Basspro.com Bassmaster Central Open on the upper Mississippi River. The morning found subimagos clinging to my hotel wall, and when I passed that wall in the afternoon, empty shells indicated the second molting had sent fully formed adults to their mating ritual.
Expounding here, Gilliland said natural lakes with abundant shoreline vegetation, attractive bottom composition and less flushing than deeper, current-centric reservoirs tend to pump out greater mayfly numbers. It’s like comparing a backyard garden to a commercial produce farm. Tomatoes are tomatoes, but who’s going to fill those baskets faster?
“The bulk of the mayfly’s life cycle is spent underwater, so having the right habitat is probably as critical as anything,” Gilliland said.
Impacts and indications
Whatever the timing, it’s hard to miss thousands of mayflies blacking out shoreline trees or blanketing boat-ramp restrooms, waterfront buildings and practically any solid structure; but don’t miss nature’s heads-up. When nymphs molt into subimagos, their former exoskeletons — “shucks” — typically float to the surface and gather in rafts that look like a handful of cigarette butts.
Once mating begins, dense mayfly swarms are often visible on weather radar. Such masses have been known to temporarily shut down bridges and roadways, as the vision-blurring aerial volume, plus road-slicking layers of dead flies, creates unsafe driving conditions.
As you can imagine, a truckload of dead bugs is equally annoying to the eyes and nose, so it’s not uncommon for municipalities to call in the heavy equipment (snowplows, front-end loaders) to remove the pungent mounds. Not fun for those tasked with the cleanup, but mayfly abundance bodes well for those tasked with fisheries management.