A stretch of 90‑degree heat this week could bring more than uncomfortable afternoons to Michigan’s shoreline communities: it may also speed up the arrival of mayfly swarms along Lake St. Clair, Lake Erie, and parts of Lake Huron.
Mayflies — often called fish flies — typically emerge when water temperatures reach about 68 degrees. Shallow Great Lakes basins warm quickly, and this week’s heat wave is accelerating that process.
According to NOAA’s Great Lakes Environmental Research Laboratory (GLERL), surface water temperatures across the Great Lakes are currently running 1 to 2 degrees Fahrenheit above the long‑term historical average.
Local meteorologists are calling the mayfly storm a “tsunami”, warning that surges could cover cars, buildings and patio furniture.
“These surges of Fish Fly Tsunamis will likely come on strong and suddenly,” wrote the Facebook group State of Emergency: Port Huron, which tracks local weather conditions. “These surges will blanket cars, buildings and outdoor furniture, leaving behind a post‑mating mess of eggs and carcasses piling up on surfaces.”
But while the swarms can be startling, experts say they’re a sign of ecological health.
“We are happy to see the mayflies, and fish are also happy to see the mayflies,” said Jo Latimore, director of the MSU Extension Center for Lakes and Streams. “They're a really important food source in the spring and early summer. Even though they can seem like a nuisance, they’re definitely a sign of healthy waterways.”
Mayflies typically peak around the Fourth of July in the Great Lakes region, but this year’s warm June is pushing the hatch earlier.
“Since it's been such a warm June, the emergences are going to be sooner than they normally would be because it all depends on the water temperature,” said Hallie Schulz, Visitor Services Manager for the Upper Mississippi River National Wildlife and Fish Refuge. “We'll see earlier emergences.”
While shoreline communities along Lake St. Clair and Lake Erie are likely to see the largest swarms due to their shallow, fast‑warming waters, healthy inland lakes and rivers across Michigan may also experience noticeable hatches.
“You may see these insects in big numbers around healthy waterways in Michigan,” Latimore said. “But they're harmless. They don't even have mouth parts. They can’t bite you — as adults they live only a day or two, and their only goal is to mate and lay eggs.”
The State of Emergency: Port Huron forecasters echoed that message.
“No need to run in fear or panic — this is not like Alfred Hitchcock’s The Birds,” they wrote. “Their whole life as an adult is 48 hours on land to mate.”
Even without the ability to bite, the insects can still cause problems. Large swarms can clog ventilation systems, reduce visibility for drivers and create slippery road conditions when crushed.
“When there’s a lot of them bouncing off your windshield, that can be a nuisance,” Latimore said. “Gas stations and other places with bright lights near the water will attract them in huge numbers.”
Experts recommend yellow outdoor bulbs, fans, and dimmer lighting to reduce attraction. For cleanup, lightly misting dead mayflies and shed skins can help prevent dust or slick residue.
Despite the inconvenience, mayfly season is also a seasonal milestone.
“I mean, it’s warmer, water’s getting warmer, everything’s warming up,” Schulz said. “And as soon as those mayflies start emerging, then you know summer is really here.”