Early last week, residents of northeast Michigan may have spotted small patches of mayflies (also known as “fish flies”) along the water or by their porch lights. While the swarm has diminished since then, more are expected to appear as summer goes on.
Mayfly season usually occurs near bodies of freshwater between June and August. During this time the insects will hatch, fly and mate within 48 hours before dying. Swarms have been emerging across the state as the weather warms up.
David Lowenstein is a consumer horticulture educator at Michigan State University with a background in entomology. He said the insects remain fairly close to water due to their short lifespans, and that hatch times and swarm sizes can vary greatly.
“You really have to be close to the stream or close to a lake (to see them),” he said. “A mile, one and a half miles can make or break between if you're experiencing a lot of fish flies or it's business as usual and you have no clue there's even fish flies active in the area.”
Lowenstein said the trigger that causes mayflies to hatch is water temperature. Once water temperatures reach 68 degrees Fahrenheit, the bugs will start to hatch and enter the adult stages of their life cycle.
According to the National Weather Service, water temperatures vary across the Great Lakes. While Lake Erie and Lake St. Clair are already seeing water temperatures as high as 71 degrees, parts of Lake Michigan and Lake Huron are still as low as 51 degrees.
Data from the National Oceanic and Atmospheric Administration (NOAA) shows that the average water temperatures in these northern lakes tends to stay below the threshold until July or August.
Despite this, smaller bodies of water in northeast counties like Alcona, Oscoda and Iosco are already close to or above the needed temperature for mayflies to hatch, and southeast Michigan is already reporting swarms.
Lowenstein said that while the insects may be annoying to some, they are actually a good sign in communities surrounded by water as they indicate good water quality. He said areas that have high oxygen concentrations are less agricultural run off tend to see higher mayfly populations, while areas polluted by agricultural run off tend to see more algae blooms.
“It's a good thing to experience fish flies,” he said. “If you don't like fish flies and you live near a lake, turn your porch lights off for the night and keep your windows closed, but beyond that it's just a part of Michigan summers and living near water. I think it's a worthwhile trade off a few days of insects that some find annoying for a sign that our waterways are doing quite okay.”
