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Mayfly hatch expected soon in Wisconsin
Kevin Skow with the National Weather Service on the possible upcoming mayfly hatch in Wisconsin.
(FOX 9) - Mayfly hatches are expected soon in the coming months along the Mississippi River, and it has the Minnesota Department of Transportation bracing for the potential impact.
MnDOT has already dimmed the lights on bridges over the Mississippi River.
Tracking mayfly hatches with radar technology
What we know:
The National Weather Service (NWS) has been monitoring mayfly hatches along the Mississippi River using radar for about 15 years. Kevin Skow, lead meteorologist at National Weather Service in La Crosse, explains that their radar is uniquely positioned to observe these hatches directly.
Speaking with FOX 9 All Day on Tuesday, Skow explained the radar’s high sensitivity allows it to pick up not just weather but lots of other things you might not expect. Skow says that the radar can be switched into a "more sensitive mode" on calmer nights when hatches typically occur.
"And the radar spins a bit slower, and it's able to scan the atmosphere for a longer amount of time and able to pick up those more sensitive signals like bugs, birds, bats," Skow said. "We can actually pick up traffic on highways with the radar. It's very sensitive."
The NWS has developed a radar archive of these hatches, providing a long-term look at how often and where the mayflies emerge. Skow describes a phenomenon called "ducting," where the radar beam gets trapped near the surface during quiet weather, sometimes picking up objects hundreds of miles away.
Swarms create safety concerns on roads and bridges
Why you should care:
Mayfly swarms are more than just a nuisance—they can create real hazards for drivers. The insects are attracted to lights along the river, coating bridges and streets and making them as slippery as a road after a snowstorm. Skow compares driving through the swarms to driving through a "mini little blizzard."
The buildup can be so severe that public works departments occasionally need to use snowplows to clear bridges. These swarms present a genuine public safety and public interest concern for communities along the river.
Mayflies spend anywhere from three months to two years developing before emerging as adults, depending on the species. Once they take flight, their lifespan is short—just a few minutes to a few days—and they usually stay close to the river. However, Skow confirms that even a light breeze can push swarms several miles inland.
Timing, unpredictability and river health
The backstory:
Hatches are largely unpredictable and can happen multiple times throughout the summer, much like cicadas. Most hatches take place between June and August, but Skow notes that only a few small hatches have been seen so far this season.
Forecasting future hatches depends on the health of the river and the success of previous generations of mayflies, which Skow says is beyond the NWS's area of expertise. The timing and intensity of hatches are influenced by a mix of environmental factors, making them difficult to predict.
While the NWS can track and archive these events, the unpredictability means that communities along the river need to stay alert during peak months.
What we don't know:
It’s still unclear when the next major mayfly hatch will occur or how intense it will be, since forecasting relies on river health and previous generations—factors outside the NWS’s expertise.