A massive mayfly hatch along the Mississippi River was big enough to be seen on the National Weather Service’s Doppler radar in La Crosse, Wis., on Saturday.
About 9:13 p.m., according to an image posted by the weather service, the newly flying bugs could be seen in the pink, purple and white bands of the radar image along the Mississippi as the insects flew from the river and nearby water areas and were carried along on south-to-southeast winds for about 10 to 20 minutes.
Dan Baumgardt, a meteorologist with the weather service in La Crosse, said they see about three to four of these emergence events around the river each year, though the warm spring contributed to the earliest one he said he’s seen.
“Usually, we see these more in mid-June to late July,” he said, adding that an early jump in water temperature likely helped the timing.
Mayflies drop their eggs on top of fresh water. The eggs then sink to the muddy river bottom, floating to the top as pods once the flies are ready to emerge and live a very short life, from a few minutes to only a day or so. Because they’re very sensitive to water pollution, heavy populations are seen as a good indicator of the health of the body of water from which they hatch.
So how did the weather service know it was looking at mayflies and not some strange rain event?
“Typically, we can tell they’re biological targets when we know there is no other weather present,” Baumgardt said, adding that other possibilities include smoke and dust, which they can often rule out by talking to local observers such as law enforcement officers.
Another clue Baumgardt cited: Large groups of insects tend to be carried by prevailing winds, whereas flocks of birds can fly against the wind.
The squishy, stinky carcasses of mayflies, which are attracted to light during their short lives that include only mating followed by dying, can cause messes for communities near water bodies. The city of Red Wing, Minn., has occasionally had to use snowplows to clear roads of inches-deep dead mayflies.