They were indeed mayflies. They showed up on the weather radar as the equivalent of a light to moderate rain storm, according to the National Weather Service. As the service described it, “the Mississippi River produced a massive radar echo as mayflies emerged from the water and became airborne. The mayflies were detectable on radar around 845 pm and reports in the towns and cities began rolling in of the swarming and piles of mayflies. … The radar detected the flies about 845 pm, emanating from the river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). ”

“By late evening,” says the weather service, “mayflies were swarming in La Crosse, La Crescent, Stoddard and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season depending on the species, this particular emergence was that of the larger black/brown Bilineata species. The radar loop [above] shows the reflected radar energy (reflectivity) from 8:35 pm to just after midnight. The higher the values (greens to yellows) indicate greater concentrations of flies. Note how the swarm is carried northward over time.”

AD

AD

Here’s the deal on them from the Fish and Wildlife Service:

As nymphs, these aquatic insects proceed through one or two years of larval development as filter feeders, consuming decaying organic matter at the river bottom. In summer, large numbers of nymphs synchronously emerge from the water at dusk and take flight as sub-adults. Within 36 hours of emergence, the sub-adults metamorphose into adults that subsequently swarm in the air to mate before returning to the water surface to lay their eggs and die. Some of these emergence events are so large and widespread that swarms can be detected by Doppler weather radar.

They’re supposed to be the bearer of good tidings ecologically. In some years, they’ve failed to breed because of pollution, so their re-emergence is supposed to be a sign of environmental health.

Unfortunately, they’re attracted to lights, so they often head for lighted highways after they hatch. Millions of them get run over or squashed, leaving behind a green slime that’s so slippery it can cause accidents. Only one accident was reported last week.

According to the Minneapolis Star-Tribune:

Sunday’s accident happened about 10:25 p.m., when a northbound car driven by Theresa L. Hunt, 24, of Ellsworth, Wis., lost control on the slickened roadway as the blizzard-like flurry of insects cut visibility, according to the Pierce County Sheriff’s Office. Hunt’s car hit another vehicle heading south driven by Scott Pauly, 52, of Rochester, and then a second vehicle, a van driven by Tanya Lapointe, 35, of Baldwin, Wis. Hunt was slightly injured, and Cynthia Pauly, 51, a passenger in one of the struck vehicles, was taken to Mayo Hospital in Red Wing for treatment of undetermined injuries.

A second, smaller swarm recorded Thursday night starts as a green band before exploding like fireworks into blue dots. The radar loop below covers the time between about 8:45 pm and 11:10 pm.