For the residents of the western Wisconsin, last Sunday started off a normal day like any other. For millions of adult mayflies, however, it was a time of frenzied mating after months spent underwater. Employees at the National Weather Service office in La Crosse, Wisconsin were stunned over the weekend when the insects swarmed across nearby cities and towns. The mass of mayflies was so large that the insects could be seen on radar.

“The radar detected the flies about 845 pm, emanating from the [Mississippi] river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). With a general south-to-north wind flow above the surface, the mayflies quickly moved north once in the air,” a release from the office stated.

By late evening, mayflies were covering cars, buildings, and even a few unlucky pedestrians in La Crosse, La Crescent, and Stoddard. According to the Star Tribune, the insects were even responsible for a three-vehicle accident near Red Wing that left one person hospitalized. There were so many mayflies on the roads that motorists reported slick conditions and low visibility. In 2012, the last time a major emergence occurred, snowplows were called out to clear the roads.

“What made this unique was the massive number of insects that were involved,” said NWS meteorologist Zack Taylor. “The signature on the radar was pretty impressive.”

Mayflies spend the majority of their lives in freshwater, only emerging as adults for a few days. The insects use this time to mate and lay eggs, after which they quickly die. The NWS found that the species involved in this particular event is the large black and brown bilineata mayflies.

You can see a video of the swarm below:

Image courtesy National Weather Service Weather Forecast Office

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