The annual onslaught of mayflies began this week along the Mississippi River in the La Crosse region.
The National Weather Service caught a swarm of them on radar, as they drifted north in the same pattern as the wind.
The radar loop showed yellow spots directly over the river as the flies emerged from the water.
The swarm morphed into a green band as it drifted north, and blue dots as the flies dispersed.
A second, smaller swarm recorded Thursday night appeared as a green band that exploded like fireworks into blue dots.
The radar captures energy reflecting off the bugs, National Weather Service spokesman Dan Baumgardt said.