Shortly after 9 p.m. on Monday night, another Mayfly hatch, or "emergence", was seen on radar here at the NWS La Crosse. This was a particularly large hatch with the radar detecting "echo" above 20 dBZ reflectivity for the width of the river valley. Mayflies were detected from near Winona, south along the Mississippi River through Guttenberg, Iowa. The radar reflectivity values will increase for more dense concentrations of raindrops, birds, or any airborne object. In this case, the Mayfly. With time the Mayflies will fly and disperse - seen as lowering radar reflectivity values at the end of the loop.
Here is the radar movie from 9-10 p.m. Monday Night (click to enlarge):