I found this post by Paul Huttner of Minnesota Public Radio News. He wrote;
You know it's getting bad when mayflies are visible on Doppler radar.
Imagine what the forecasters on duty thought when they peered at the Doppler screen last Saturday evening at the La Crosse NWS. Shades of bright pink showed up over the Mississippi River. There were no storms in the area, so what were the radar returns?
Mayflies. Yup. Mayflies.
As the mayflies hatched on the Mississippi, the dense swarms reflected the radar beam back toward the Doppler receiver. The radar was in the very sensitive "clear air mode" which results in greater sensitivity and shows things like dust, birds, and well...mayflies.