Mayflies flying along the Mississippi River in Wisconsin were numerous enough to show up on NEXRAD radar.
NEXRAD radar is usually cleaned up a huge amount before it's released--things like ground clutter and bird returns are removed.
I grabbed a processed return from the archive at UCAR here.
It's not as dramatic as the processed return shown in the news story. I wonder where they got that particular image.
Massive insect swarms are nothing new for residents of the Midwest near lakes and rivers. On occasion, I've seen swarms of gnats take up residence in the lees of buildings along the lakefront in Chicago thick enough to resemble smoke.
Another common anomaly on NEXRAD radar is the Sun rising or setting--since it's a source of radio waves--strong enough to be detectable.