Radar captures mayfly swarm as thousands of bugs emerge from Mississippi River to mate

This July 20, 2014 photo provided by the National Weather Service in LaCrosse, Wis., shows a massive swarm of mayflies captured on radar as they emerge from the Mississippi River and drift north with the wind. A NOAA spokesman says they know the images show insects when the swarm appears over the river with no warning, but they don’t know what kind of bug it is until people start calling _ mayflies in this case. (AP Photo/National Weather Service) (The Associated Press)

MILWAUKEE – Mayflies have begun hatching in the Mississippi River, emerging from the water in swarms that show up on radar like thunderstorms, coat roads and other surfaces and leave behind a slimy mess.

The National Weather Service in La Crosse captured a massive swarm on radar Sunday night as flies emerged from the river to mate and drifted north with the wind.

The radar loop shows yellow spots directly over the river as the flies emerge from the water. The swarm morphs into a green band as it drifts north, and blue dots as the flies disperse.

A second, smaller swarm recorded Thursday night appears as a green band that explodes like fireworks into blue dots.

National Weather Service spokesman Dan Baumgardt says the radar captures energy reflecting off the bugs.