By Tracy Perlman

LA CROSSE, Wis. (WCCO) – La Crosse, Wis. was covered Sunday night when millions of mayflies from the Mississippi River hatched.

The bugs made their way to buildings, lights and everything in sight.

READ MORE: Garbage Truck Dumps Burning Trash In Richfield Neighborhood

The cloud of bugs was so massive it got picked up on radar!

“It’s not surprising. They’re just such large numbers that you can see their bodies on radar,” entomologist Petra Kranzfelder, said.

Kranzfelder said she’s not surprised. The size of this weekend’s mayfly appearance up and down the Mississippi River isn’t unusual.

It’s a good sign to see them. They’ll only reproduce in areas with high-quality water.

“They provide food for fish and birds and bats. And the other nice thing is they transfer nutrients, like phosphorus and nitrogen, from the river to the land,” Kranzfelder said.

READ MORE: Next Weather: Severe Thunderstorm Watch Issued For Northeastern Minnesota

Two years ago, mayflies swarmed over Hastings, Minn. The blanket of bugs posed a risk similar to snowy streets. Bridges were closed to keep drivers safe.

The Pierce County Sheriff’s Office said the bugs may be to blame for a bad crash along Highway 63 near Trenton Township.

Police say the driver lost control. The road was slippery and visibility was limited because of mayflies.

Kranzfelder said we may see smaller swarms over the next few days since mayflies tend to emerge at the same time. But nothing like they saw in La Crosse.

Mayflies are typically seen in the end of June as the weather warms up. Our cool spring delayed their arrival.

Once you see them, that’ll be it for the year. Mayflies fly around in swarms looking for a mate. They quickly lay their eggs and die a day or two later.

MORE NEWS: Kayakers On Minnesota River Found Human Skull That Turned Out To Be Nearly 8,000 Years Old

“I would suggest, it’s a natural phenomenon and we should take some time to enjoy it especially since it doesn’t last long,” Kranzfelder said.