"It’s like ice on this bridge," says reserve officer Stephen Thayer of the Sabula Police Department in Iowa. He was radioing in a call for the Department of Transportation to come and clear the bridge, but it wasn’t due to wintery weather — the Savanna-Sabula Bridge was impassable in the middle of summer because there were just too many bugs, reports Aarik Woods for the local TV station, WQAD.

A swarm of mayflies, also called shadflies, had claimed the bridge over the Mississippi, explains Patrick Hogan for Fusion, which also posted the video captured by Thayer’s mother Teena Franzen. She rode down to see the sight. Her video shows alarming numbers of the insects sitting on the car’s window shield and still more flying around the interior.

The mayflies were piled so high that two motorcycles slipped while trying to cross. No one was hurt, but the Iowa DOT needed to clear the bridge with snowplows and then sprinkle sand because "It was still pretty slippery," Thayer told WQAD.

Fortunately, mayflies are pretty harmless. They live in fresh water during their immature stages and emerge as adults in the spring and summer. Often, all the mayflies in a particular area mature at once and "hatch," much to the delight of fish who like the gobble them and fly fishing enthusiasts who like to try and catch the excited fish. Their lifespans are very short and focused on reproduction — the females of one species called Dolania americana live only five minutes after their final molt.

The rapid, overwhelming emergence is thought to be a survival strategy. Predators stuff themselves and still many bugs keep on flying. They certainly overwhelmed motorists on the Savanna-Sabula Bridge.