Teena Franzen recorded the incident while riding with her son in a Sabula police car.
"I've lived in Sabula since '75, I've never seen that many before, ever," Teena Franzen told WQAD.
A pair of motorcycles lost traction trying to move through the piles of mayflies. No injuries were reported.
"The biggest thing I noticed was... (mayflies) piled knee-high in front of their headlights from just sitting there," said Reserve Officer Stephen Thayer of the Sabula Police Department.
"We had Iowa DOT came in with a snow plow and actually plow them off and then sand it because it was still pretty slippery. And then Illinois plowed their side," Thayer added.
The Iowa Department of Transportation will keep lights on the bridge turned off over the next few weeks to keep the bugs off the bridge.