Transit officials in Iowa were forced to bring out the plows last week to clear away a thick swarm of mayflies covering a bridge.

"Shadfiles, also known as mayflies, are in the height of their hatching season and are covering the bridge, making it very slick," the Department of Transportation said in a Facebook post last week.

People were urged to use extra caution in the area during the hatching period.

"Dick Banowetz, our supervisor in the area said the flies were ankle deep on the bridge," the post said.