Officials shut down the Veterans Memorial Bridge between Columbia and Wrightsville for about 45 minutes shortly after 10 p.m. Sunday.
It was reopened shortly before 11 p.m., a Lancaster County-Wide Communications supervisor said.
It was the second night in a row the bridge was closed. The cause, both times, was swarms of mayflies.
On Saturday night, they caused three motorcycle crashes due to poor visibility as well as slick conditions resulting from dead mayflies covering the roadway.