Confused mayflies wreak havoc on a Pennsylvania bridge
The primitive aquatic insects think the illuminated road is the river
Mayflies swarming a central Pennsylvania bridge over the Susquehanna River are a good thing, and a bad thing. Before the 1972 Clean Water Act, the river was too polluted to support the primitive aquatic insects. So their comeback is a sign that the water is healthier, says forensic entomologist John Wallace of nearby Millersville University.
But those swarms have become a nighttime menace for people driving or walking across the Columbia-Wrightsville bridge — thanks to the 2014 installation of large, 1930-era lamps along the two sides of the bridge. Soon after the lights were added, adult mayflies of the species Hexaginia bilineata began invading — causing blizzard-like conditions on the 2-kilometer overpass. The swarms were so intense in 2015, the bridge was closed following three accidents, and bulldozers were brought in to remove knee-deep piles of insect carcasses.