Sign of the Apocalypse? Flies Invade Lancaster County

The swarms caused traffic accidents and bridge closures. Here's why it could actually be good news.

It’s like something out of a movie: twice in the past five days, swarms of mayflies have shut down the Veterans Memorial Bridge over the Susquehanna River in Lancaster County. On Saturday night, the bridge closure came after the low visibility from thousands and thousands of flies caused three motorcycle accidents (the injuries were reportedly minor).

Ryan Robinson of Lancaster Online reports today of “a surreal scene” that sounds like an entomological version of the frog scene from Magnolia. Fire Chief Chad Livelsberger told Robinson, “It was like a blizzard in June, but instead of snow, it was mayflies.” There was an inch-think slick of dead flies on the bridge, making cars skid and wheels spin as though the roadway were covered with ice. But the flies weren’t all dead, apparently, since they swarmed again on Sunday night, prompting a second shutdown of the same bridge.

From Lancaster Online:

“They were getting in our mouth,” [Columbia Borough Fire Chief Scott Ryno] said. “We had to close our eyes. We had to swat them away. Even when we got back, it felt like bugs were crawling in you.”

Mayflies flew in their shirts and holes in their vehicles, Livelsberger said. More than a hundred mayflies flew into his personal vehicle in the time it took him to open the door and get inside.

“We had to rip our radiators off” to clean the bugs out, he said. …

Livelsberger grew up in Wrightsville and has served as chief since 2012 and in other positions at the fire company since 1998.

“I never saw anything like it,” he said of Thursday night’s mayflies invasion.

The swarms have made national and international news, and even landed on End Time Headlines.

But Jon Gelhaus, a professor in Drexel’s Department of Biodiversity, Earth and Environmental Science and the curator of entomology at the Academy of Natural Sciences of Drexel University, says what seems to be a harbinger of doom is actually the opposite: Swarms of mayflies are good news.

“Mayflies tend to be found in cleaner waters,” Gelhaus says. “We use mayflies and their diversity and abundance to track water quality. When waters get polluted in various forms with chemicals or erosion or siltation, mayflies are one of these groups that drop out of these systems.” Pennsylvania, he says, tracks the presence of aquatic insects like mayflies to measure how healthy the water is for supporting ecosystems. “What this is showing is that the river is clean enough to support large populations of mayflies, so it’s actually a good sign that these systems are cleaner than they used to be.” The reason that mayfly swarms are increasingly seen, says Gelhaus, is due to various anti-pollution laws that control what we put in our waterways. They have created healthier, more diverse aquatic communities.

As for whether the swarms will be back, Gelhaus can’t say for sure because he doesn’t know which species these particular flies are. But these adult mayflies, when they emerge from the water, do so solely to reproduce, for a few days per year at the most. Emerging together as an entire population makes it easier to find a mate and to swamp predators, from whom they have no defenses. When this happens, says Gelhaus, “you get hundreds of thousands, or a million, just huge huge numbers, that are landing everywhere, including the roads. Then the smashed bodies get kind of slick, and it’s going to be hard to brake.” He also notes that many aquatic insects, when they emerge, are attracted to lights; “if there were lights on the bridge,” he says, “that could have contributed.” The bridge was, in fact, recently renovated with new lights. I’m guessing the mayfly swarm was an unintended consequence.

#mayflies #wrightsvillebridge A photo posted by Mark (@420chee) on