Synchronous fireflies, known as Photuris frontalis, blink in the woods near the Congaree River on Wednesday, May 15, 2024, in Columbia, S.C. Congaree National Park holds an annual event for visitors to view the fireflies, which blink for a few weeks every May and June.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

It’s twilight on a warm May evening at the Congaree National Park in Hopkins, South Carolina. A wooden boardwalk weaves through cypress knees, and towering loblolly pines.

A viewing platform holds the lucky few that won the park lottery this year – an opportunity to see thousands of fireflies, blinking in synchrony, for a few short weeks this spring.

Hrudaya Reddy and her husband Shiva Vanamala traveled here from San Francisco, Calif. Vanamala says he recalls seeing swarms of fireflies in a forest as a child in Karnataka, India. “It sounds a little fantastical to me now – I’m not sure if that was a dream or if it really happened.”

As dusk turns to dark, the dream becomes real: Thousands of fireflies start to flash together in a rhythm. The couple say it’s beyond their expectations. “It’s like magic is happening,” Reddy says. Vendalam describes them as “shooting stars on the ground. Pretty incredible,” he says.

According to the National Park Service, there are just three types of fireflies in North America that are synchronous, meaning they coordinate their belly lanterns to flash at exactly the same time.

The species here in Congaree National Park is Photuris frontalis, also known as “snappy syncs,” named for their quick, steady flash. “It’s constant, like a metronome,” says Lynn Frierson Faust, author of Fireflies, Glow-worms and Lightning Bugs, a field guide that covers the eastern and central U.S. and Canada.

Seeing synchronous fireflies is a rare, ephemeral treat. The insects emerge from the ground to flash and mate for only a few weeks. The natural phenomena draws thousands of visitors to Congaree each year. Park staff are working to protect the fireflies and their habitat so their populations can thrive, and so scientists can unravel the secrets of their unified blinking.

Michael Dunleavy, of Columbia, S.C., holds his daughter Aurora, 4, as they watch fireflies at Congaree National Park on May 16, 2024, in Hopkins, S.C. Visitors had to enter a lottery to get tickets to see the fireflies during their mating season.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

Discovering the species

There are more than 2,000 different kinds of fireflies around the world, more than 170 in the U.S. and Canada, “and we’re still discovering more,” says Sara Lewis, professor emeritus of biology at Tufts University and co-chair of the International Union for the Conservation of Nature’s Firefly Specialist Group.

Scientists got wise to the presence of synchronous fireflies in the U.S. in the 1990’s, thanks to the efforts of Faust, a citizen naturalist. “Growing up in east Tennessee, we called them lightning bugs. They’re just part of summer,” she says.

In the early 1990’s, Faust read an article in a science news magazine that said there were no synchronous fireflies in the Western Hemisphere. “I thought, ‘Ours are synchronous – who do I tell this to?’” she recalls.

She wrote a letter to researchers, who came to Tennessee and studied those fireflies for the next twenty years.

Firefly researcher Lynn Frierson Faust came to the park to view the synchronous fireflies. Decades ago, she helped researchers learn about synchronous firefly species in the U.S.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

Faust is now a leading expert herself. She consults with National Park staff and researchers. This is her busiest time of year, from late May to early July. “Generally speaking – very generally – every species of firefly becomes active in an eight-week period all over eastern North America,” she says.

She drove six hours from her home by the Great Smoky Mountains in Tennessee to catch peak fireflies at Congaree – a few consecutive nights where the synchronous swarm is most active.

‘Hi, I’m a male’ – snappy syncs flash to find mates

Faust is here with researchers like Orit Peleg, a biophysicist and computer scientist from the University of Colorado in Boulder, who’s looking into how synchronous fireflies communicate. It’s still a mystery how they know when to flash to stay in beat with their fellows, without a central leader.

The behavior is thought to be an elaborate ritual by the males to get the ladies to notice them. As they fly low over the ground, flashing their lights, “they’re basically saying, ‘Hi, I’m a Photuris frontalis firefly, I’m a male, and I would like to mate. They’re looking for a response from a female,” Peleg says.

Synchronous fireflies, known as Photuris frontalis, blink in the woods near the Congaree River on Wednesday, May 15, 2024, in Columbia, S.C. Congaree National Park holds an annual event for visitors to view the fireflies, which blink for a few weeks every May and June.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

But a single firefly blinking on his own isn’t very visible. “One of the ways that fireflies deal with that is to synchronize their flashes; that immediately increases the signal,” says Peleg, “We’re trying to understand how they manage to do it.”

At first, the flashing seems random. “As the sun sets, you’ll start seeing little flickers here and there,” Peleg says. “[Then] you’ll see more and more fireflies coming out – perhaps a hundred flashes in your field of view.” As more fireflies join in, it becomes a wave of blinking lights that spreads.

Peleg says the waves may start with a few fireflies – a small group around them see their blinks and instantly light up as well. “We can actually trigger that communication with an artificial LED light,” she says, citing ongoing work by a researcher in her lab. The synchrony can originate in different parts of the swarm.

Protecting endangered fireflies for the future

The synchronous fireflies at Congaree was a local secret for a long time. “It was always a beautiful, fantastic, gorgeous natural light show,” says Jon Manchester, a park ranger at Congaree. About ten years ago, when he started working there, “It seemed to be a non-event. The first time I saw them, I was pretty much the only one out there,” he says.

Now, the word is out. In the years before the COVID-19 pandemic, “there were nights when we had 2,000 people crammed into just a small area right outside the visitor center,” Manchester says. “It wasn’t great for the fireflies. And it also wasn’t a great experience for people.”

When COVID shut down the park in 2020, the Park saw a research opportunity. Staffers collected data from synchronous firefly populations around the park, and found that the ones near the main viewing area were “visibly decreased,” compared with the ones deeper in the woods. “That’s when we realized there was a problem,” Manchester says, “There was a need for us to scale back how many people are coming in here because we want to have fireflies for the future.”

So the Congaree started a lottery system in 2021, and now limits visitors to about 400 people each night to see the fireflies at their peak. It’s modeled after one at the Great Smoky Mountains National Park – where another species of synchronous firefly, Photinus carolinus, can be seen during a viewing eventin early June.

Visitors, carrying red flashlights, look for fireflies at Congaree National Park on Thursday, May 16, 2024, in Hopkins, S.C. The red lights are less disturbing to the insects than regular white lights.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

And the Congaree kicks the visitors out at 10 p.m. – to give the fireflies some peace and time to lay their eggs in the soil, so that in the future, there will be more mind-blowing firefly light shows to come.

Photuris frontalis are seen from Florida to Alabama and Tennessee, and sometimes as far north as Maryland and Delaware. As far as fireflies go, “their populations are doing well,” Lewis, with IUCN, says.

Firefly enthusiasts hope these “wow”-factor fireflies can inspire people to care about other species of lightning bugs in the U.S. which are not as well-studied, and of which an estimated 10% are considered in danger of extinction. “[We need] many eyes in many places to be able to gather more information,” says Lewis, who’s involved with the Firefly Atlas, a citizen science project where volunteers can help survey for fireflies and report sightings.

For firefly lovers, there are other ways to protect them, says Lewis. You can make your yard an inviting habitat by avoiding pesticides on the lawn and leaving some leaf litter on the grass. And you can cut down on light pollution, like bright outdoor spotlights, which can make it hard for fireflies to see their potential mates twinkling.

To look for fireflies, find a place away from light and away from any insect spraying, says Faust. In the eastern and central U.S., they’re usually most active in June, though their peak periods are temperature-dependent.

“A lot of these [other] species, they flash 20 minutes at night and that’s it – and they only live two weeks,” Faust says, “So if you’re not standing in the right place at the right time, looking in the right direction, you’ll miss them.”

‘So hopeful, so peaceful’

From the viewing platform, at Congaree, Nancy Canterbury, from Cary, North Carolina, squints at a cluster through the trees. The fireflies give off a cool, white light – different from the fireflies she caught in jars growing up in eastern Pennsylvania and Ohio. “They’re faster than I remember too,” blinking about twice every second, she observes.

Clinton Wessinger and Pamela Bennett, from Charleston, South Carolina, are perched on a corner bench. “It looks like when you’re coming down out of the sky, your plane has landed and you can see the lights flashing on the runway. It’s like being guided in – except there’s like a thousand of them,” Wessinger says. “There’s something so hopeful, so peaceful,” Bennett says, “It just feels good.”

These swarms of fireflies will flash in synchrony for hours, long after the last visitor has left.

Firefly researcher Lynn Frierson Faust, of Knoxville, Tenn., holds a male firefly at Congaree National Park. Like most fireflies, this species of synchronous fireflies, photuris frontalis, are sensitive to human impact on their habitat.

Sam Wolfe for NPR

hide caption

toggle caption

Sam Wolfe for NPR

Editing and visual design by Carmel Wroth.