This article was originally published in bioGraphic.

One dusky June evening, two days before the 2022 Pennsylvania Firefly Festival, the biologist Sarah Lower sat on a back porch, watching the sky for a specific gradation of twilight. A group of Lower’s students from Bucknell University hung around her, armed with butterfly nets and stopwatches for counting the time between firefly flashes—a way to differentiate between the multiple lightning-bug species that live here at the edge of Pennsylvania’s Allegheny National Forest. This postindustrial expanse of second-growth trees and hills pimpled with oil wells also happens to rank among the world’s best places to see fireflies.

Once the cloudy sky blushed red from its last glimpse of the setting sun, I set out with Lower and her students toward the forest edge. Moving from habitat to habitat as the evening deepened, Lower narrated which species we saw and their different behaviors. Her students, meanwhile, netted their way down a wish list of research samples.

First up was Photinus macdermotti, a firefly species that emits two quick flashes. Just a few feet away, near a pond ringed by cattails where a beaver lazed face up, the students caught Photinus marginellus, a quick single flasher. Males buzzed around one patch of goldenrod, blinking quick winks at the sitting females who deigned to flash back. Like other species of fireflies, males of P. marginellus typically flash in flight, while females wait below on blades of grass, shooting answering flashes at only the most compelling suitors.

At first, these early-evening species looked almost like pixels of static. But the darker it got, the more they came to resemble dust motes twinkling in invisible sunbeams.

Half an hour later, we moved on. Heading across Pennsylvania Route 666 and past a modest farmhouse, we reached a small path leading down to Tionesta Creek, which parallels the road. By now the air had chilled. Twilight drained away the last notes of color, a dullness almost immediately punctuated by a yet-undescribed firefly species from the genus Photuris, nicknamed “Chinese lanterns” by Lower and her team. Each flash set the fireflies aglow for long beats of unearthly green so bright they illuminated surrounding vegetation. A student snagged one in a net, marveling at its size—several times larger than the species they’d already collected. Irritated or alarmed, the captured firefly switched to a faster pulse, reminiscent of a car alarm.

“These are the ‘I’m angry’ lights,” Lower explained.

Clumsy in the dark but reluctant to spoil our night vision with flashlights, we meandered along the creek to where a bridge spanned the water, overlooking an island spiked with conifers. From the base of the island to the tree canopy, a galaxy of fireflies shone in drifts or brief flashes, complemented by a starry sky overhead. Their flashes merged with the stars into a doubly scintillating reflection in the water below. It was a dazzling scene, and one that hundreds of people would soon flock here to see as the Firefly Festival got under way.

Around the world, firefly tourism is surging in popularity. The interest gives scientists like Lower hope that funding and conservation will follow, because fireflies—like other dark-dependent invertebrates—are succumbing to our society’s penchant for sterile lawns and careless nighttime lighting. But the choice to open any of the world’s most spectacular firefly sites to the public focuses these same pressures to a sharp point. When the founders of the Pennsylvania Firefly Festival chose to share their backyard’s magic with the world a decade ago, did they further imperil the local firefly population? Or, by giving people like me the chance to stand on a bridge, balanced between galaxies, did they play a small role in protecting one of our most beloved summer spectacles?

On another June night, in 2012, a group of visitors arrived at Ken and Peggy Butler’s bed-and-breakfast, out past reliable cell service in Forest County, Pennsylvania. Peggy was a school therapist, Ken a money manager, and they had moved out into the northwest corner of the state for the quiet and the fly-fishing.

These visitors were not the Butlers’ typical bed-and-breakfast guests. The roving band of firefly scientists lugged microscopes and butterfly nets into the Butlers’ garage, then spent the next six weeks venturing out in tick-proof gear each evening, surveying fireflies where the Butler’s grassy backyard melted into the half-a-million-acre national forest. What they found was nothing short of astonishing—a wonderland of evolutionary biology amid the quiet, unimposing hills of rural Pennsylvania.

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One theory holds that bioluminescence emerged on Earth half a billion to 2 billion years ago in organisms to which oxygen was toxic. This theory holds that some life forms evolved a chemical process that could consume and detoxify any offending molecules while popping out a little bit of light as a harmless by-product.

Whatever its primordial purpose, bioluminescence has since emerged or reemerged at least 94 times across the tree of life, according to recent counts. The specifics of how different single-celled organisms and larger creatures accomplish their own glow-up tricks vary, but a general pattern holds across many examples. Bioluminescent organisms like fireflies have enzymes called luciferases (from the Latin lucifer, meaning “light-bringer”), which they apply inside specialized lantern organs, alongside a pinch of oxygen and a little bit of energy, to another class of compounds called luciferins. Et voilà: A photon of light comes out.

Most creatures who adapt this ancient chemistry to their own ends reside in the ocean: electric-blue crustaceans, fish that use dim lights to cloak themselves from predators, and deep-sea squid that scintillate like alien spacecraft. A few, like New Zealand’s glowworms, live in caves. Fireflies, conversely, are easy to see, flickering at the edge of backyards, captured in jars, shining in the childhood memories of millions as a stand-in for nostalgia or wonder. Perhaps because they’re the type of bioluminescent creature people are most likely to encounter, fireflies hold a special allure—often they’re a gateway to an underappreciated, imperiled cosmos of nocturnal biodiversity.

To date, scientists have described more than 2,000 species of fireflies. Some are active during the day, communicating via pheromones. But the most well known come out during the evening or night to inscribe bursts of light into the air like species-specific autographs. The researchers who first came to survey the species in the Butlers’ backyard included Lower, who was then a graduate student, and Lynn Faust, an independent naturalist and firefly expert. The team reported at least 15 species in all, the insects living practically on top of one another.

Two species in particular stood out. The researchers spotted clouds of one famous and rare firefly, Photinus carolinus, which flashes in synchronous bursts, causing larger groups of them to light up in near unison in a wave that moves across the forest. Then they discovered what appeared to be a new species, the one they nicknamed “Chinese lanterns,” flying like lazy sparks above a campfire for long beats of electric lime green. Both these and the synchronizers, wrote Faust in the survey report, “easily reached the ‘WOW!’ level.”

For the Butlers, the choice now was whether the scientists should be vague or precise about the location of the firefly wonderland. “If you decide you don’t want to pursue anything with this, we will keep it quiet,” Faust told the Butlers. “You can just go about your lives as normal as possible.”

The Butlers evaluated their options. Make the report as specific as you like, they said. How many people could possibly come?

Faust knew the answer to that question. She had begun her own path to the forefront of firefly science not as a credentialed academic but as a young mother in 1992, when she invited scientists to her family’s cabin in the Great Smoky Mountains in Tennessee to study a spectacle her family had long referred to as “the light show.” As those scientists soon published, Faust’s family’s private light show was a proven example of synchronous fireflies.

Before long, people wanted to see for themselves. Many people. The synchronizers in Great Smoky Mountains National Park became an annual event on par with Fourth of July fireworks, drawing more than 26,000 tourists a year. Visitors clomped through the forest, often crushing female fireflies underfoot or disorienting the insects with their flashlight beams. “I have crouched in the dark woods, illuminated by the rhythmic flashes, and wept over the unintended consequences,” Faust wrote in her 2017 book, Fireflies, Glow-worms, and Lightning Bugs, one of the few authoritative field guides to North American fireflies.

She also felt, however, that many of these clompers would otherwise never go out in the dark with eyes and hearts open to nature. Was sharing the Smoky Mountain fireflies with the world the right call? “It depends on which night you get me,” she told me recently.

Humans’ fascination with fireflies has long been smothering. In the early 20th century, hunters in the Japanese countryside stuffed fireflies into cages and shipped them to major cities such as Tokyo to glimmer out the rest of their lives as doomed mood lighting. Another wave of lighting-bug lust occurred in mid-century America, when a chemical company eager to harvest bioluminescent enzymes dispatched community groups and Boy Scouts as firefly collectors. And in China, 17 million fireflies were sold in 2016 alone, many over the eBay-like website Taobao, to customers who used them as living gifts, decorations, and Valentine’s Day–esque love tokens. (The chemical company stopped soliciting fireflies in the 1990s, and Taobao banned the sale of fireflies in 2017.)

Just going to see fireflies poses less obvious risk to them. But scientists have amassed some alarming reports. In Thailand, for example, where boats ferry tourists past mangrove-swamp forests pulsing with synchronous fireflies, scientists have documented shorelines eroding, gas leaking into the water, and camera flashes disturbing firefly courtship. At one popular Thai site, scientists have estimated that the population of one synchronizing-firefly species is down 80 percent since tourism began.

In a rural town in Mexico’s Tlaxcala state, where a new synchronizing-firefly species was formally recognized in 2012, tourism has since ballooned to some 120,000 visitors a year. And in North America, too, firefly tourism is on the rise. In Faust’s beloved Great Smokies, even after years of trying to throttle crowds—the National Park Service has instituted an online lottery to limit the number of visitors—some guests still head off into the forests and lie on the ground.

Tourism is far from the only threat to fireflies. As with many insects, data on lightning-bug populations are spotty, outside of a general, anecdotal sense that they’re blinking out. But insects overall are in crisis. Numerous studies suggest that within many insect groups, abundance is dwindling by 1 to 2 percent each year. An International Union for Conservation of Nature (IUCN) group found in 2020 that fireflies face three primary threats.

The first is habitat loss, which eradicates all but the hardiest lightning bugs from developed areas, leaving species like the big-dipper firefly—the pigeon of the firefly world. Second, like other insect populations, fireflies also seem to be suffering collateral damage from pesticides used in agriculture. And, finally, on top of that is light pollution: the glare of each streetlight, LED-outfitted billboard, front-porch lamp, and every other fixture left on in the night. A recent global study estimated that the collective glow of all this wasted light is making the night sky about 10 percent brighter each year, bathing ever more of the planet’s nighttime surface in light. Such artificial lights threaten to drown firefly bioluminescent courtship signals in much the same way loudspeakers blaring out static would disrupt birdsong. The entomologist Avalon Owens, who studied fireflies for her Ph.D. dissertation at Tufts University, has found that even ambient light pollution can cause some firefly species to blink less often, transforming what should be call-and-response dialogues into a series of missed connections.

Our effort to understand how quickly fireflies are disappearing is also hampered by our relative ignorance of them. North American fireflies spend much of their lives as larvae wriggling through soil, where they hunt down worms and snails, inject their prey with enzymes, and slurp up the resultant puddle of goo. Once they emerge as short-lived adults, some species are known only by a specific flash that a naturalist described seeing in a dark jungle decades ago. When the IUCN published its first firefly-conservation-status survey in 2021, focusing on 132 species in North America, it classified 18 as threatened. But it categorized 70 more only as data deficient, meaning we don’t know enough about them to say how imperiled they might be.

“Compared to what the monarch people can do, it’s so sad,” says Owens. Unlike butterfly hobbyists, who go out in clubs during the daytime and have collected decades of data on population abundances, firefly surveying has historically been a solitary activity. “Each couple of decades, you get, like, one eccentric person who spends every night in the middle of the woods,” she adds.

“Five years ago we basically knew nothing,” says Sara Lewis, a biologist at Tufts. For years, Lewis designed careful lab experiments to understand firefly reproductive structures and behaviors. Then “a switch went off in my head, and I was like, wait, what difference does it make to know [these specific details about] a group of animals that could be extinct in 50 or 100 years?” Today, Lewis co-leads the IUCN’s efforts to keep firefly populations alive.

As some firefly populations fade to black, though, general and scientific interest is swelling. More people want to see fireflies for themselves, driving firefly tourism, and more scientists want to better understand firefly biology both for its own sake and for future conservation work. Perhaps the Butlers didn’t have to make the same stark choice Lynn Faust made in the Great Smoky Mountains. Perhaps tourism and science could complement each other. Maybe people could love fireflies neither too little nor too much but just the right amount.

The Butlers’ path to sustainable firefly tourism was rocky. The summer after Lynn Faust’s report on the Allegheny National Forest fireflies was published, the Butlers hosted the first Pennsylvania Firefly Festival—a free, two-night event in the grassy field behind their house. They had food trucks, face painting, and music. Some 400 people came. The next year was similar. Then, in 2015, David Attenborough and his crew came to the property to film a documentary called Life That Glows, hiring Lower and Faust as on-site firefly wranglers. “Then we knew: This is serious,” Peggy says.

After Attenborough’s film, things got out of hand. A thousand people showed up in 2016. Cars filled the field, and as they pulled out, every pair of headlights beamed into the woods, grinding the synchronous display to a halt. “It was like, this is gonna break us,” Peggy says. “This is going to kill us because it’s going to kill the fireflies.”

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Since then, the Butlers have taken steps to rein in the enthusiasm. First they started charging admission, which they funneled to a nonprofit called the Pennsylvania Firefly Festival, which supports research and sponsors graduate students. With advice from Lewis, they installed bleachers and red-rope lighting to keep visitors from trampling female fireflies and their habitat. After the pandemic forced a pause, they went even smaller: They sold just 100 tickets in 2022, divided into two nights.

At the same time, the Butlers built up closer ties with the scientific community, converting their bed-and-breakfast into something more like a hostel for visiting researchers. Among the scientists who kept coming back was Lower, who is studying the many firefly species that restrict their activities to the day and communicate with pheromones. Lower and her collaborators recently isolated the first known firefly pheromone, and she was at the Butlers’ in 2022 to determine what scents fireflies are using to attract one another, and whether light- and smell-based flirting are mutually exclusive.

The Butlers have also hosted research on how artificial light stifles fireflies. In recent years, ecologists have demonstrated that many species are more sensitive to blue colors of light. When Owens came here to test the least harmful colors of artificial light for fireflies in 2019, though, she found that amber-colored lights—darlings of the dark-sky environmental movement because most species, humans included, seem less bothered by them—are especially disruptive to fireflies. Red lights are still a good choice, Owens says, but the best strategy remains the most obvious: Just use light sparingly overall.

The research happening at the Butlers’ is just one part of a worldwide firefly renaissance. Setting aside habitat loss, light pollution, and pesticides, the known ranges of many firefly species seem to be expanding, Faust says, because more people are out looking. Starting from the “discovery” of synchronizing P. carolinus fireflies in the Smoky Mountains in the 1990s based on Faust’s reports, naturalists and scientists have recognized other P. carolinus outposts up and down the Appalachian Mountains. (The Xerces Society maintains a map of places that accept visitors to view these and other species.)

The same scientists whom Faust had summoned to Tennessee later documented synchrony in another American species, Photuris frontalis, which soon drew its own research scientists and crowds, which in turn helped spark the passion of new enthusiasts. After surviving a life-threatening car accident, for example, the North Carolina State University entomologist Clyde Sorenson told me he pursued research on fireflies for the pure joy of it. In 2019, Sorenson documented firefly synchrony on North Carolina’s Grandfather Mountain, and he has since been tracking down an undescribed “ghost” firefly species that emits faint green signals.

With firefly tourism on the rise as well, a team convened by Lewis published a set of recommendations in 2021 for how to manage the upswing of interest. Although tourism is unlikely to lead to global extinctions, it can certainly extirpate local populations, she says. The final report recommends robust habitat protection and education programs, including etiquette guides. For guests, that means carrying no artificial light sources and staying on marked trails; for hosts, it means limiting total visitor numbers, fencing off paths, and minimizing lighting. These are all steps that the Butlers have taken as part of their journey from wide-eyed enthusiasts to conservation advocates.

A few weeks before the 2022 Pennsylvania Firefly Festival, Ken and Peggy Butler visited their first international scientific conference, in Portugal. From the time their plane touched back down in the U.S. to the start of the festival, their days were packed with answering emails, wrangling volunteers, and accommodating an in-home guest list that had ballooned to festival presenters, interns, the troop of Bucknell researchers, and the latest visiting journalist.

Finally, a few hours before the 2022 festival’s first night, Ken and Peggy slowed down long enough to chat with me on their porch about their own learning experience. Sarah Lower listened in, pausing at one point to snatch another day-active, presumably pheromone-emitting firefly buzzing around us and slot it into a vial.

I asked whether the Butlers regretted the answer they had given to Faust a decade ago, when the choice to publish their location propelled the rest of their summers—and a sizable part of their lives—into firefly-land. “I’m a firm no,” Ken said, and Peggy agreed.

Once the festival began, local musician Matt Miskie played a set of songs, including one written for the event: “We’re out tonight,” the chorus goes, “beneath the Allegheny skies.” (He’s “the Jimmy Buffett of Western Pennsylvania,” Ken explained.) There was a merch table and exhibits: The astronomer Diane Turnshek, who had recently helped the city of Pittsburgh change its street lighting to limit light pollution, set up a booth promoting dark-sky environmentalism. Don Salvatore, a firefly naturalist and educator from New England, gave a Boston-accented presentation on firefly courtship. And then groups set out to see fireflies, guided by volunteers and the Bucknell students.

Even with the Butlers’ dedication to protecting fireflies and encouraging responsible tourism, nothing is perfect. That first evening got too cold, causing the synchronous fireflies to slow down and eventually stop flashing. One little girl, scared of the dark, had chosen to wear sneakers that burst out purple flashes with every step. An elderly woman sat in reverence and reminiscence at the edge of the woods, listening as Peggy explained firefly life history, but the car that fetched her back pierced the forest with its headlights.

And though the Butlers can control what happens on their own property, some of the most enticing firefly-viewing locations—like the magical bridge over Tionesta Creek—are public spaces subject to the choices of the entire community.

The nominal marquee show started at about 10 p.m. that evening, behind the house in the darker shadows of the woods. I stood in shivering silence, shoulder to shoulder with Miskie and a few other festival volunteers as a forest clearing’s worth of synchronous Photinus carolinus fireflies alternated between paparazzi bursts of quick white flashes and long, coordinated beats of collective quiet. A few straggling Chinese lanterns floated through their midst on their own tempo, unperturbed. Afterward, it was very, very hard to fall asleep.