Synchronous fireflies in a meadow at the 2021 Pennsylvania Firefly Festival. (Peggy Butler)

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Every year in late June, Peggy Butler and her husband, Ken, welcome visitors to rural northwestern Pennsylvania for the chance to glimpse the rare and beguiling Photinus carolinus. This firefly species flashes synchronously, creating dazzling spectacles of light. The abundance of fireflies on their property in Forest County — there are at least 17 species in addition to the synchronous firefly — led the Butlers to found the Pennsylvania Firefly Festival.?

Launched in 2013, the annual event has become so popular that the Butlers had to institute a lottery system to protect the fireflies that visitors so desperately wanted to see. This year, 2,500 people applied for just 130 spots.?

The intense interest in the festival highlights how much Americans love fireflies. Summer nights spent watching fireflies (and debating whether or not they should be called lightning bugs) are a cherished tradition across the country. But this tradition could be threatened by climate change, according to a new study.?

Researchers found that climate change is among the most serious threats to firefly populations in the United States. To understand what determines firefly abundance, researchers analyzed more than 24,000 surveys conducted by citizen scientists from 2008 to 2016 using the program Firefly Watch.?

Other studies have established that short-term weather affects fireflies, which makes sense because their life cycles last between one to two years, with most of that time spent as larvae living in the soil, where they are particularly vulnerable.?

“What we were really surprised to find is it’s also long-term weather patterns, like averages and things that are expected to change with climate change, that are actually the number one drivers of firefly populations,” said Darin McNeil, the study’s lead investigator and an assistant professor of wildlife ecology and management at the University of Kentucky. Increasing temperatures have a negative impact on fireflies, McNeil said, and as some places become hotter and drier, their firefly populations could disappear.?

Like the giant panda, fireflies are a charismatic ambassador for their less iconic brethren, shining light on the plight of declining insect populations worldwide. “There are a lot of insect species that are in dire need of conservation work and scientific study,” McNeil said. “But it’s sometimes hard to get people excited about dung beetles, for example.”?

Recent research has shown that insect populations fell by 45 percent in the last 40 years, a collapse so extreme that scientists have branded it “the insect apocalypse.” Anecdotal evidence suggests firefly populations are also in decline, and 14 species of fireflies in North America have been assessed as threatened. The Bethany Beach firefly, found only in coastal Delaware and Maryland, is critically endangered.?

For many people, even those who are afraid of other insects, fireflies evoke wonder, magic and nostalgia, a symbol of fairy tales and childhood. “They’re friendly little bugs. They don’t bite. They don’t sting,” Peggy Butler said. “That light really attracts people to ask, ‘How do they do that?’ And it sparks their curiosity.” She paused, laughing. “Sorry for all the puns.”?

Catching fireflies and marveling at their light is a shared experience across time and geography. “They’ve been written about and sung about for centuries,” said Sarah Lower, a co-author of the study and an assistant professor of biology at Bucknell University. “I mean, how fascinating is it that an insect produces its own light?”

Conservationists hope that by learning more about and educating people about what hurts fireflies, they will also help other insects who are harmed by the same things. Other than climate change, fireflies are threatened by pesticides, light pollution and unrestricted development.

The Allegheny National Forest, where the Pennsylvania Firefly Festival is held, is largely sheltered from some of these scourges of modern life. “It’s very wild here. We have half a million acres of national forest where we live,” Butler said. “Fireflies prefer very dark places. There has to be a lot of moisture and undisturbed soils.”?

The Butlers avoid pesticides, rarely mow and limit the use of outdoor lighting during the summer. Fireflies flash to attract mates, and light pollution interferes with their signaling.

At the festival each year, guides lead small groups of attendees into the forest to witness the synchronized flashing of Photinus carolinus. It is often a powerful and emotional experience, especially for people who have never seen a firefly before. “When we take them back into the forest, into the pitch darkness, and they see that synchronous activity, they’re wowed,” Butler said. “You can hear the murmurs. You can hear the ‘Wow, oh my gosh. Look at that.’”

Photinus carolinus was previously thought only to live farther south, in the Great Smoky Mountains. In the Smokies, these insects’ mating displays are a “big phenomenon,” Lower said, making the region a beacon for firefly tourists. It is not known if Photinus carolinus exists elsewhere in northern Appalachia, though Lower theorizes that it’s possible, and they have been found in other parts of Pennsylvania.?

Lower was a member of the team that first confirmed the presence of Photinus carolinus in the Allegheny National Forest in 2012, and she is now on the board for the Pennsylvania Firefly Festival, advising the organization about how to keep the event sustainable.

A map of firefly abundance and ideal climate conditions generated for the recent study shows hot spots in Pennsylvania, and McNeil called Pennsylvania “the heart of firefly country.” This is perhaps why the firefly is the state insect. But what climate change will mean for Pennsylvania’s beloved fireflies in the long-term is still unclear.?

Because of climate change, Pennsylvania is becoming hotter and wetter,with increased rainfall and more intense storms. Butler has seen how flooding can affect the fireflies on her property. “In 2015, we had a big flash flood in our area, and it wiped out the undergrowth about two weeks before the festival in the forest right behind us,” she said. “That year, and for two to three years after that, we didn’t see the synchronous fireflies in those areas at all.”?

Lower said it’s too soon to predict what will happen to fireflies in specific places because of climate change, and outcomes will be different for each species and will depend on their habitat requirements. “The thing I’m most concerned about is if it changes really rapidly,” she said. “We actually don’t have a very great idea of how far fireflies can travel.”

McNeil said firefly populations may appear stable to people in Pennsylvania as climate change accelerates, even though what is actually happening is a “turnover in species.” There are more than 100 species of fireflies in the United States, and some of those species, like the common Big Dipper firefly, named for its J-shaped flight trajectory, may thrive, even as other, less adaptable species are lost.?

Lower and McNeil encourage people to manage their properties in ways that benefit fireflies, by leaving leaf litter on lawns over the winter, for example, and to participate in community science around fireflies. The efforts of residents across the United States made this study possible and could make others possible in the many areas of firefly research that are understudied. Collecting data about fireflies in your own backyard “can go a long way toward their conservation,” McNeil said. “And if you bring your kids out to count fireflies, those are future conservationists.”?

Fireflies may be small but their presence—and diverse abundance—is important. “We could probably lose all of our fireflies, and it would not impact your day to day much, aside from the summer nights being a bit less magical,” McNeil said. “But fireflies serve as what we would call a bioindicator, telling us something about the broader health of our ecosystem.” Their disappearance is a warning, not only for insect populations but for us. “The question is,” McNeil said, “how many species can we lose before we see huge consequences to human society?”?