Quick Take
- One of the rarest firefly displays in North America was hiding in Pennsylvania until just over a decade ago, and few knew it existed. See the math explained →
- Synchronized flashing looks like teamwork, but researchers found the real reason males do it is something more competitive. Discover the real motive →
- A Pennsylvania bed and breakfast accidentally became ground zero for one of nature's rarest light shows, and what happened next turned into a cautionary tale for nature lovers. Plan your viewing window →
- Seeing the display is surprisingly accessible, provided you do not miss the one narrow window that makes or breaks the entire experience. Meet the five species →
If you visit the Allegheny National Forest in Pennsylvania in early July, get ready for a spectacular sight that happens in only a handful of places worldwide. As the sun sets over the mountains, the darkening hemlock and beech forest lights up with flickering fireflies. While most fireflies (or “lightning bugs”) emit light individually as a form of male signaling, the Alleghenies are among the few places where fireflies flash in synchrony. Every summer, members of the species Photunis carolinus—also found in the southeastern U.S.—gather to put on this light show.
Synchronous Firefly Flashing
The biological reason for firefly synchrony remains elusive. But researchers have made progress in uncovering the mathematical mechanism behind the synchronous rhythms. In a recent study, computer scientists from the University of Colorado at Boulder isolated captured male P. frontalis (another synchronous species from the Southeast U.S.) in a dark tent and experimented with flashing LED lights to observe the fireflies’ responses. They found that male fireflies synchronized their flashes with the LEDs, provided the LED flashes occurred shortly after the male’s own burst of light. The behavior fits the pattern of a mathematical “phase-response curve.”
The effect is mesmerizing. In an interview, math graduate student Owen Cardwell, who witnessed a P. frontalis light show, recalls, “It was cool, like you just sit in there and the fireflies are blinking around you in sync. It’s not like anything else. Normally with fireflies, there’s a blink here, a blink there, but this was all at the same time.”
Where Are the Synchronous Fireflies?
Synchronous fireflies occur in the Allegheny National Forest.
©zrfphoto/iStock via Getty Images
Five species of firefly are known to synchronize their flashes, three in North America and two in Asia. Until 2012, Photinus carolinus was known only from the southeastern states, with light shows reported from Elkmont, Tennessee, and the Great Smoky Mountains National Park. A 2012 survey of fireflies in Pennsylvania, however, confirmed reports of P. carolinus in the Allegheny National Forest.
Males of the 15 other firefly species recorded in Pennsylvania flash individually, creating seemingly independent displays. In contrast, according to the survey, P. carolinus males streak through the forest simultaneously, leaving trains of flashes that resemble holiday lights suspended for a split second. After 6 to 15 seconds of darkness, they repeat the performance, continuing this cycle for up to two hours on a peak night.
In the southeastern P. carolinus populations, the display peaks from late May into early June, while in the Allegheny population, the peak occurs from late June into early July. An annual Pennsylvania Firefly Festival celebrates the uniqueness of the behavior.
Why Do The Males Flash?
Male fireflies have light organs on two of their abdominal segments, as shown here.
©khlungcenter/Shutterstock.com
Fireflies are beetles that, in most species, emit light from bioluminescent organs (“lanterns”) on their abdomens to attract mates. Males have two lanterns, while females have one. In P. carolinus, male fireflies flash 5 to 8 times, then pause. During this pause, any females observing the display flash back twice. Flashing patterns are species-specific, so a female will respond only to a male of her own species.
In a 1998 study in the Journal of Insect Behavior, researchers found that female fireflies selected males who flashed earlier and more intensely. They concluded that ‘flash synchronization’ is a competitive display rather than a cooperative behavior intended to maintain female responsiveness.
However, a female P. carolinus can mate several times during her short lifespan. Therefore, males benefit from continuing to flash their courtship signals while females are present. A 2010 study in BioOne found evidence that males emerge first from their burrows each night, then wait for females. When females emerge, the male courtship signals intensify, with more flashes per flash train.
After mating, females lay eggs in the leaf litter and soil. These eggs hatch into larvae that prey on invertebrates such as snails and slugs. After two years, the larvae pupate and mature into adults, which live for a few weeks—just long enough to flash and mate.
People have to be careful not to love something so much that they destroy the habitats and the insects themselves with human impact and activity.
Peggy Butler, founder of Pennsylvania Firefly Festival
How Can I Watch?
Rangers guide people to the best spots to see fireflies without disturbing them.
©Great Smoky Mountains National Park from Gatlinburg, TN / Public domain – Original / License
Watching synchronous fireflies is straightforward. “I would do it again,” adds Cardwell in the interview. “It’s not that hard to see them.”
However, timing is everything. Flashing activity, according to a NatureServe page, “typically begins 37-43 minutes after sunset.” One visitor to the Allegheny National Forest reported that the activity was concentrated between 10:30 and 11:15 PM. Seeing the synchronous fireflies during their peak activity has become so popular that tickets are required—and they are hard to come by.
During the original annual Pennsylvania Firefly Festival, for example, 150 lucky people won tickets by lottery to see the synchronous fireflies from the private property of a couple, the Butlers, living in the Allegheny National Forest. Once the synchronous firefly population was discovered, their bed and breakfast became the hub of firefly viewing.
Quickly, the number of people visiting the site skyrocketed to levels that were unsustainable for both the landowners and the fireflies. Because female fireflies emerge from their burrows onto the ground, they are at risk of being trampled. Now, the Butlers limit both the number of visitors and the duration of visits.
“People have to be careful not to love something so much that they destroy the habitats and the insects themselves with human impact and activity,” explained Peggy Butler. “Tourists need to educate themselves about the fireflies and the places that they are going to go in order to see them.”
As of 2026, the Pennsylvania Firefly Festival is held at the Market Village in downtown Tionesta, with guided tours at the Tionesta Lake Visitor Center. Tickets for guided tours are distributed by lottery and sell out quickly. The next event will be announced HERE. Alternatively, if you chat with the park rangers, you can likely glean some insider tips about where and when to see the dazzling display.
While it’s reportedly quite difficult to capture photos or video of the synchronous displays, you can get a sense of them HERE. Enjoy the sparkle!
