Grandfather Glows: Bioluminescent Evenings on Grandfather Mountain

Have the rare opportunity to see Synchronous Fireflies, Blue Ghost Fireflies and Glowworms, weather and conditions permitting (rain dates provided). Tickets go on sale May 23 at 10 a.m.

The Photinus carolinus is the only species of firefly in North America whose individuals can synchronize their lighting display, meaning they can flash in unison. The Grandfather Glows nighttime viewing events will take place on the lower half of mountain.

“The light show that the synchronous fireflies put on is actually a mating ritual,” John Caveny, Director of Education and Natural Resources at the Grandfather Mountain Stewardship Foundation, said. “There is a call and response going on between the males and females of the species. The males are flying around, and the females are in the grass. One group of males will emit a flashing pattern as they try to find receptive females, and the females will respond by replicating that flashing pattern, which creates the synchronous effect.”

Photinus carolinus famously resides in the Great Smoky Mountains National Park, where it is a major draw for visitors from late May through early June. During that time, hopeful spectators enter a lottery to win tickets to witness the phenomenon.

In 2019, the existence of Photinus carolinus’ was discovered on Grandfather Mountain by Dr. Clyde Sorenson, an entomologist from N.C. State University. While hosting a workshop on the mountain, Sorenson was invited to stay the night in the park’s guest cottage near the Woods Walk & Picnic Area. Around 9:30 p.m., he saw a few fireflies flashing and knew right away that it was something special.

“As it got dark, the numbers steadily went up, and between 10 and 10:30 p.m., there were several hundred all around the guest cottage and Woods Walk, flashing synchronously,” Sorenson said. “I knew what to look for, and apart from the flash patterns, which are species specific, there are neurological features that are distinct. They were doing what they were supposed to be doing, and when I looked at them in my hand, they had all the diagnostic features for that same species.”

Sorenson later confirmed his findings with East Tennessee naturalist Lynn Faust, a preeminent expert on the subject, who, among numerous other publications, wrote a field guide on fireflies, which Sorenson described as “one of the best resources to anyone who is interested in learning more about these critters.”

Sorenson is particularly excited, because synchronous behavior is rare in fireflies.

“There’s only a handful of species all around the world that do this, and for a long time, this particular species, the phenomenon of seeing large numbers of them synchronizing has been associated tightly with just a couple geographical areas,” he said. “But the species goes all the way from New York to Georgia. Where they have been most widely known and recognized for so long is at Elkmont in the Great Smoky Mountains National Park. But that’s at 2,200 feet. Where I saw them (at Grandfather) was at 4,200 feet.”

What makes Grandfather Mountain such a unique location, Sorenson said, is its elevation range. Starting at a relatively low elevation of 3,000 feet, the mountain reaches up to nearly 6,000 feet. And while conducting a research survey near the Mile High Swinging Bridge, Grandfather Mountain’s director of education observed the same fireflies blinking synchronously.

At the top of the mountain, they blinked in a slower cadence, because the temperature was colder. Males were observed flying low to the ground, and females were perched along the ground. The groups of fireflies would totally sync light rhythms with each other. During one survey, more than 1,000 fireflies were observed from one overlook on the mountain.

“They could start down at the bottom (of the mountain) very early in June and continue well into July, as you go up the mountain,” Sorenson said, “which presents a really unique opportunity for, perhaps, a lot more people to see them over a much longer span of time. Generally, at any given location, they’re really active for about two to three weeks, and then they’re done for the year. But again, because Grandfather has that great elevational span, it’s possible that the display could last for many weeks.”

This would be a boon for the scientific community and spectators alike, he said, as it would offer another opportunity to study Photinus carolinus and observe one of nature’s most marvelous light displays.