Written by Lindsey Taylor, Hosted by Jason Epperson, Storytelling by Abigail Trabue

Excited voices turn to whispers as twilight finally takes hold. Visitors spread out along a gravel road that runs through the heart of an Appalachian forest. A few glowing red flashlights are all that illuminate the trail as people search for a good spot to settle in. Anticipation hangs in the humid air. For a while, everyone searches the forest, eyes straining in the dark. It gets even quieter. And then, a single spark of light illuminates among the trees, earning a couple gasps from the audience. It gently floats through the air for a few seconds before the light blinks out.

Other lights begin to illuminate and drift among the trees. Some appear on the forest floor. They grow in number until there are thousands of glowing yellow lights, looking as if visitors had stepped into the night sky itself. Once there are enough, something even more spectacular begins to happen: the lights begin to glow in time with each other. At the top of the hillside through the trees, hundreds of fireflies blink their lanterns, with more joining them down the hillside with every flash as they create a cascade of light to the bottom of the forest. Then the lights at the top dim and the darkness follows all the way down the hill until the entire forest is dark again.

The synchronous fireflies in the Great Smoky Mountains.

North American Fireflies

In 1680, one of the earliest Western accounts of coordinated fireflies flashing was recorded by a Dutch physician while traveling down the Meinam River in Siam. He wrote, “A whole swarm of these insects, having taken possession of one Tree, and spread themselves over its branches, sometimes hide their Light all at once, and a moment after make it appear again with the utmost regularity and exactness.” 

More than 200 years later and the synchronized flashing of fireflies was still a mystery. In 1917, Philip Laurent theorized in a published science journal that it was not the fireflies flickering together in unison, but in fact the observer’s blinking eyelids.

It wasn’t until 1992 that synchronous fireflies were discovered in North America. Scientists long believed that it was a behavior characteristic only of Southeast Asian fireflies. A reader of Science News thought it was strange that an article about synchronous fireflies in Asia mentioned nothing about the synchronous fireflies that she had seen near her home in Tennessee. She wrote a letter to Steven Strogatz, a Cornell mathematician who studies synchronization, and said, “I am sure you are aware of this, but just in case, there is a type of group synchrony lightning bug inside the Great Smoky Mountain National Park near Elkmont, Tennessee. These bugs ‘start up’ in mid June at 10pm nightly. They exhibit 6 seconds of total darkness; then in perfect synchrony, thousands light up 6 rapid times in a 3 second period before all going dark for 6 more seconds. We have a cabin in Elkmont, and as far as we know, it is only in this small area that this particular type of group synchronized lightning bug exists. It is beautiful.”

Scientists confirmed the synchronized fireflies in the Great Smoky Mountains in 1995 and have since found other populations in the Congaree Swamp in South Carolina and other places in the Appalachian mountains. It’s possible that there are other types of fireflies blinking in unison throughout the United States, waiting to be discovered. We happened to see a swarm of them in a Army Corps of Engineers recreation area just outside of Montgomery, Alabama in our first year traveling full-time. It was, simply, exhilarating. 

Fireflies are actually a type of beetle. Maybe you know them as “lightning bugs,”which is what we called them as a kid in my neighborhood. As a larvae, fireflies eat snails and smaller insects. It takes a single firefly at least a year, and up to two, to mature into an adult. but adult fireflies will only live for around 21 days. In that time, they don’t even need to eat.

There are 19 species of fireflies that live in Great Smoky Mountains National Park, and two main groups. One group is more active during the day instead of the night and does not flash at all. The second group has 13 species that use light during their mating season after dark. Fireflies use the light patterns as part of their mating display, and each species of firefly has a unique “flash pattern” so males and females can recognize each other. It’s also a great way for us to be able to tell different species of fireflies apart. In most fireflies, this light flashes greenish-yellow, and in only one does it flash blue light. Generally, male fireflies will fly around flashing their light while the female responds with a flash of light on the ground.

Science Behind the Light

Photinus carolinus, are the only firefly in America whose individuals synchronize their flashing light patterns together. What makes this light appear? Oxygen is combined with the chemical luciferin and then an enzyme called luciferase to make light in the lanterns of their abdomens. This chemical reaction is taking place every time you see a firefly flash. It is also highly efficient and releases little to no heat. When you light an incandescent light bulb. After awhile, the bulb becomes too hot to touch. More than 90% of the energy created is lost as heat, leaving only 10% left as light. This makes the process and chemical reaction especially inefficient. Fireflies’ lanterns are almost exactly the opposite. Their light bulbs are referred to as “cold” light, which means nearly 100% of the energy is given off as light instead of heat.

No one is sure why this type of firefly flashes synchronously. It could be because males are competing for mates and want to be the first flash. It could also be because if the males flash together, they have a better chance of being noticed. Or, it may offer some safety against predatory firefly species that mimic others’ flashing patterns to lure their prey. Sometimes the fireflies flash in unison, at other times they flash in waves across hillsides. The synchrony occurs in short bursts that are followed with periods of darkness.

Photo by Radim Schreiber, Firefly Experience

New research published in the fall of 2020 is helping us understand more about how fireflies are able to synchronize their lights. A team from the University of Colorado-Boulder has discovered that the fireflies may not actually be flashing according to an instinctive rhythm. Instead, they may actually be observing what nearby fireflies are doing and adjusting their behavior to match it.

To learn more about firefly behavior, the research team from Colorado drove to Great Smoky Mountains National Park and placed two 360-degree cameras in a wooded area. This new approach allows the team to track locations of the fireflies around the camera and then map them. The team also set up a pop up tent nearby. They brought a few fireflies in at a time to observe their behavior when they weren’t surrounded by hundreds of other fireflies.

When the researchers put a single male firefly into the pop up tent on his own, he would flash without rhythm, a few bursts here and there. Once the team put more than 20 fireflies together, they started to synchronize. This means that the synchronized firefly displays seem to be more social than a behavior they are born with.

Each year, the mating season only lasts for around two weeks. We aren’t sure why the date that fireflies start their mating season varies, though it could be related to temperature and soil moisture. No one is able to exactly predict when the fireflies will begin flashing. On the first day of the mating season, only a few individuals will begin flashing, then more and more will join the synchronous display each day until a “peak” is reached. After the peak, fewer and fewer fireflies will be flashing each night until the season is over. Historically, this peak date has happened anywhere from the third week of May to the third week of June since 1993.

Firefly Tourism Today

Today, firefly tourism in the United States is booming, but fireflies are declining around the world. More than 2,000 species of fireflies are found across the globe and fewer than 200 of those are in the United States. The main threat to fireflies is habitat loss, which is mostly due to development and leaves fewer marshes and meadows for fireflies to mate in. Artificial light also disrupts the fireflies’ mating season, who need the darkness to signal to their mates. And as most of the firefly’s lifetime is spent as a larvae feeding on slugs and snails, they are vulnerable to pesticides and weed killer used in lawns.

In the Great Smoky Mountains, the park runs trolleys during the firefly event to reduce light pollution in the parking lot. The lottery also helps maintain the number of visitors each night to a manageable amount. There are even scientists dedicated to researching how we can better protect fireflies during ecotourism, by conserving habitat, involving local communities in their protection, and providing training programs for guides and interpretive materials for visitors.

Seeing the Synchronous Fireflies

To see the synchronous fireflies in Great Smoky Mountains National Park, visitors need to enter a lottery system through recreation.gov. The lottery applications generally open for just a few days in April or May for firefly viewing in early June. Watch the official National Park Service website in the spring for updated details about the lottery each year.

Remember that the quality of viewing each night can be affected by the environment. Fireflies may display less following a large rainfall on a misty evening, and cooler temperatures below 50 degrees Fahrenheit may also cause the fireflies to stop. The phases of the moon have even affected the timing of the display. Nights with a bright moon may cause the fireflies to start flashing later in the evening than usual.

There are a few steps you can take to ensure the synchronous fireflies event is enjoyable for everyone. Using flashlights during the light show will impair visitors’ night vision and can also disrupt the fireflies. Cover your flashlight with red cellophane or use a flashlight with a red filter to protect your eyes and the fireflies. You can also help protect fireflies and their habitat by not catching them and staying on the trail at all times.

Want more TN National Parks? Visit The Great Smoky Homestead or Cataloochee– the Center of the World!