By Bill Napper
Imagine this: You’re hiking the Overalls Loop Trail in Bernheim Forest just after dark. It’s a surprisingly warm March night, with bare winter trees and a stunning starfield overhead. As you walk, the stars twinkle through the branches.
Suddenly, something catches your eye. Was that a star, or… something else? A flash, like a lightning bug? But that’s impossible in March… isn’t it?
You keep walking, and there it is again! You stop, perfectly still, staring at where you thought you saw movement. Nothing. Just as you’re about to convince yourself it’s your imagination, another flash appears from the corner of your eye.
Frustrated, you’re ready to give up when you decide to take one last, deliberate look. And then… BAM! There it is—not high with the stars, but among the trees—an unmistakable flash of light.
Something’s alive out here in March! The forest has just revealed its secret, and you’ve witnessed something magical that few have ever seen.
Winter Canopy Lights: Bernheim’s Rare Display
My first encounter with Bernheim’s Spring Treetop Flashers (Pyractomena borealis), in March 2016, a phenomenon that occurs annually, weather permitting, was a truly magical experience. Unlike the familiar fireflies of our backyards, these are a rare treat to observe. Their elusive nature stems from their late-winter appearance, a time when few venture into the woods after dark, and the cold weather conditions.
Bernheim Forest boasts a remarkably diverse firefly community, with additional species being identified every year or two. While the warmer months offer ample opportunities to witness this diversity through Bernheim’s firefly viewing programs, I want to highlight the less commonly observed, early-season phenomenon: the Spring Treetop Flasher.
From Glow Worm to Sky Light: The Firefly’s Journey
Like most flashing fireflies, they begin life the previous year, overwintering as larvae. These firefly larvae, often called “glow worms” locally, are frequently unrecognized as fireflies—much like caterpillars are not always recognized as future butterflies.
Spring Treetop Flashers, like all fireflies, are actually beetles and undergo complete metamorphosis (egg, larva, pupa, and adult)—similar to butterflies! Nearing the end of their larval stage, these remarkable creatures leave the protective warmth of the leaf litter and embark on a journey to find their ancestral tree—the perfect host for their upcoming transformation into adult fireflies.
Hanging Tough: Enduring Extremes
In late winter, despite freezing temperatures, these fireflies begin their critical upward journey. These discerning creatures seek out large hickories, tulip poplars, or oaks, often the same trees their ancestors chose for generations, because finding the perfect tree is essential for their survival.
Once they locate their colony tree, they secure themselves to the south-facing side of the trunk, at a height of about 3 to 10 feet above the ground—a strategic position that provides optimal conditions to capture the sun’s warmth. Then, with remarkable adhesive abilities, they glue their tails to the bark and hang head-down, a position they’ll maintain through extreme weather conditions. These resilient insects endure temperatures plummeting well below freezing, withstand snow and icy rain, and even survive if direct sunlight heats their chosen spots to a scorching 100°F!
Clinging to the bark, they patiently endure until late February or early March, awaiting the mysterious environmental signals that will trigger pupation—the brief, transformative stage leading to their final metamorphosis into the luminous, flying adults we recognize as fireflies.
Why Risk the Cold? A Trade-off of Dangers
Why would fireflies take such huge risks by enduring extreme winter weather? While summer might offer an easier life in terms of temperature and comfort, they face a far greater threat: predation. The harsh winter, with its freezing temperatures and unpredictable conditions, ironically offers a degree of safety from the many predators that thrive in the warmer months. For these Spring Treetop Flashers, the gamble of enduring the cold is a calculated trade-off, a desperate measure to avoid an even greater chance of becoming a meal.
The Spring Treetop Flasher’s Radiant Romance
Beneath the canopy of ancient hickories and towering tulip poplars, an extraordinary love story unfolds in the world of Spring Treetop Flashers. These remarkable fireflies demonstrate one of nature’s most devoted courtships—a tale of patience, protection, and profound dedication. Male fireflies emerge from their pupae about two weeks before females, but this head start isn’t spent idly. These determined suitors immediately embark on a crucial quest: finding their future mates while they’re still developing. Day after day, they meticulously search the rough bark of their colony tree, seeking female larvae and pupae with singular focus.
Nocturnal Knights: Glowing Guardians
When a male discovers a female pupa, his existence transforms. He becomes her unwavering guardian, remaining constantly by her side and fiercely defending her from rivals. These battles for love aren’t merely symbolic—males engage in passionate combat for the privilege of becoming her lifelong partner. Devoted males may stand guard for up to two weeks, neither eating nor resting, their entire focus dedicated to this single relationship.
As the female finally emerges from her pupal case, her dedicated guardian, who has spent weeks defending her, mates with her immediately. But unlike many insects, where males depart after fertilization, the Spring Treetop Flasher’s devotion continues. He remains steadfastly by her side as she lays eggs, shielding her from predators during this vulnerable time—a rare example of paternal care in the insect world. With their romance complete and the next generation secured, both partners ascend to the highest branches, spending their final short weeks together.
Given the vastness of the forest relative to their size, many of these small fireflies are not lucky enough to find each other early, so they will fly up into the tops of the trees and use their bioluminescence to visually flash their love songs in the Bernheim dark forest. The forest at night becomes a twinkling spectacle! The males, like tiny, flying stars, streak along the tree edges, each sending out a warm, golden-yellow pulse every two or three seconds. It’s a visual symphony in the dark. The females are the discerning audience, perched patiently on the trees. If a flash catches their fancy, they’ll flash right back – a glowing ‘yes!’ in the night.”
Overhead – Hidden In Plain Sight
Adult Spring Treetop Flashers live briefly but brilliantly in the treetops of their Bernheim Forest home, glowing against the night sky. This lofty lifestyle explains why, in Bernheim Forest, no adult has yet been officially photographed—they dwell so high among the leaves that they remain just beyond our reach, their remarkable love story continuing undisturbed above us.
Witnessing this ethereal display requires perfect timing and conditions, as nature dictates the schedule—flashing ceases below 50°F or during high winds and rain—making each sighting all the more precious for its rarity.
While most of us may never witness this luminous courtship dance against a star-filled winter sky, its very existence reminds us why Bernheim Forest’s protected habitat is so vital. As stewards of this remarkable ecosystem, Bernheim safeguards not just the fireflies whose love stories unfold overhead, but countless other secretive residents who find sanctuary within these wondrous woods. Join us throughout the year to discover what other hidden wonders await in this extraordinary place we are privileged to enjoy and protect.
