Nature’s notebook

A mysterious firefly display in the Uwharries

The bioluminescent glow of a firefly. Photo: Radim Schreiber

Two years ago, over Memorial Day weekend, I witnessed a phenomenon in the Uwharries that has proven difficult to explain. I stepped out of the house after dinner and looked toward the grove of majestic oaks surrounded by our fields of native warm season grass. Thousands of lights were flashing in the canopy. It was so bizarre, I instinctively blinked in the exaggerated manner of a cartoon character. My vision didn’t clear. The canopy looked like a crazy disco Christmas tree.  For a moment, I wondered if I was hallucinating. The rest of the world was still in order – the soft spring air on my skin, the whoosh of a car heading down the road, the embers in our grill still smoking – so I hadn’t entirely lost touch with reality. I called my husband outside and asked him to tell me what he saw. His description matched mine.

In summer, we’re accustomed to seeing fireflies – or lightning bugs as we like to say in the Uwharries – hovering over our fields. These are likely the big dipper firefly (Photinus pyralis), the most common firefly in the East, which tolerates all sorts of habitat, even suburban backyards. But I had never seen fireflies congregate en masse in the tree canopy. (We did see a much diminished version of this display on a recent weekend in May.) I figured an internet search would quickly turn up other accounts of something so dramatic, but I tried link after link in vain. 

I did learn that fireflies are beetles, not true bugs or flies. There are more than 150 species in North America. The greatest numbers occur in the Southeast; we have the warm and humid conditions they prefer. Areas near water sources, on the margins of forests and fields, provide the most species diversity. In the Appalachians, there are synchronous fireflies that flash in unison, like a wave going around a football stadium, and blue ghost fireflies that emit pulses of light for up to a minute, creating rivers of luminescence as they waft through the forest. As enchanting as these spectacles must be, they didn’t describe the event I’d witnessed. I finally reached out to several entomologists, bombarding them with a litany of questions. (I have to pause here and note that I have never encountered a more responsive or gracious group of scientists.) They reassured me there are indeed firefly species that prefer the tree tops to grasslands, namely the Pyractomena borealis and those in the genus Photuris. The ones I witnessed were possibly Photuris tremulans and/or Photuris versicolor. To determine the exact species, a knowledgeable entomologist would need to observe the timing, pattern and color of the flashes, and probably examine a specimen in hand. Lynn Faust, who researches fireflies in the Smokies, has bestowed the common name “Christmas tree firefly” on P. tremulans in her forthcoming book, a moniker that corresponds with my initial impression.

Synchronous fireflies light up in the Great Smoky Mountains National Park. Photo: Radim Schreiber, www.fireflyexperience.org

Photuris species are particularly interesting. Fireflies use their bioluminescence – the chemical reaction that makes their lanterns glow – to communicate with predators and with each other. They use their light to ward off attacks, as if to say, “I’m toxic; I taste bad; don’t eat me.” Males use it to announce their availability and females to signal their interest. Female Photuris fireflies take this communication to another level. These so called “femme fatales” can change their flash patterns to attract males of other firefly species. When he comes over to mate with her, she pounces on him and eats him. Photuris fireflies lack the compounds that make other fireflies toxic to predators, but she can absorb these from her prey and even pass them along to the eggs she lays to help protect her babies. While this aspect of the Photuris lifecycle has been well documented, we know little about their mating ritual because it usually occurs so high in the trees.

In researching this topic, I found a recurring theme: There’s much we still don’t know about fireflies. Isn’t it ironic that we know relatively little about an insect so beloved by humans? Seth Bybee, a professor at Brigham Young University, is spearheading an effort to sequence the genome of the big dipper, which it’s hoped will help shed some light, so to speak, on firefly behavior. I had an epiphany after corresponding with him. There are economic incentives to understanding how to limit the proliferation of household and agricultural pests. As a result, we know much more about the insects we despise than about those we cherish.

In another great irony, we are inadvertently ridding ourselves of the very species we enjoy having around. Firefly populations seem to be in decline. As with most species, habitat loss is a factor. We might see big dippers in our backyards, but other fireflies rely on more pristine natural areas. Even big dippers are vulnerable to the fertilizers and pesticides we slather on our lawns. Fireflies are also especially sensitive to light pollution, which can disrupt their ability to reproduce. In the case of fireflies, we need to know more in order to save them. A deeper understanding has certainly enhanced my appreciation of the mysterious firefly displays I’ve witnessed in the Uwharries.

(Special thanks to entomologists Marc Branham, Seth Bybee, Jason Cryan, Lynn Faust and Sara Lewis.)


FOR MORE FIREFLY INFORMATION:

Sara Lewis’ book Silent Sparks: The Wondrous World of Fireflies would make a fine summer read.  Also watch her TED Talk

Firefly photos and videos at www.fireflyexperience.org 

For more on the effort to sequence the big dipper genome

Learn how to communicate with fireflies

For tips on helping fireflies survive their night in a jar on a kid’s bedside table

To see the synchronous firefly display in the Smokies

To see some firefly videos plus more photographs like the ones in this article