As evening falls, a familiar, ethereal glow begins to illuminate the Southeast’s thick woodlands and rippled mountains. Individually and then in small groups, yellow and green lights begin to appear, flashing at a regular interval in the air or on the ground.
Before long, dozens or even hundreds of ghostlike, glowing dots are floating through the forest like will-o’-the-wisps as much-loved beetles employ a built-in lighting system in search of a mate.
“Fireflies.”
“Lightning bugs.”
“Glow-worms.”
Whatever you call them, these terms all refer to soft-bodied beetles in the scientific family Lampyridae that are best known for their ability to emit a bioluminescent glow.
[Editor’s note: While “firefly” and “lightning bug” are often used interchangeably, folks in Appalachia very definitively lean more towards team “lightning bug, according to a dialect map created by North Carolina State University’s Department of Statistics. For the rest of this article, therefore, we’ll be sticking with “lightning bugs,” which we think is fine as frog hair.]
The Lampyridae family encompasses about 2,000 species found worldwide on almost every continent except Antarctica. About 30 species are found in Tennessee.
Lightning bugs are a ubiquitous and beloved fixture of Appalachia, the region of North America where they tend to be most heavily concentrated thanks to an ideal climate. Nevertheless, various species exist across the continent.
Unfortunately, like all insects, lightning bug populations have been declining. “Where have all the lightning bugs gone?” has become an increasingly common question from people who grew up watching these insects and are now seeing fewer of them in their backyards.
To answer that question, we’re delving into the life cycle of these beetles, the chemical processes that fuel their glow, the threats they face and a few ways humans can help.
A grow-up glow-up
Like most other insects, the lightning bug undergoes a complete metamorphosis throughout its life. Beginning as an egg, it hatches into a larval phase, resembling a grub or maggot. During this time, it lives underground and feeds on small invertebrates.
Next, it pupates before emerging as a fully formed adult, after which it sets its sights on finding a mate to start the next generation.
Lightning bug populations are most concentrated in the Southeast thanks to its climate. During their larval stage, they tend to eat things like slugs, snails and larval cicadas. Those invertebrates all prefer places with wet springs, warm summers and mild winters, which this region has in abundance.
How and why do lightning bugs glow?
These beetles’ bioluminescence, aka their “lightning,” is created through an internal chemical reaction fueled by an enzyme called luciferase. That enzyme reacts with magnesium, adenosine triphosphate (ATP) and oxygen to produce a bright light.
It isn’t just adult lightning bugs that glow. Some species also bioluminesce as larvae, which is where the term “glow-worm” originates. (These should not to be confused with the similarly named — but entirely unrelated — New Zealand Glowworm, which is actually the larval stage of Arachnocampa luminosa, a fungus gnat.)
Adult lightning bugs primarily glow to attract a mate, though they can also use that light to lure prey or confuse predators. Some lightning bugs utilize different coded flashes to communicate with other members of their species.
“They have different patterns that they use to flash at one another to say, ‘Hey, I’m this species of firefly (lightning bug),’ and then a male will come back with ‘Hey, I’m also this species,’” says Senior Entomologist Rose Segbers. “That way they can identify one another.”
Finding another member of the same species isn’t just crucial for reproduction. Some lightning bugs like to eat other lightning bug species, so locating a potential mate that is a good match is also essential for safety.
Still, the system isn’t foolproof.
“It’s kind of like a passcode, but like any biological process, it can be hijacked,” Segbers says. “There are some species of firefly that will mimic the glowing patterns of other species to attract males, and then they will eat them.”
A Common Eastern Firefly.
Where have all the lightning bugs gone?
First of all, it’s not just lightning bugs that are going dark.
“All insect populations are on the decline due to habitat loss,” Segbers explains.
Human development has threatened insect habitats, and a changing climate has led to greater extremes in temperature and weather patterns. These changes impact tree communities and reduce the availability of food for lightning bugs.
As larvae, lightning bugs are voracious eaters, since that’s the life cycle where they’re focused solely on growing.
“A lot of people noticed there were fewer cicadas during this last cicada boom than there were in the past,” Segbers says, referencing the 2024 dual-emergence of Brood XIX and Brood XIII.
“That’s also a side effect of an overall decrease in plant life and mature trees,” she adds.
Since they frequently eat cicada larvae, fewer cicadas means fewer lightning bug larvae making it to adulthood. The slugs, snails and other invertebrates eaten by larval lightning bugs are also affected by extreme swings in temperature and precipitation, further reducing the availability of food.
Since most lightning bugs have multi-year life cycles, these changes can impact their abundance far into the future.
How can people help lightning bugs?
“The thing about insects is that, because they are very small, what you do in a relatively small environment can have a much larger impact on them than it would on, say, a deer, which can be both good and bad,” Segbers says.
One of the best ways to help lightning bugs is to plant native species in your yard and let your grass grow a little bit more than usual since a perfectly manicured lawn is not their ideal habitat. Limiting the use of weed killers and pesticides will also benefit lightning bugs, their larvae and their food supply living in the soil beneath the lawn.
Looking out your window, you may have seen plenty of lightning bugs glowing above your yard this year and wonder what all the fuss is about.
This year seems to have brought a resurgence of lightning bug populations fueled by a mild winter, wet spring and hot summer. There’s no guarantee that this trend will continue in the future, however, and it needs to be reliably tracked to be understood, Segbers says.
“We can’t really fix what we can’t measure,” Segbers says. “So there are some citizen science projects that people can do to help measure firefly abundance.”
iNaturalist is an app used for monitoring populations for practically every species on the planet, and it’s a great starting point for anyone who wants to help provide data to scientists studying lightning bugs.
There’s also Firefly Atlas, a collaborative effort launched in 2022 to track lightning bug distribution in the United States and Canada. Users can report sightings to help scientists better understand how the species’ populations are changing over time.
