For insects that glow, artificial light is killing the vibe

Bella Isaacs-Thomas Bella Isaacs-Thomas

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Artificial light is bad news for a wide range of wildlife, but it can be a particular challenge for a unique subset of species: those that make their own glow.

“With bioluminescent creatures, you can make a really direct line between what light pollution does and their survival,” said Avalon Owens, a Rowland Fellow at Harvard University who studies fireflies and other insects.

Though humans have been lighting up the darkness for a long time, it’s only fairly recently – with the advent of electricity and, later, LED bulbs – that brighter nights have started to cause significant consequences for the natural world, including animals that aren’t bioluminescent. Artificial light can meddle with important behaviors and biological cycles – like hunting or circadian rhythms – that are informed by light exposure.

“The cycles of light and dark have been so stable for so long that [wildlife] don’t have a response to suddenly having incredibly bright lighting in the middle of the night,” said Jeremy Niven, a professor of zoology at the University of Sussex.

There is plenty of glittering marine life, but the phenomenon on land is largely limited to bioluminescent fungi and a handful of animals. That includes glow-worms and fireflies, which belong to the same taxonomic family, Lampyridae.

READ MORE: What too much artificial light steals from our night skies

For both of those insects that depend on bioluminescence to mate, recent research shows that artificial light can disrupt courtship.

Since humans are the problem, we could also choose to be the solution. Although fully addressing light pollution would require a lot of large-scale change, there are some steps individuals can take to make a difference in their homes and communities.

Glow-worms and fireflies possess the same biochemistry. They produce an enzyme called beetle luciferase that fits together with a smaller molecule called luciferin, said Sara Lewis, professor emerita of biology at Tufts University and co-chair of the International Union for Conservation of Nature’s Firefly Specialist Group. That fitting together kicks off a process that chemically excites the smaller molecule, which then gives off light as it relaxes.

The common European glow-worm is a fairly typical sight across southern England, and its range extends through parts of Europe and into northern China. Their name is a bit of a misnomer. Niven noted that although the females do resemble “a big maggot,” glow-worms (and fireflies, for that matter) are actually beetles.

Animation by Megan McGrew/PBS NewsHour

They prefer open, grassy habitats where the females’ soft green glow can more easily be spotted by males, Niven said. In his region, he added, glow-worms can be spotted anywhere from farmed fields to public parks and even golf courses. When night falls, female glow-worms stay put on the ground as their abdomens glow for several hours, while males take flight in search of them.

The female glow-worms Niven studies don’t have much control over their glow, but fireflies are a different story.

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There are around 170 firefly species in the United States and Canada, and though they all look similar to the naked eye, “there’s this incredible hidden diversity” in their lifestyles, communication methods, flying abilities and what time of night they emerge, Owens said. They use different precisely timed flashes to telegraph their sex and species, Lewis noted.

Male fireflies in flight give off an advertising flash with the hope that a female looking up from the ground will signal interest with a flash of her own, Lewis said. After that, the male will flash again, and the female may or may not respond.

This exchange of signals – what researchers call a flash dialogue – “is unique to the lightning bug fireflies,” Lewis said.

So how do the bright lights that dot our world interrupt these insect love fests? Niven and his colleagues published a paper in June that explored the ways light at various intensities affects male glow-worm behavior. They placed these males in a Y-shaped maze with a faux LED female glowing green in one prong and a white light in the other.

They kicked the experiment off in total darkness, save for the green glow – conditions under which the males had no problem finding their fake mate. They then upped the white light incrementally before returning to pitch black. As the brightness increased, the researchers observed multiple effects on the males, who became increasingly unmotivated in their quest for the female, Niven said.

“It’s not just that they’re not reaching the females, it’s that even when they reach the females, they take longer to do so,” he said.

A firefly female is marked with fluorescent powder in preparation for tracking her movement and mate success in the field. Photo courtesy Avalon Owens

One observation particularly surprised the researchers. Male glow-worms have a head shield that’s not in use when they’re looking for a female. But in the presence of artificial light, males often pulled their heads in and stopped trying to search for a mate entirely.

“One of the things that we really noticed with the increasing light was that they simply stopped moving and literally kind of tucked their head underneath their head shield just to protect themselves from the light,” he added. “Rather than making an incorrect choice, they just stopped and stayed where they were.”

For fireflies, various colors of artificial light have different effects — reds and blues are more tolerable, while whites and ambers are more disruptive, Owens said. But females go dark completely when lights are switched on overhead, she noted. When males don’t see any response flashes, they get “dejected” and start flashing less themselves.

“Because the females stop responding, the whole courtship dialogue totally breaks down,” Owens said, adding that there’s evidence that when the fireflies “don’t flash at each other, they don’t mate.”

Owens conducted an experiment that involved putting fireflies in close quarters with one another under a few different lighting conditions. Under bright lights, males often walked over females but didn’t attempt to mate — instead, she said, they appeared to totally lose interest.

It’s possible that the light caused temporal disorientation (i.e. they didn’t know what time of day it was), which interfered with the fireflies’ circadian rhythms by tricking the males into believing environmental conditions weren’t right for “courtship and reproduction,” Owens explained.

Not too long ago, the light sources we used at night were much dimmer than the ones we have now. Compared to the candles and oil lamps of the past, artificial lights today are bright, easily distributable and able to produce a lot of light with very little energy, Niven said.

“We can have solar-powered lights in our gardens to illuminate paths. We can have lighting on the side of our houses to illuminate our patios or backyards. So it’s everywhere in a way that it wasn’t [before],” he added.

Some research suggests that glow-worm populations are going extinct in parts of England, which Niven noted isn’t particularly surprising given the documented effects of artificial light.

It’s harder to make a definitive claim about firefly numbers given a lack of quantitative research, Owens said. There’s evidence that fireflies that are active at dusk are less sensitive to light pollution compared to species that prefer total darkness. These are known as crepuscular species (as opposed to nocturnal species), like the Big Dipper firefly, one of the most common species in the United States.

Not too long ago, the light sources we used at night were much dimmer than the ones we have now. Photo via Getty Images

But there are anecdotal reports about some populations dwindling or even disappearing in certain areas, noted Lewis. Light pollution is the second greatest threat to fireflies, just after habitat loss, she added.

“For these nocturnal fireflies, [if] you put up a streetlight or a security light on somebody’s house, there aren’t going to be any fireflies,” Lewis said. “If they can move out of that area, they will.”

READ MORE: This global challenge invites people to document the wild side of their cities

If you want to help make your local habitats more firefly-friendly, less light is always better, Owens said. It’s easy to get caught up in which light colors are better or worse for insects, but she warned that most LED lights are simply too bright. Red light may generally be less offensive to fireflies, but a “super bright red LED” is still going to do damage.

“There’s no good light, period,” Owens said. “And if you’re going to have one, have it on when you’re there, [and] turn it off when you’re gone.”

Dark sky reserves are places that offer protection for nocturnal ecosystems and stunning views of starry skies, and are certified through the International Dark Sky Association. There’s one close to Niven’s workplace that he said offers an “interesting intersection of people” who are passionate about either stars or glow-worms.

In the dark-sky movement, using lamp shades to direct light downward is a common piece of advice for helping reduce sky glow, Owens added. But that doesn’t help our insect friends on the ground.

Lewis said that researchers have learned a lot about the threats facing fireflies over the past five to 10 years, and the fact that they’re widely admired makes them a useful “gateway bug” to get people interested in insect conservation. These unique creatures are not only transfixing to watch on a hot summer night, but predate humans and have survived for hundreds of millions of years.

“How amazing is that, that we still have them?” Owens said. “It’s really pretty magical and worth preserving.”

Left: Artificial light can interfere with important ecological systems, like firefly courtship. Photo via Getty Images

By Bella Isaacs-Thomas

By Isabella Isaacs-Thomas

By Isabella Isaacs-Thomas

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Bella Isaacs-Thomas is a digital reporter on the PBS NewsHour's science desk.