As dusk deepens the shadows at the edge of the forest, a tiny beacon lights up the darkness. Soon the twilight is filled with drifting lights, each twinkling a message in a peculiar semaphore: ‘Man seeks woman for short union.” This courtship takes place on summer nights around the world between beetles of the Lampyridae family, commonly known as fireflies.
However, the darkness in which fireflies have always pursued their love affairs has been broken by the glare of artificial lights. People’s love of lighting has meant that many of the Earth’s habitable surfaces suffer from light pollution at night. In recent years, scientists who study fireflies have heard from people concerned the insects may be declining, said Avalon Owens, an entomologist at Tufts University.
“There’s that feeling of doom. They don’t seem to be where they used to be,” she said.
So little is known about how fireflies live that it’s difficult to assess whether they’re in danger — and if so, why, said Dr. owens But in a study published Wednesday in the Royal Society Open Science journal, she and Sara Lewis, a professor of biology at Tufts University, shed light on how fireflies respond to artificial lighting. Experiments in forests and fields, as well as in the laboratory, showed that some North American fireflies mate with wild abandon regardless of lighting, while others in bright light do not complete a single successful mating.
Fireflies seem to rely primarily on flashes of light to find each other, meaning light pollution could compromise their ability to see mates. In the four common species examined in the study, females hide on the ground and watch as males migrate through the sky. When a female responds to a male’s blink with her own, the two engage in a dialogue that may result in meeting and eventually mating. In earlier work, Dr. Owens and Dr. Lewis found that shining light on female fireflies of the species Photinus obscurellus made them less likely to respond to male calls.
In a forest west of Boston, the scientists played the role of female fireflies and responded to male Photinus greeni with green LED lights. The lights either stood in the dark or glowed like from a street lamp. The scientists found that more than 96 percent of males preferred darkness. Then, in laboratory experiments with P. obscurellus, they observed that while dim light had little effect on successful mating, none of the firefly pairs mated in brighter light. The insects found each other, some even crawled over each other, but something stopped them from continuing.
“This is really important because we’ve all wasted our time counting lightning bolts, and none of that matters when they’re literally next to each other and not mating,” recalls Dr. owens “It’s pretty worrying.”
She speculates that the fireflies interpret the light as day, waiting to mate in weaker conditions—essentially waiting for a night that never comes.
In a field in Tionesta, Pa., Dr. Owens something that complicated the doom and gloom of the lab experiments. Bruce Parkhurst, a firefly enthusiast who lives in the area, alerted them to the introduction of bright outdoor lighting at a visitor center, so Dr. Owens and her colleagues studied the behavior of local fireflies in the adjacent field.
Over the course of many July nights, they caught and tagged females of two species – P. pyralis and P. Marginellus — and placed them in areas of the field in a spectrum from brightly lit to completely dark. Females in bright areas tended to emerge later and further into the shadows, suggesting that if the insects found the light uncomfortable, they would simply move into the darkness. But even where the light nearly blinded the researchers, fireflies of both species somehow found each other and successfully mate.
“They only mate left, right and center,” said Dr. owens “She doesn’t care at all. It’s crazy to be on the field and see it.
In a group as large and diverse as fireflies — more than 2,000 species worldwide — adapting to different degrees of darkness may mean different responses to light pollution, the researchers suggest. Of the four species in the study, P. obscurellus, the insect that never mates in bright light is also least active in the twilight and prefers the deep night. What doesn’t bother one group at all could destroy another.
Could there be a version of artificial light that’s friendly to all fireflies — a wavelength of light that works for humans and light-sensitive insects? dr Owens has been pursuing the idea for some time, but a generally benign option has remained elusive.
The best solution might be something simpler and more radical: being more aware of outdoor lights and using them more sparingly. While the study suggests that fireflies may be able to flee light pollution into havens of darkness, the nighttime symphony of tiny lights may be a thing of the past if there is no dark place left for them.