What sprawling light pollution does to fireflies

As the sun sinks below the horizon, two scientists perch quietly beside a darkening field and wait for flashes of light.

They have traveled far outside the city in search of research sites, because they study a group of creatures that have been driven to the margins of human settlements: fireflies.

Avalon Owens ’13, a fellow at The Rowland Institute at Harvard, and Abby Robinson, a postdoctoral researcher in the Owens lab, have come to this field near Gloucester to observe the mating behaviors of fireflies, a diverse array of insects that evolved over millions of years — when the only glow in the nighttime sky came from the moon and stars. Now these icons of summer nights are dwindling thanks to the modern human’s aversion to darkness.

“We’re trying to understand how light pollution influences ecosystems,” Owens said. “We focus on insects, but the real question is, What is light pollution doing ecologically, and how can we fix it?”

Fading lights

Surprisingly little is known about global firefly populations, which have been largely neglected by conservation efforts.

Part of the larger order of beetles, fireflies have diversified into about 2,500 species around the world. They occupy varied niches from wetlands to grasslands to forests. Different species are active at different times of the night or different times of the year. In North America, some 167 firefly species have been identified.

“Because most people view fireflies as a monolith,” Owens warned, “we are in danger of having this hidden diversity disappear before our eyes.”

Most experts see the firefly’s plight as part of the so-called insect apocalypse. Populations are declining for reasons such as loss of habitat, pesticides, climate change, and invasive species. Another lesser-known threat is light pollution.

Two centuries ago, the advent of electric lighting began to transform our world. As human settlements expanded, the darkness of night gave way to artificial daytime. Stars disappeared from view and the moon dimmed. Nocturnal animals saw their world fundamentally altered.

In the last decade alone, the night sky has become twice as bright. By conservative estimates, nearly half of the United States, 88 percent of Europe, and 23 percent of land outside the poles is significantly polluted by artificial light.

For many nocturnal insects, artificial lights are a fatal attraction. Some studies suggest that one-third of insects drawn to artificial lights die before morning due to exhaustion or predation.

Fireflies are vulnerable for another reason — they depend on darkness for reproduction.

With most North American fireflies, males cruise over their habitat and flash with patterns that uniquely identify their species. Females lie in the vegetation below, flashing responses at males of interest. This dialogue continues until the male finds the female and attempts to mate — or the female loses interest and goes dark.

In studies conducted by Owens, males in illuminated areas flashed only about half as much as they did in darkness, while females went nearly completely dark. Owens suspects this sex difference arises from different points of view: Males look down at the dark ground while females look up into the glare of artificial lights.

Sparks of interest

Owens came to her subject slowly. “I didn’t really like insects as a kid,” she recalled. “I didn’t like being outside, actually. I grew up in Idaho, which has a lot of dark skies — totally lost on me at the time.”

As a Harvard College undergraduate, she concentrated in organismic and evolutionary biology (OEB) and intensively studied Chinese. “I just thought the OEB concentrators seemed really cool and seemed like they were having a really good time,” she said. “I guess it was almost a little aspirational — they seemed outdoorsy. I wasn’t outdoorsy, but I wanted to be.”

After graduation, Owens won a scholarship to pursue a master’s in entomology at National Taiwan University. She also hosted a YouTube channel called Insect Island, where she expounded in Mandarin on entomology and cultural topics and attracted more than 40,000 subscribers.

Afterward, she earned her doctorate at Tufts University and wrote her dissertation on fireflies under Sara Lewis ’76, a biologist who specializes in fireflies and conservation.

Lewis described Owens as a creative scientist working across disciplinary boundaries. “Her research has made a major contribution to understanding the environmental impacts of light pollution, and she has become a leader in trying to reduce this pervasive yet insidious threat to biodiversity,” said Lewis, now professor emerita at Tufts and co-chair of the International Union for the Conservation of Nature’s Firefly Specialist Group. “And on top of all that, she’s a talented mentor and is adept at communicating scientific concepts to the general public.”

In a 2020 study, Owens described how light pollution is driving insect declines. The same year, she joined Lewis and other colleagues in a comprehensive report on threats to firefly species around the globe.

In 2022, Owens won a five-year fellowship at the Rowland Institute, which supports high-risk, interdisciplinary research for promising early-career scientists. She founded a lab devoted to light pollution.

“We study artificial light as a conservation threat,” she explained. “There aren’t very many ecologists doing that right now. You’re seeing more awareness of this as an ecological issue, but it’s still somewhat niche. The nocturnal ecosystem is really ignored.”

In 2024, the Owens team reported that some moths are becoming less attracted to light traps. At one site in Delaware, blacklight traps suggested that corn earworm moths declined precipitously over 25 years, but pheromone traps showed the populations remained stable.

One explanation for this discrepancy is that insects simply were becoming indifferent to light or facing “light competition” from many sources: streetlights, buildings, automobiles, and so on. Because light traps are commonly used to monitor insect populations, these findings raise questions about the accuracy of long-term surveys, and whether the “insect apocalypse” is really as dire as portrayed.

In other words, light pollution has blinded researchers, too. “Maybe it’s interfering with our ability to measure declines,” Owens said, “but I also think it’s a driver of declines based on everything I see.”

Owens spent the first years of her Rowland fellowship studying moths and recently returned to fireflies, the subject of her doctoral work. She seeks to better understand how fireflies are coping with the urban environment — how they have adjusted their distributions, the timing of their activity, and other behaviors.

Last summer, her team examined firefly populations in three urban areas: Boston, Providence, and Worcester, plus rural areas on the outskirts of these cities. On summer nights, two-person teams fanned out for repeated surveys at 14 sites in Greater Boston, including Fresh Pond, the Arnold Arboretum, and a residential property near Central Square in Cambridge.

A few trends already are apparent. In the laboratory, fireflies in illuminated settings flash more brightly — just as birds sing louder when forced to compete with noise pollution. Species that are active at dusk seem more tolerant of light pollution than those requiring complete darkness. “That could explain why the types of fireflies that we see in cities tend to be the dusk-active ones,” Owens said.

She has argued that scientists suffer from “diurnal bias” — a preference for studying species that are active during the daytime — and have overlooked environmental impacts on nocturnal species, particularly the devastating effects of artificial lighting.

In addition to her research focus, Owens advocates for more insect-friendly lighting, such as dimmers, timers, and motion detectors, to ensure lights come on only when humans need them. Fortunately, light pollution is one of the easiest threats to reduce. It can be adjusted immediately and leaves behind no lasting effects.

Searching in the dark

One evening in late July, Owens and Robinson traveled about 45 miles from campus to one of their rural research sites, the Manship Artist Residency near Gloucester, on Cape Ann. The property, featuring two abandoned granite quarries, is the former home of sculptor Paul Manship, whose works included the Prometheus statue at midtown Manhattan’s Rockefeller Center. Manship named his property Starfield — a nod to both its clear views of the celestial sky and its abundance of summer fireflies.

The scientists set up two tripods with GoPro video cameras that record 360-degree views. Afterward, a software system analyzes the video and identifies firefly species by their flash patterns. The cameras are positioned exactly one meter apart, allowing researchers to triangulate the exact position of each flash.

Each night, they follow the same protocol: At the exact time of sunset — 8:03 p.m. on this occasion — they turn on the cameras and keep them running until the sun drops 12 degrees below the horizon. For this trip, that amounted to 79 minutes of recording time.

Their target was the little gray firefly, Photinus marginellus, a dusk-active species found around Massachusetts. Earlier in the summer, they studied another, more elusive nocturnal species called Photinus obscurellus, or the obscured firefly.

These videos document firefly activity at each site, allowing researchers to compare locations and assess how the two species of fireflies are responding to urbanization.

“We had thought maybe we would go into August, but it seems like they’ve been winding down quite quickly,” Robinson said as the researchers sat in a pergola overlooking the study area.

“They may have peaked early because of the heat wave,” Owens added. “But honestly, we don’t have tremendously good data — except for here, actually.”

About 25 minutes after sunset, a glow finally appeared in the brush. Then another flash and another. The little gray fireflies were courting.

Late in the evening — the firefly equivalent of bar closing time — males of some species such as the obscured firefly will add additional flashes to their pattern. “My understanding is that it’s like desperation,” Robinson said. “They’re trying to get the females’ attention, so they add a couple more blinks.”

But the little grays had no such tricks, and soon their lights subsided. A nearly full moon rose above the trees, and the windows of the artist colony glowed in the distance. The flooded quarry loomed like a dark void. The fireflies made their last flashes of summer — and time will tell whether they continue to do so in the years ahead.

By the time the cameras turned off, it was clear that summer firefly season was coming to an end.

In the months ahead, the team will analyze their data and prepare a publication. Owens hopes the findings will shed light on how fireflies manage to persist — or why they fail to — when dark nights are becoming scarce.

“These fireflies are in urban areas where there is light pollution,” she said. “We know that light pollution makes it hard for them to engage in courtship. How can it be that they still exist?”