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Night light, habitat loss & pesticides threaten Brazil’s bioluminescent insects

  • Brazil’s diverse habitats house a remarkable variety of firefly species, many of which are habitat specialists, thriving in unique ecological niches but vulnerable to environmental changes.
  • A new study from the Cerrado shows a drastic decline in the diversity of fireflies and other bioluminescent beetles in areas affected by habitat loss and pesticide use over 30 years and suggests that ALAN — Artificial Light At Night — might also pose a threat to these insects in the future.
  • Global research has also pointed to habitat loss, pesticide use and light pollution as the main threats to firefly populations, singling out the latter as the fastest-growing threat in southeastern Brazil.
  • While protected areas offer some refuge against habitat loss and pesticide use, the subtler impacts of light pollution combined with a lack of fundamental knowledge about fireflies and other bioluminescent beetles remain ongoing obstacles to effective conservation efforts.

“Be honest with me: Now you want to study fireflies too, right?” said Luiz Felipe Silveira, a professor at Western Carolina University and one of the world’s leading experts in — of course — fireflies.

At that point in our online conversation, Silveira was enthusiastically showing me the results of a study he published with seven other Brazilian scientists in 2022. The article, published in Zoologica Scripta, focused on the evolution of a single genus of firefly (Luciuranus), but it spoke volumes about the diversity of the entire group in the Atlantic Forest.

“Of all eight species we studied here,” Silveira explained, “none co-occur at a fine scale. That is, they can inhabit the same mountain, for example, but they are never really occupying the same spaces at the same time.”

Each firefly lives in its own little niche, a specific set of preferred climatic conditions, such as temperature and humidity. Many species have wingless females, a trait that restricts their mobility and further ties them to their particular environment. They are, as ecologists put it, habitat specialists. Not every firefly fits this lifestyle, but many in South America do. Scientists have catalogued around 350 fireflies in Brazil alone, a number Silveira guaranteed was a drastic underestimation.

Fireflies lighting up the night at Serra dos Órgãos National Park, Teresópolis, Rio de Janeiro. Image courtesy of André Alves.

“Brazil has an incredible diversity of habitats. It is very likely that many endemic species are just waiting to be discovered,” he said.

Amazing as it was to hear about such hidden wealth, it wasn’t their diversity that led me to the interview. “So,” I asked, “all that specialization and low mobility is that what makes them so vulnerable to habitat loss and pesticides?”

“Absolutely,” Silveira confirmed. “And their larvae live in the soil and are frequently associated with bodies of water, which is an extra reason to worry about pesticides.”

As research is starting to show, endemism and specialization come with a cost. These species are very vulnerable to the main threats hitting fireflies in Brazil and beyond: habitat destruction, pesticides and artificial light.

Trouble in the Brazilian Cerrado

Fireflies comprise just one family (Lampyridae) in the roster of bioluminescent beetles — insects capable of producing light using an enzyme called luciferase to oxidize a compound called luciferin. Click beetles (Elateridae) and railroad worms (Phengodidae) are also common sights in the nightscape of many ecosystems, including the Cerrado, a savanna-like biome that dominates central-west Brazil. It’s here that larvae of a particular species of click beetle, Pyrearinus termitilluminans, create a unique spectacle.

The larvae hide on large termite mounds and emit light to attract prey, lighting up the mounds’ silhouettes with bright green dots. A breathtaking phenomenon, but according to Vadim Viviani, a professor at the Federal University of São Carlos and an expert in bioluminescence, a sight that is now becoming rare.

“In the 1990s, we would see many of these termite mounds full of fireflies and other bioluminescent insects, even in areas of pasture,” Viviani said in a press release. “Now, sugar cane is grown in most of the areas and we hardly see any.”

Viviani is the lead author of a three-decade-long survey published in the Annals of the Entomological Society of America last November that found bioluminescent beetles are declining in the Cerrado. The researchers surveyed different areas of the biome, including Emas National Park (ENP) and nearby farms in the municipalities of Mineiros and Costa Rica, in the state of Goiás. Spread over approximately 100,000 hectares (around 247,000 acres), ENP is one of the largest and best-preserved remnants of Cerrado in Brazil.

Bioluminescent larvae glowing in termite mounds to attract prey at Emas National Park. Light pollution from Chapadão do Céu can be seen in the background. Image courtesy of Vadim Viviani.

Over the last 30 years, the researchers recorded 51 species of bioluminescent beetles across all the study areas. Nearly half, 24 species, were found within ENP, making it, unsurprisingly, one of the richest sites. In the 1990s, adjacent farms near the park had similar diversity. Between 1990 and 1996, scientists recorded 23 species at Sta Cruz farm, a property then mostly covered by cerradão (a mix of cerrado and tropical forest), a few pastures and a gallery forest. But by 2000, when the farm changed its name to Bacurí, “large areas of cerradão had already been cut for agricultural and coal production,” according to the study. Between 2010 and 2021, scientists recorded only eight species in the site.

Scientists also found signs of declining populations of railroad worm beetles on the farm. Before 1996, scientists recorded an average of 10 beetles every surveyed night, belonging to nine different species. In 2005, they saw only around three railroad worms a night, of two different species. After 2008, despite 12 nights of effort, the scientists didn’t find a single railroad worm.

“The decline in this family was especially evident,” Viviani said.

But land use change — from pastures and natural cerrado to soy and sugar cane — wasn’t the only problem uncovered by Viviani’s research. Near the headquarters of ENP, something else was affecting the number of railroad worms found. In the first couple decades of the survey, the scientists could attract several beetles such as Euryopa clarindae and Mastinocerus nigricollis using light traps set along the close-by Formoso River. After 2010, though, the scientists caught few railroad worms.

As the habitat remained intact and relatively distant from croplands, the scientists were left with a single suspect: halogen night lamps installed in several spots inside the park. The researchers suggested that this might be the first evidence of ALAN — Artificial Light At Night — affecting adult railroad worms, and worry that this finding may herald a future problem.

Near ENP, three urban centers are expanding: Chapadão do Céu, Chapadão do Sul and Mineiros.

“Until 2000, these surrounding cities were too small to cause any visible effect inside the park,” the authors wrote. “Nowadays, however, the ALAN reaches quite considerable intensities, which are evident mostly during cloudy nights.”

Mineiros, the largest of the three cities, reached 70,000 inhabitants in the 2022 census, a 38.32% increase from 2010.

“The increasing levels of artificial light coming from nearby urban centers at night may threaten several bioluminescent species inside Emas National Park,” Viviani said. “The problem merits special attention and further studies.”

Firefly larvae of the Psilocladus genus. Firefly larvae “live in the soil and are frequently associated with bodies of water,” says Luiz Felipe Silveira. Image courtesy of André Alves.

A terrible trifecta

For some years now, ecologists and entomologists have been warning about the decline of insect species all over the world, and fireflies are no exception to this problem. In 2020, Sara Lewis, professor emerita at Tufts University and an expert in insect behavior, led a different kind of study concerning firefly conservation. She presented firefly researchers all over the globe with a questionnaire, asking them to grade different firefly threats. Out of 49 responses, three winners emerged: habitat loss, pesticide use and light pollution.

It is unsurprising that habitat destruction came out on top, since scientists have identified the destruction of habitat, time and time again, as one of the five major global threats to Earth’s biodiversity.

Pesticide use is also not surprising, as it has been linked to insect decline in many parts of the world. Few studies have tested the direct impact of pesticides on bioluminescent beetles, yielding varied results. While evidence suggests that organophosphate insecticides weren’t very toxic for larvae of the Japanese firefly Luciola cruciata and their prey, a study evaluating the susceptibility of the Korean Aquatica lateralis to 10 different insecticides showed mortality rates jumping by a shocking 80-100%.

Pesticide compounds can be easily carried through air and water, affecting areas adjacent to fields. Viviani and his colleagues, for example, found railroad worms absent from parts of the Bacurí farm that had remained uncleared for agriculture. Despite good habitat, pesticides, carried by the wind from nearby farms, remain the number-one suspect.

Unlike pesticides and habitat destruction, however, light is hardly a typical driver of biodiversity loss. Yet, it was ranked among the three greatest threats to firefly populations worldwide in Lewis’ study.

“Light affects these bioluminescent animals through competition,” explained Stephanie Vaz, a postdoctoral researcher at the Federal University of Rio de Janeiro and coordinator of the South American branch of the IUCN Firefly Specialist Group. “Males and females that rely on a luminous signal to find each other can be rendered basically blind. Larvae that need their light for self-defense or to attract prey can also be affected.”

The small, ephemeral glow these tiny animals emit is no match for a halogen lamp or, perhaps much more seriously, an artificially illuminated night sky.

“There are studies outside of Brazil showing that females can stop glowing altogether in face of so much light competition, or have their circadian habits changed, acting as if it was always day,” said Vaz.

In 2021, Vaz led a study showing that light pollution is the fastest growing potential threat to firefly conservation in the Atlantic Forest. It outpaces urbanization and deforestation in south-eastern Brazil, and its effects are unhindered by the boundaries of protected areas.

“Light pollution can penetrate forests regardless of vegetation loss,” Vaz explained. “They may be able to provide partial shelter, but species living on forest edges — particularly the ones with low mobility, with wingless females or sedentary larvae — can be especially harmed.”

The impact of light pollution seen from Serra dos Órgãos National Park, Teresópolis, Rio de Janeiro. The brighter line crossing the horizon, around 40 km (25 miles) away from the park, is the city of Rio de Janeiro, which forms an urban complex inhabited by 11.8 million people together with neighboring municipalities. Image courtesy of André Alves.

Combating these impacts isn’t easy. Habitat loss and pesticides largely follow agricultural expansion, a trend that threatens many insect-rich ecosystems in Brazil and beyond. Further research and testing may help identify pesticides that are less harmful for these species or tougher regulations could help preserve biodiversity.

“The use of less toxic pesticides might be an option to mitigate their effects,” Vaz said. “Their application should also steer away from critical environmental features, such as groundwater.”

Light is a more delicate matter, as scientists are just beginning to explore its impact.

“There are research efforts trying to identify the kinds — LEDs, fluorescent and so on — and colors of lights, which impact these animals the least,” Vaz said. “The use of lamp covers to direct light downward, reducing spread, could also potentially mitigate the impact of artificial illumination installed in wild areas.”

The first barrier to the conservation of this highly diverse group of beetles, however, is still basic knowledge.

“In order to conserve, we need to know who we are conserving,” Vaz said. “Efforts to identify species are increasing now, and we are putting together a database to understand what conservation measures we can propose for the species that we are discovering.”
Banner image of an adult firefly of the Photinus genus. Image courtesy of André Alves.


Lewis, S. M., Wong, C. H., Owens, A. C., Fallon, C., Jepsen, S., Thancharoen, A., … Reed, J. M. (2020). A global perspective on firefly extinction threats. BioScience, 70(2), 157-167. doi:10.1093/biosci/biz157

Silveira, L., Souto, P., Khattar, G., Takiya, D. M., Nunes, V., Mermudes, J. R., … Macedo, M. (2022). Unlocking the evolution of abdominal specializations in Luciuranus fireflies (Coleoptera, Lampyridae). Zoologica Scripta, 51(6), 708-723. doi:10.1111/zsc.12566

Vaz, S., Manes, S., Gama‐Maia, D., Silveira, L., Mattos, G., Paiva, P. C., … Lorini, M. L. (2021). Light pollution is the fastest growing potential threat to firefly conservation in the Atlantic Forest hotspot. Insect Conservation and Diversity, 14(2), 211-224. doi:10.1111/icad.12481

Viviani, V. R., Rosa, S. P., Prado, R. A., Pelentir, G. F., De Souza, D. R., Reis, R. M., … Costa, C. (2023). Inventory and ecological aspects of bioluminescent beetles in the Cerrado ecosystem and its decline around Emas National Park (Brazil). Annals of the Entomological Society of America, 116(6), 386-403. doi:10.1093/aesa/saad029

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