Artificial light at night, habitat loss, and pesticide impacts in the Cerrado savanna ecosystem of Brazil appear linked to a notable decline in the abundance and diversity of fireflies and other bioluminescent beetle species over three decades, a new study suggests. Shown here are examples of luminescent click beetles and their larvae, known as fireworms: Fulgeochlizus bruchi (A); Pyrophorus punctatissimus overhead view (B), underside view (C), and larva (D); and Pyrearinus termitilluminans adult female (E) and larva (F). (Adapted from image originally published in Viviani et al 2023, Annals of the Entomological Society of America)

By Ed Ricciuti

Ed Ricciuti

Even the grandeur of stars in the heavens on a moonless night pales before the fairy-like blinking of fireflies as they try to attract mates in the darkness. But, their light show is being dimmed by the glare of ALAN, the acronym for “artificial light at night.”

Fireflies—more than 2,000 species of beetles grouped in the family Lampyridae—are increasingly threatened by ALAN, which scrambles their senses and forces them to use more energy to flash more brightly. Many other bioluminescent beetles, notably the rove beetles (Staphylinidae) and glowworm beetles (Phengodidae), are in the same fix. Competition with ALAN comes atop pressure from habitat pollution and loss, plus other environmental stresses that take a toll on these bearers of living light.

A new study published this month in the Annals of the Entomological Society of America documents the decline of beetles that light up the night in a region of Brazil that is notable for them, even in a country with about 500 species, the most diverse assemblage of bioluminescent beetles in the world. For 30 years, a research team has been inventorying and tracking the decline of bioluminescent beetles in the biodiversity-rich Cerrado, South America’s largest savanna and Brazil’s second largest biome after the Amazon.

The most threatened ecosystem in Brazil—even more so than the Amazon forest—the Cerrado has such fertile land that its exploitation sparks less public outcry than that of the Amazon, says Vadim Viviani, Ph.D., the Federal University of San Carlos (UFSCar) and lead author of the study.

The biodiversity-rich Cerrado is South America’s largest savanna and Brazil’s second largest biome after the Amazon. Examples from the variety of habitats illustrated here include the cerrado along the main road of Emas National Park (A), termite mounds field inside the park (B), a marshy field near the forest of the Formoso river (C), the Formoso river and its gallery forest (D), pasture areas followed by gallery forest and sugarcane farm (E), and a road cutting fragments of cerradão at Água Doce farm (F). (Image originally published in Viviani et al 2023, Annals of the Entomological Society of America)

Expeditions by the research team from UFSCar as well as the Federal University of Itajubá and the University of São Paulo focused on two national parks that contain some of the Cerrado’s last original habitats and on farms, ranches, and sugar plantations that surround them. While small farmers and Indigenous peoples have utilized the region’s rich resources productively for people and nature, vast landscapes of Cerrado vegetation has been replaced by industrial-scale farming for soy cotton, sugar cane, and palm oil and cattle ranching. A decline in the number of bioluminescent species was dramatic on ranches and agricultural plantations outside the parks, the researchers found.

They say that producing evidence of the decline was an unintended result. “Although the original purpose of the expeditions was to gain a better knowledge about the biochemical, biological, and ecological aspects of bioluminescent species in Cerrado ecosystem, the collections and observations consistently made in the same places and same period of the year … allowed us to highlight a decline of richness, concomitantly with the replacement of the surrounding original cattle pastures in late 90s by croplands and sugar cane,” Viviani says.

Scattered about some habitats in the Brazilian Cerrado are termite mounds that glow from the bioluminescent click beetles that are squatters in the huge nests. In panel A, artificial light from a nearby town is visible in the sky. (Image originally published in Viviani et al 2023, Annals of the Entomological Society of America)

As the sun set, teams of two to three researchers patrolled the shrub-studded savanna and associated habitats such as the dry forest called the cerradão and wetter tropical and gallery forests. They recorded a total of 51 bioluminescent species. These insects use their living light to seek prey and for defense as larvae and as mating signals when adult. Females of one group of fireflies, scientists have found, even have a fatal “come hither” look. They mimic mating signals of other species and, when males respond, eat them.

The study describes how some protected areas are a bioluminescent wonderland, with fireflies blinking yellow-green, click beetles shining green, and glowworm beetles, well, glowing orange and red. Scattered about were luminous termite mounds radiating light from the click beetles that are squatters in these huge nests.

Where people have destroyed original habitats, however, the glory has faded. A sharp decrease in species was found where sugar cane replaced the cerradão. “In the farms [around the parks], however, a declining number of bioluminescent species was recorded” in places where cattle and sugar cane had replaced dry forest, the researchers write. Glowworms in particular suffered on sugar cane plantations, with males losing attraction to light traps. Artificial light from nearby urban areas also may pose a threat to bioluminescent species within parks.

On one farm, 23 species were collected between 1990 and 1996, while only eight were found from 2010 to 2021. Even within the parks, there were signs of species loss. During the 1990s, individuals of several species swarmed to light traps at a park headquarters. No adult male glowworm beetles showed up after 2010, even though appropriate habitat around headquarters remained healthy. The reason, the researchers theorize, was the installation of halogen lamps around the facility. Their supposition was bolstered by the fact that light traps away from the artificial lighting continued to attract those males, seemingly the first direct evidence that ALAN impacts glowworms.

The condition of bioluminescent beetle populations, especially glowworms, qualifies as a bio indicator of the health of ecosystems they inhabit. “Habitat decrease, and use of pesticides (especially on phengodids), are the major impact factors responsible for bioluminescent beetle decline,” says Viviani.

“Our inventory, published in this manuscript, is the first one regarding this specific group of beetles (bioluminescent), and aims to show what we had in terms of biodiversity in the Cerrado, and what we are losing,” he adds.

The Cerrado originally occupied a vast area in central Brazil. Since the 1980s, however, it has been under severe pressure from agriculture. Viviani recounts that, in the late 1980s when his research team first made expeditions into Emas National Park, a UNESCO World Heritage Site in the Cerrado, much of the region outside the park was already dedicated to pastures and cotton, but large swaths of original Cerrado remained. But since then, pastures have been gradually and almost totally replaced by soy monoculture and sugar cane, with severe reduction of the original Cerrado. The park, Viviani says, “now is just a small island of Cerrado inside an ocean of monocultures.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.