UNIVERSITY PARK, Pa. — Catching fireflies is an iconic summer experience for many people living in North America, but the flickering beetles are on the decline. New research has identified factors that may be contributing to declining populations.
The study, which is available online now and will be published in the June 15 issue of Science of the Total Environment, suggests that fireflies are sensitive to various environmental factors, from short-term weather conditions to longer climatic trends.
The team — including researchers from Penn State, the University of Kentucky, Bucknell University and the U.S. Department of Agriculture Agricultural Research Service — used advanced artificial intelligence-based machine learning techniques to analyze more than 24,000 surveys conducted through the citizen science initiative Firefly Watch, now called Firefly Atlas, to arrive at their conclusions.
The researchers said the study marks the first comprehensive attempt to apply a data-driven approach to understanding firefly population dynamics on a continental scale.
Study co-author Christina Grozinger — Publius Vergilius Maro Professor of Entomology and director of the Insect Biodiversity Center in Penn State’s College of Agricultural Sciences — said while scientists previously knew that urbanization, agricultural intensification and climate change can affect biodiversity, less was known about how these complex factors interact and what people can do in their own backyards, towns and cities to support biodiversity.
“In this study, we integrated large-scale datasets on species abundance, land use, soil type, weather and climate using AI-based machine learning models to precisely model and predict firefly abundance patterns at the local scale across the eastern U.S.,” Grozinger said. “We were very fortunate to have a multi-year citizen science data set that spanned more than 24,000 observations.”
Fireflies thrive in temperate conditions, the researchers said, with wet and warm summers creating the ideal breeding environment and cold winters supporting the survival of immature stages such as eggs, larvae and pupae.
However, as global temperatures rise, these conditions become less predictable and, often, less hospitable. Changes in precipitation patterns, another critical factor for firefly survival, have led to either overly dry conditions that reduce larval survival or excessively wet conditions that can flood breeding grounds and disrupt life cycles.
“Subtle changes in climate patterns, especially related to temperature, are significantly impacting firefly breeding cycles and habitat quality,” said Darin McNeil, study principal investigator and an assistant professor of wildlife ecology and management at the University of Kentucky who began the study while completing a postdoctoral fellowship at Penn State in Gronzinger’s lab.
McNeil said that artificial lights at night could disrupt firefly populations, possibly affecting both adult and larval stages.
“Firefly larvae, which live in the soil, are particularly vulnerable to changes in light exposure, and artificial light could alter their developmental cycles and survival rates,” he said. “Firefly larvae, which are predatory, also require moist soil conditions because the humidity supports soft-bodied invertebrates like snails and slugs that firefly larvae use as prey.”
Urban growth, which includes establishing impervious surfaces such as sidewalks, buildings and roads, poses a significant threat to firefly populations by invading natural habitats and decreasing available breeding areas. Light pollution from streetlights and commercial signs is particularly disruptive, as it interferes with the fireflies’ bioluminescent communication that is essential for mating.
The researchers found that fireflies are markedly less common in areas with significant nighttime light pollution and that the decline in firefly populations is not uniform across all species or regions. Some species adapted to dryer environments or those with specific breeding patterns may be less affected by certain changes, whereas others are more vulnerable. This highlights the complexity of natural ecosystems and the need for tailored conservation strategies.
The research team also made significant strides in identifying potential conservation measures to mitigate these impacts. These include reducing light pollution, preserving natural habitats and implementing wildlife-friendly agricultural practices that support fireflies.
“In the future, further research is encouraged, particularly in exploring the long-term trends of firefly populations and the efficacy of various conservation strategies,” McNeil said. “Focused studies that survey particular firefly species, rather than the firefly community as a whole, will be important. This approach will be crucial in ensuring that future generations can also enjoy the natural wonder of fireflies lighting up the night sky.”
To create similar large-scale data sets for other insect species, the research team is leveraging technology and AI to develop automated monitoring systems through a U.S. National Science Foundation-funded INSECT NET graduate training program at Penn State.
Those interested in learning more about the land use, weather and climate conditions in their locations can use Penn State's Beescape tool, which also provides location-specific habitat quality scores for pollinators.
The project team also included Sarah Goslee, USDA-Agricultural Research Service; Melanie Kammerer, EcoData Tech-nology; Sarah Lower, Bucknell University; and John Tooker, Penn State.
Penn State’s Insect Biodiversity Center, the Huck Institutes of the Life Sciences, and the College of Agricultural Sciences via the National Institute of Food and Agriculture Hatch Appropriations provided funding for this project.