The Mystery of the Disappearing Fireflies

For as long as humans have observed the night sky, fireflies have illuminated summer evenings with their magical glow. But anecdotal reports from across North America have suggested that firefly populations may be declining in recent decades. To solve this mystery, scientists have leveraged a massive citizen science dataset to uncover the complex environmental factors influencing firefly abundance – and identified climate change as a potential threat.

Fireflies, or Lampyridae, represent over 2,000 species found worldwide. While only a handful produce light, bioluminescent courtship displays make fireflies one of the most charismatic insect families. Yet surprisingly little was known about broad-scale patterns in their populations until now. “Fireflies are challenging to study because different species signal at different times and inhabit diverse habitats,” explains biologist Darin McNeil of the University of Kentucky. “We needed big data to understand the ecological needs of these insects across a continent.”

Enter Firefly Watch, a citizen science project started in 2008 by the Museum of Science in Boston. Volunteers record firefly sightings during evening surveys, providing data on location, date, weather conditions and estimated abundance. Over its decade-long history, Firefly Watch has amassed survey records from 24,683 unique sites across the eastern U.S. “This dataset allowed us an unprecedented view of firefly populations across large spatial and temporal scales,” says McNeil.

 

“Fireflies seem exquisitely sensitive to temperature and moisture levels, which climate models predict will be disrupted by our warming world,”

McNeil and an interdisciplinary team set out to analyze the Firefly Watch data using machine learning algorithms, uncovering patterns in firefly occurrence related to climate, weather, soils, land use, and more. Their findings, published recently in the journal Science of the Total Environment, represent the most comprehensive assessment to date of factors influencing firefly populations in eastern North America.

The researchers first built models to account for detection bias based on factors like date, time of night, temperature and moonlight. They then analyzed how broad-scale climate patterns relate to firefly abundance, mapping hotspots and coldspots across the landscape. Somewhat surprisingly, the best climates overall maximized firefly populations in the upper Midwest, mid-Atlantic and parts of the Great Plains—regions not traditionally thought of as firefly strongholds.

More locally, the team found firefly numbers were strongly driven by complex interactions between soil conditions, weather, and land cover characteristics. For instance, abundances rose with increasing soil moisture but declined sharply in sandy soils unable to retain water. Warmer conditions also supported fireflies, but only up to a threshold—excess heat reduced populations, likely due to impacts on larvae or prey insects.

Notably, results pointed to climate change as a growing threat. “Fireflies seem exquisitely sensitive to temperature and moisture levels, which climate models predict will be disrupted by our warming world,” says McNeil. The team’s analysis suggests some regions may gain fireflies as conditions become more optimal, but overall populations face decline as temperatures surpass tolerable limits.

Some findings challenged past assumptions. Contrary to expectations, the models detected no negative effects of pesticide use after accounting for correlated land cover. And surprisingly, higher abundances occurred in cropland compared to forest—though the researchers note this pattern requires deeper study.

Results did align with several known stressors. Urbanization heavily reduced firefly numbers, as impervious surfaces replaced permeable grounds needed by larvae. And light pollution from artificial night lighting seemed to disrupt courtship behavior by interfering with bioluminescent signals.

“This work provides the empirical support we’ve long needed to evaluate driving factors behind reported firefly declines,” says McNeil. “It suggests conservation priorities around mitigating climate change impacts and preserving dark, moist habitats close to the soil surface.” The team stresses both consistent monitoring and adoption of insect-friendly land management practices will be key to protecting North America’s fireflies into the future.

Citizen scientists nationwide contributed the raw material making this novel understanding possible. “Fireflies are charismatic, easy for anyone to observe—yet their complexity was invisible without big data analyzing population trends across environments,” says McNeil. “This highlights how citizen science can solve ecological mysteries previously out of reach, with implications for better targeting conservation of not just fireflies but many threatened species.”

As night falls each summer, fireflies still signal from meadows, marshes and backyards coast to coast. But with climate pressures intensifying, their future depends more than ever on action. By uncovering keys to the luminous beetles’ survival, this landmark study lights the way for science-based strategies ensuring fireflies continue illuminating childhood wonder for generations to come.

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Reference(s)

1. DOI: 10.1016/j.scitotenv.2024.172329 

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CLIMATE CHANGE | ENTOMOLOGY | ENVIROMENT