The future of fireflies on a light-polluted planet

For the overwhelming majority of evolutionary history, the brightest sources of light on earth have been the sun, the moon, the stars, the sky, and sometimes fireflies. Two centuries ago, the development of electric lighting sparked a transformation of our visual environment that recent advances in LED technology have only accelerated (1). Inexpensive, ultrabright artificial lights now extend the day, eclipse the moon, and obscure the stars even in nominally protected areas (2), effectively eliminating swaths of suitable habitat for the over half of all animal species known to be active at night (3).

As the Milky Way has faded from view (4), many of these species too have vanished from places they were once found (5). But could light pollution really be to blame? When I began my Ph.D. research in 2016, the environmental costs of light pollution were nebulous. We had plenty of anecdotal reports of individual-level responses to artificial light, often ominously maladaptive: moths self-immolate in candle flames, seabirds strike lighthouses, deer freeze in headlights, baby sea turtles crawl into oncoming traffic… But it was hard to know how worried to be because the population-level consequences remained unclear. Similarly, while recent research had shown that male fireflies flash less around artificial light (6), we knew almost nothing about what this meant for the future of these charismatic insects, nor what to do about it.

The firefly displays that amaze us (7) are constellations of flying males, each patrolling a patch of habitat while producing chains of species-specific flash patterns that act as courtship advertisements. Sedentary females concealed in the grass below flash back at males that spark their interest (8). Their courtship dialog continues until the male finds and tries to mate with the female, or the female loses interest and goes dark. Male fireflies are frequently present in excess and attempt to mate with almost anything that looks good—including blinking LEDs and their top predators (9)—making their response to artificial light less salient. Instead, female fireflies decide whether mating will occur and so determine the fate of future generations.

I have documented the impact of artificial light on multiple aspects of firefly courtship and reproduction, my primary goal always being to understand how light pollution mitigation can aid firefly conservation. Here are some of my findings.

First, we have been blinded by male bias. I filmed firefly courtship dialogs under dim and bright downwelling light of five colors (cool white, warm white, blue, amber, and red) and immediately saw that by only observing the impact of artificial light on firefly displays (again, mostly males) we had massively underestimated its consequences for species as a whole (10). The males flashed about half as much under illumination as they did in darkness, which was in line with what I and others have seen in the field. But the females went almost completely dark. They recovered some amount of interest under dim blue and red illumination but were still much less inclined toward flirtation than they had been in darkness.

Why are female fireflies so much more sensitive to artificial light? I believe it is because males are generally looking down, hunting for female flashes on the ground, while females spend their evenings looking up at the stars. Male flashes superimposed against downwelling light may be completely undetectable and at minimum should appear much dimmer, i.e. less compelling. Females may then prefer to wait to meet the right male under the right conditions, while males continue to self-advertise (if with less enthusiasm) in the absence of any form of encouragement whatsoever. Their persistence is misleading: if you see a nice firefly display in a brightly lit park, you are likely to think that population is doing well—but in a few years it could just as easily vanish completely.

We have also been tricked by the firefly mimicry complex. There are 167 firefly species described from North America (11); I usually see at least five in a night. By light of day, individuals of different species appear essentially identical, having converged on a red, black, and yellow motif that warns predators of their toxicity (12). At night, they distinguish themselves through diverse repertoires of species-specific flash behaviors as well as temporal partitioning: some species are active right at sunset, while others emerge only in complete darkness.

These two groups do not respond to light pollution in the same way. I found that artificial light disrupts the synchronous displays of nocturnal Photinus carolinus fireflies (13) and impedes courtship and mating in nocturnal P. obscurellus (10, 14), but does not meaningfully affect dusk-active P. pyralis or P. marginellus (14). It is possible that these and other dusk-active species might even benefit from life in eternal twilight. If so, it will doubtless come at the cost of their nocturnal counterparts. Because most people view fireflies as a monolith, we are in danger of having this hidden diversity disappear before our eyes, with few able or willing to notice.

But we can fix this if we try. Fireflies are small, but as flagship species (15) for light pollution mitigation they have potential to cast a very large shadow indeed. It would be a shame if well-meaning dark sky conservation efforts unintentionally snuffed them out. I tested a range of “dark sky-friendly” artificial lights in my studies, including amber ones approved by the International Dark-Sky Association, and found that fireflies are more disturbed by amber light than by light of most other colors—including standard white light!—probably because it most resembles the color of their bioluminescence.

I have since collaborated with the Xerces Society on firefly-friendly lighting guidelines (16) that emphasize win-win mitigation measures such as dimmers, timers, and motion detectors. If we can only ensure that our lights are not on when and where no one but the fireflies are there to see them, we might be amazed and inspired by firefly displays for generations to come. Better yet, our stewardship will benefit all creatures in need of dark nights.
 

1. Kyba et al. 2023. Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022. Science.

2. Guetté et al. 2018. Worldwide increase in Artificial Light At Night around protected areas and within biodiversity hotspots. Biological Conservation.

3. Hölker et al. 2010. Light pollution as a biodiversity threat. Trends in Ecology and Evolution.

4. Falchi et al. 2016. The new world atlas of artificial night sky brightness. Science Advances.

5. Hallmann et al. 2017. More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS One.

6. Owens et al. 2022. Behavioral responses of bioluminescent fireflies to artificial light at night. Frontiers in Ecology and Evolution.

7. Lewis et al. 2021. Firefly tourism: Advancing a global phenomenon toward a brighter future. Conservation Science and Practice.

8. Lewis and Cratsley 2008. Flash signal evolution, mate choice, and predation in fireflies. Annual Review of Entomology.

9. Souto et al. 2019. How to design a predatory firefly? Lessons from the Photurinae. Zoologischer Anzeigerool

10. Owens and Lewis 2021. Narrow‐spectrum artificial light silences female fireflies. Insect Conservation and Diversity.

11. Fallon et al. 2021. Evaluating firefly extinction risk: Initial red list assessments for North America. PLoS One.

12. Berger et al. 2021. Insect collections as an untapped source of bioactive compounds: Fireflies as a proof of concept. Insects.

13. Owens et al. 2022. Costs and benefits of “insect friendly” artificial lights are taxon specific. Oecologia.

14. Owens and Lewis 2022. Artificial light impacts the mate success of female fireflies. Royal Society Open Science.

15. Jepson and Barua 2015. A theory of flagship species action. Conservation and Society.

16. Fallon and Owens 2020. Conserving the jewels of the night: Firefly friendly lighting practices. Xerces Society for Invertebrate Conservation.