Narrow-spectrum artificial light silences female fireflies (Coleoptera: Lampyridae)

The rapid spread of artificial light at night (ALAN) poses myriad problems for nocturnal biodiversity. Bioluminescent taxa that communicate via glowing lures, warning signals, or courtship advertisements may have their signals obscured by artificial illumination, unless the added light is spectrally tuned to reduce its impact. Previous research has demonstrated that broad-spectrum white light inhibits the production of courtship advertisements by male firefly beetles, yet the impact of varying wavelengths of artificial light on both members of the courtship dialogue is largely unknown. We exposed courting pairs of Photinus obscurellus fireflies to five colours of downwelling illumination, at two intensities, and recorded changes in male flash rate and flash intensity, female responsiveness, and the flash pattern composition of both sexes. All artificial light treatments significantly suppressed courtship activity. Bright amber light had the greatest impact, suggesting that ALAN which most overlaps the spectrum of firefly bioluminescence will most disrupt firefly courtship. Our findings contradict similar investigations of non-bioluminescent taxa and raise concerns about the possibility of a universal standard for ecologically sustainable ALAN.