Mitigating the light pollution problem via spectral adjustment: color-biased phototaxis in male glow-worms

Light pollution is a widespread threat to dark-adapted species. Attraction to artificial light can have severe, even fatal, consequences with population level repercussions, especially for nocturnal insects. Since most insects are particularly drawn to short-wavelength light, the recent switch to LEDs with a greater emission in the blue range has exacerbated the problem. One potential mitigation measure is to adjust light spectra toward longer wavelengths. However, the effectiveness of this measure may vary among species that differ in their spectral sensitivities. In glow-worms (Lampyris noctiluca), nocturnal beetles with presumably declining abundance, especially blue and white artificial light hamper males' search of yellowish-green glowing females. Here, we assessed whether easing the search by switching to longer wavelengths impacts males' phototactic behavior. We recorded the movements of males in an arena illuminated at one end by either white, yellow, red, or no artificial light. Males displayed positive phototaxis toward yellow and red light, and negative phototaxis toward white light, with the latter also being associated with reduced activity. Therefore, males' attraction to longer wavelengths that resemble the female glow could be an evolutionary trap in human-modified environments with artificial lights of that color range. These results show that different wavelengths can influence disparate behaviors even within a single species and adjustment of outdoor lighting systems, by filtering out the blue part of the spectrum, has limited capacity to solve the light pollution problem.