Short- and mid-wavelength artificial light influences the flash signals of Aquatica ficta fireflies (Coleoptera: Lampyridae)

Abstract

Urbanization can radically disrupt natural ecosystems through alteration of the sensory environment. Habitat disturbances are predicted to favor behaviorally flexible species capable of adapting to altered environments. When artificial light at night (ALAN) is introduced into urban areas, it has the potential to impede reproduction of local firefly populations by obscuring their bioluminescent courtship signals. Whether individual fireflies can brighten their signals to maintain visibility against an illuminated background remains unknown. In this study, we exposed male Aquatica ficta fireflies to diffused light of varying wavelength and intensity, and recorded their alarm flash signals. When exposed to wavelengths at or below 533 nm, males emitted brighter signals with decreased frequency. This is the first evidence of individual-level light signal plasticity in fireflies. In contrast, long wavelength ambient light (≥ 597 nm) did not affect signal morphology, likely because A. ficta cannot perceive these wavelengths. These results suggest long wavelength lighting is less likely to impact firefly courtship, and its use in place of broad spectrum white lighting could augment firefly conservation efforts. More generally, this study demonstrates benefits of bioluminescent signal plasticity in a "noisy" signaling environment, and sheds light on an important yet understudied consequence of urbanization.

Fig 1. Plastic effects of 533 nm ambient light exposure on firefly flash morphology.

(A) Example of change in intensity of flashes emitted by A. ficta male under different illumination conditions (indicated above each section), recorded in units of average counts per 100 ms over 4 min. (B) Changes in signal intensity over 4 min of separate A. ficta male under the same illumination conditions (indicated above each section).

Fig 2. Effects of exposure wavelength and intensity on firefly flash morphology.

Change in average (± 1 SE) flash intensity (A) and frequency (B) of light signals emitted by A. ficta under illumination by eight wavelengths and two intensities of ambient light (N = 6). Base values for average flash intensity (5420 counts) and flash frequency (43.167 peaks/min) obtained from 1 min dark recordings have been subtracted out.