Artificial light at night has life stage-specific effects on biological rhythms in a specialist insect

Artificial Light at Night (ALAN) has infiltrated many once-dark nightscapes, introducing a novel cue on biological clocks. Recent research has uncovered effects of ALAN on behavior, physiology, and fitness across taxa. However, questions remain about ALAN’s effects on key biological rhythm functions like diapause and sleep, and how disruptions to these rhythms can be linked to fitness declines. Further, most studies test effects of ALAN at the adult stage only, but little is known about the specific effects of ALAN across multiple life stages. We measured the outcomes of ALAN exposure on diapause termination (i.e., adult eclosion), sleep-wake cycles, and fitness-related traits (i.e., survival, body size, and fecundity) in a specialist insect: the walnut husk fly (Rhagoletis juglandis). We tested for life-stage specific effects of ALAN by implementing four experimental groups: pupal ALAN exposure only, adult ALAN exposure only, pupal and adult ALAN exposure, and pupal and adult dark night exposure (control). We found that pupal flies exposed to ALAN eclosed later than dark night pupae. Also, all adult flies exposed to ALAN had altered sleep-wake cycles compared to dark night adults. Further, flies that were exposed to ALAN as pupae only slept more during the transition from day to night compared to control flies. Finally, we found no fitness effects of ALAN. This work highlights the ability of ALAN to impact crucial functions like diapause and sleep, including through pupal exposure that has lasting effects into adulthood.