The effects of spectrally distinct nighttime dim light on sperm production in <scp> <i>Drosophila</i> </scp>

Abstract Urbanization imposes a range of abiotic stressors on organisms, with artificial light at night (ALAN) among the most significant. Although numerous studies have shown that ALAN disrupts circadian rhythms and consequently affects a wide range of traits, its impacts on male reproductive capacity and survival, as well as their wavelength dependence, remain largely unknown. To examine how different wavelengths of ALAN affect male reproductive capacity and survival, and whether urban‐derived lines have evolved mechanisms to mitigate such effects, we compared genetically distinct isofemale lines derived from multiple urban and rural source populations of Drosophila suzukii to evaluate the impacts of each wavelength of ALAN on emergence time, wing length and sperm head size. Flies were reared under controlled conditions with red, green or blue light exposure at night. Blue light delayed emergence in individuals from both populations, while red light led to an increase in wing length. Sperm head size decreased under blue light in urban flies and under all wavelength treatments in rural flies. Notably, individuals from urban‐derived lines showed greater variation in emergence time but tended to show reduced variability in wing length and sperm size, suggesting an adaptive strategy to buffer morphological traits while adjusting developmental timing. These findings reveal that the biological impact of ALAN is not limited to blue wavelengths, as even red light, which is generally assumed to have little or no effect on insects due to their limited sensitivity to long wavelengths, can alter reproduction and development, highlighting the complex ecological consequences of urban light pollution.