Dim light pollution prevents diapause induction in urban and rural moths

Light pollution is increasingly affecting biodiversity and may also disrupt seasonal adaptations. Even dim artificial light, such as skyglow-which can spread far beyond urban areas-can interfere with using photoperiod as a seasonal cue.Here, we test how light pollution impacts diapause induction and whether urban evolution counteracts it, by using common-garden experiments with a common, widespread geometrid moth (Chiasmia clathrata). We raised offspring from urban and rural populations from North- and Mid-European countries in treatments with and without dim light at night.The dim light treatment strongly increased direct development overall-with no evidence for urban adaptation to it-but distinctly more so in Mid- than in North-European populations.Because diapause induction is critical for surviving winter, these results indicate that dim but widespread light pollution may have detrimental effects on insect populations, especially so at mid-latitudes, and may hence explain part of the ongoing, large-scale insect declines globally.Synthesis and applications. Latitudinal variation in sensitivity to light pollution means that its contribution to insect declines-and its conservation importance-should likewise vary among regions. In mid-latitude regions, where populations seem more sensitive to light pollution, mitigating light pollution should be a high priority for insect conservation. Reducing skyglow from cities should benefit both urban populations-which were just as susceptible as rural populations in our study-and rural populations-because skyglow extends far beyond the geographic boundaries of cities.