Effects of single and combined exposure to artificial light at night and an insecticide on a leaf beetle

Although multiple environmental stressors often co-occur and interact in complex ways, most research to date has focused on the effects of single stressors on organisms. The extent of artificial light at night (ALAN) and the use of pesticides have rapidly increased in past decades. Individually, both factors have been shown to elicit changes in animal behaviour and to impair fitness-related traits, while effects of combined exposure have, to our knowledge, not yet been assessed. Here, we used a full factorial design to determine the effects of single and combined exposure to ALAN (25 lux) and a sublethal dose of the pyrethroid insecticide γ-cyhalothrin on the leaf beetle Phaedon cochleariae. We tracked developmental time until adult eclosion, food consumption by larvae, adult mass, survival, mating latency, egg number and hatching rate of the unexposed offspring. ALAN alone led to a higher female mass, but a lower male mass and offspring hatching rate. Under insecticide exposure alone, the developmental time of individuals was prolonged and the survival reduced. Furthermore, during insecticide exposure food consumption by larvae was reduced, but it increased after exposure at least in females, suggesting sex-specific compensatory feeding. Contrary to our predictions, we mostly found additive effects of the combined exposure, but an interactive effect on female relative consumption rate. This may be due to adaptation to the stressors, masking effects due to early mortality or behavioural avoidance of stress exposure. Overall, our results highlight the need to further examine combined stressor scenarios to develop effective mitigation strategies.