Salty suitors: High larval sodium intake reduces adult lifespan and influences reproductive behaviour in a lepidopteran herbivore

Abstract One potential predictor of adult reproductive success is physical condition, which can be influenced by earlier developmental stages, such as nutritional limitations during juvenile growth. Sodium is a vital element for animals, essential for physiological function and individual‐level development and behaviour. When animals are sodium‐limited, they at times engage in behaviours such as cannibalism or geophagy to obtain sodium. At high levels, sodium may be toxic and result in adverse physiological effects. How dietary variation in sodium availability during larval stages influences adult mating behaviour and reproductive output has received relatively little attention. Previous studies within Lepidoptera have reported varied results. We used the bordered patch butterfly, Chlosyne lacinia (Geyer, 1837; Nymphalidae), to investigate the role of larval dietary sodium uptake on adult body size, lifespan, mating behaviour and reproductive success. Body size did not differ between treatments. Individuals raised on host plants with high‐sodium concentrations had shorter lifespans than those reared on low‐sodium host plants, indicating that sodium in high concentrations in larval diets can be harmful. Females raised on lower sodium diets took longer to start mating regardless of males' larval access to sodium, but copulation durations did not differ. Males and females raised on host plants with higher sodium concentrations mostly did not pair differently than individuals raised on host plants with lower sodium concentrations and did not differ in their immediate reproductive success. Females raised on high‐sodium host plants avoided mating with males raised on high‐sodium host treatments, suggesting that larval sodium acquisition could influence reproductive decisions made by adults. Reproductive success in insects is shaped by both intrinsic behavioural strategies and extrinsic environmental factors. During mating, individuals make complex choices to maximise fitness, and environmental conditions experienced during juvenile stages—such as dietary sodium exposure—can influence reproductive decisions in adulthood. Understanding the role of sodium in these life‐history traits is increasingly important in light of global ecological change: drought‐driven reliance on saline irrigation is accelerating soil salinisation worldwide, which may in turn disrupt insect–plant interactions and destabilise multi‐trophic dynamics. Collectively, these findings underscore how adaptive behaviours and environmental stressors interact to shape insect populations in a rapidly changing world.