Seasonal climatic variability shapes immune responses and infection risks in the common bluetail damselfly.

Understanding how a changing climate influences host-parasite interactions is important to predict disease-driven extinction risks. Insect immune responses are sensitive to seasonal climatic factors such as temperature, humidity, and rainfall, however, the influence of seasonal climatic fluctuations on insect immune responses and parasite prevalence remains poorly understood. To address this gap, we studied seasonal variation in immune response and endoparasite (protozoan gregarine) prevalence (proportion of infection) in Ischnura heterosticta damselflies. Damselflies may experience increased food availability during warmer seasons, providing greater energetic resources for the metabolically costly synthesis of melanin a key component of the prophenoloxidase immune response; therefore, we predicted higher melanisation and lower parasite (gregarine) prevalence in warmer than cooler months. In accordance with our prediction, we found stronger melanisation and lower gregarine prevalence in summer. We further found that melanisation increased with air temperature and decreased with rainfall and humidity while gregarine prevalence decreased with air temperature and increased with humidity and rainfall in females but not in males. Our study provided evidence that natural seasonal variation in climate can impact patterns of immune response, host-parasite interactions and infection prevalence across seasons. While short-term warming during favourable seasons may enhance host immune response, long-term or extreme climate change might disrupt host-parasite relationship by altering resource availability, humidity patterns, or insect thermal limits, thereby contributing to seasonal declines in host populations.