Too Hot to Live? Ectoparasites, Disturbance and Body Size as Drivers of Upper Thermal Limits in Damselflies

The increase in global ambient temperatures poses a challenge for ectotherms. Heat tolerance in these organisms is sensitive to stressors such as parasitism and environmental disturbances (e.g., urbanization). Moreover, the combined effects of multiple stressors acting on thermal tolerance can synergize in a positive or antagonistic manner. In this study, we compared the critical upper thermal limits (CTmax) of Hetaerina americana and H. vulnerata (Zygoptera: Calopterygidae), two damselfly species, under exposure and non-exposure to a nylon immune challenge, while considering average annual temperature, body size, and habitat type (disturbed or preserved), as covariates along a thermal-altitudinal gradient encompassing 10 sites. Additionally, in two disturbed sites, we evaluated the CTmax of the damselfly Argia plana (Zygoptera: Coenaegrionidae) as a function of body size, infection status (infected or uninfected), and water mites’ intensity. No significant changes in CTmax were found after exposure to the nylon implant for both Hetaerina species. However, a negative relationship was found between CTmax and average annual temperature in preserved sites. Furthermore, individuals from disturbed sites exhibited significantly higher CTmax values compared to those from preserved sites. In A. plana, individuals with higher mite intensity and larger body sizes exhibited higher CTmax values. These results suggest that damselfly populations experiencing multiple stressors, such as high ectoparasite intensity and disturbed habitats, may tolerate higher temperatures through immune response activation in response to regional temperature increases.