| dc.description.abstract | Rising temperature is an existential threat to biodiversity, particularly to ectotherms since they cannot regulate their internal body temperature. It can greatly affect the survival of ectothermic animals by altering essential physiological processes like immune responses that normally keep pathogens in check. In this study, we exposed adult red four beetles to five different temperatures (22, 26, 30, 34, and 38°C) for 12 days and infected them with Bacillus thuringiensis (Bt) to observe how different temperatures can affect host immune responses and survival. Host survival showed a significant decline with temperature rise, being the highest (53.13%) at 22°C, and the lowest (10.94%) at 38°C. Moreover, the rate at which the beetles died showed a significant rise as the temperature increased, underscoring a positive correlation between temperature and mortality. There was a significant survival difference between sexes where females were more susceptible to infection. In vitro growth of Bt exhibited a temperature-dependent increase in the growth rate that showed a strong correlation between Bt growth and heightened mortality rate of the host at higher temperatures. The temperature did not have any effect on phenoloxidase and antibacterial activity of the host; however, we found a significantly lower phenoloxidase activity in the females which potentially explains their heightened mortality. Antimicrobial peptides, cecropin-2, defensin-2, defensin-3 are components of the Toll, IMD, and Toll+IMD signaling pathways, respectively, and cactus is a negative regulator of the Toll. RT-qPCR analysis of these genes showed differential expression at different temperatures, although the expression trends were not universal for all genes. Defensin-3 showed a trend of down-regulation at high temperatures. Similarly, cactus was marginally significantly downregulated at 38°C whereas a temperature-dependent rise was observed in hsp27, particularly at 38°C. Collectively, these results show a detrimental effect of temperature on host survival, where high temperatures facilitate faster pathogen-mediated killing by inducing heat stress and weakening the host immune response. In conclusion, lower temperatures substantially increase the chance of host survival by slowing pathogen growth and facilitating a better immune response, thereby benefiting the survival of the insect host. | |
