Functional trait differences between Tribolium species shape responses to the shared pathogen, Beauveria bassiana

Understanding how different hosts respond to a shared pathogen is essential for predicting host-pathogen dynamics and coexistence. In this study, we conducted a series of complementary bioassays to examine how two congeneric species of Tribolium beetles differed in their functional traits when exposed to the entomopathogenic fungus Beauveria bassiana in the absence of competition. The mean fungal concentration required to kill 25% of the beetles and the time to sporulation onset were significantly greater for T. castaneum than T. confusum (9.24% and 23% greater, respectively). There was no difference in infection probability between the two species from the lowest to highest fungal concentrations, but the mean proportion of individuals infected increased by 87% for T. castaneum and 54% for T. confusum. Moreover, the relationship between beetle density and number of individuals killed (i.e., pathogen functional response) fit a Holling type II curve for both species. The two key parameters in the Holling model, attack rate and handling time, did not differ significantly between beetle species. Surprisingly, adult beetles that were exposed to the same range of pathogen concentrations for 24 h did not exhibit a significant reduction in reproductive output (adult offspring produced), likely due to the short timeframe of the study. Together, our results suggest that T. confusum is less resistant to B. bassiana infection compared to T. castaneum. Our findings underscore important interspecific differences in host functional traits and we discuss how variation in host responses to pathogens may influence competitive interactions, coexistence outcomes, and biological control efficiencies.