Multilevel variation in pathogen susceptibility and grooming behavior in ant societies.

Ant societies rely on a combination of individual and collective defenses to mitigate pathogen exposure and transmission. Certain morphological, physiological, and life-history traits are expected to influence ant susceptibility to pathogens. In species with an exclusively subterranean lifestyle, such as the yellow meadow ant Lasius flavus, a weakly sclerotized and depigmented cuticle is assumed to increase vulnerability to contact-transmitted entomopathogens, potentially favoring a compensatory investment in grooming behavior. However, little is known about how survival and sanitary behavior are related, and how they vary across biological levels: between species, colonies, or nestmates. In this study, we exposed L. flavus workers to spores of the entomopathogenic fungus Beauveria bassiana and compared their survival with that of five closely related, pigmented, epigeous Lasius species. Although pathogen susceptibility and grooming behavior differed among species, L. flavus workers unexpectedly showed levels comparable to those of most epigeous species. Mortality also varied significantly across L. flavus colonies and between functional groups of workers, with foragers experiencing higher mortality than intranidal workers, likely due to immunosenescence. Across Lasius species, both self-grooming and acidopore-grooming correlated positively with survival, suggesting that these behaviors contribute to pathogen resistance at the interspecific level. Among L. flavus colonies and functional groups of workers, the weaker, non-significant correlation between survival and grooming suggests that other immune traits may prevail at this level. Overall, our findings highlight both individual- and colony-level variation in pathogen susceptibility and emphasize the interplay between immune defenses and behavioral strategies in shaping species-specific adaptations to pathogenic threats.