Can caffeine improve the survival of the ant Formica fusca under pathogenic infection by the fungus Beauveria bassiana?

Caffeine, a widely occurring plant alkaloid, has been shown to exhibit antimicrobial properties and influence insect behaviour. This study investigated the potential of caffeine to mitigate the effects of the fungal pathogen Beauveria bassiana in colonies of the ant Formica fusca . Using field-collected colonies, we examined the impact of a naturally relevant caffeine concentration (100 ppm) on ant survival, foraging behaviour and gene expression under fungal exposure. Caffeine consumption delayed peak mortality and altered foraging dynamics in exposed ants, although it did not prevent fungal spore germination at the tested concentration. Transcriptomic analysis revealed significant upregulation of immune- and metabolism-related genes in exposed ants, alongside changes in genes linked to feeding behaviour and symbiotic interactions. While caffeine was a deterrent to sham-treated ants, exposed ants did not exhibit avoidance/preference. Additionally, caffeine demonstrated antifungal activity at higher concentrations, suggesting potential disruption of fungal development. These findings highlight the complex interplay between dietary compounds, pathogen defence and ant behaviour, emphasizing the role of plant secondary metabolites in shaping insect–pathogen interactions.