Recruitment of minor workers for defense against a specialized parasite of Atta leaf-cutting ant fungus gardens

Social insects that obligately depend on mutualists are known to defend both themselves and their partners from exploitation. One example is leaf-cutting ants, which defend the mutualistic fungus they cultivate for food from potentially virulent specialized microfungal parasites (genus Escovopsis). Mechanisms employed by the ants to reduce the impact of Escovopsis include grooming the mycelium of their fungal cultivar and weeding out infected parts of the fungus garden. These behavioral defense tasks are performed by specific size castes: minor and major workers, respectively. While minor workers are efficient at removing Escovopsis spores through fungus grooming, they are significantly less mobile than major workers. This implies that if major workers were to facilitate the movement of minor workers from uninfected to infected sites within colonies, defense against Escovopsis potentially would be faster, and consequently more efficient. Using dual-chamber sub-colony set-up experiments, we explore the role major workers play in the movement of minors from uninfected to infected chambers in two leaf-cutting ant species, Atta cephalotes and A. sexdens. We find that the presence of major workers results in an increase in the movement of minors when gardens are infected with Escovopsis. This suggests that minor workers may be recruited to help defend the fungus garden from infection, providing an example of how colony social structure influences defense against pathogens in a social insect.