Immune response and survival of a leafcutter ant with a compromised immune system during an entomopathogenic challenge

Social insects employ diverse strategies, including individual and collective sanitary care as well as colony-level hygiene, to prevent, manage and control pathogen transmission. The immune system constitutes a key component of the individual defensive repertoire, but its activation entails trade-offs with other life history traits due to its high metabolic cost. In this study, we investigated how different immune strategies interact in social insects. We challenged workers of the leaf-cutting ant Atta cephalotes with an entomopathogen to compare hygienic behavior and survival rates in the presence and absence of a previously activated immune encapsulation response. Entomopathogenic-challenged ants: (a) increased the production of infrabuccal pellets, formed by collecting detritus and fungal conidia and mixing them with antimicrobial compounds as a prophylactic behavior to avoid conidia germination; and (b) exhibited a reduced encapsulation response. Although the encapsulation response of insects is typically initiated within a few hours after infection, in our experiment neither pellet production nor encapsulation response depended on the timing of the prior immune challenge. Moreover, simultaneous activation of the immune system with both a foreign body (nylon implant) and entomopathogenic fungal conidia significantly reduced survival rates. Our findings suggest that when the ant immune system is chronically challenged to encapsulate a foreign body, its defensive capacity against a fungal pathogen is weakened.