Effects of entomopathogenic fungi on the mortality and immune system of the leaf-cutting ant Acromyrmex subterraneus subterraneus

Acromyrmex leaf-cutting ants use the antibiotics produced by ectosymbionts (Actinobacteria) to suppress the growth of Escovopsis spp., which are the specialized parasites of attine fungus gardens. However, the spectrum of activity of these bacteria and their interactions with insect immune systems have not been thoroughly studied. The objective of this study was to evaluate whether the presence of symbiotic actinobacteria covering the entire cuticle of Acromyrmex subterraneus subterraneus (Forel) (Hymenoptera: Formicidae) workers provides protection against two fungus species, Metarhizium anisopliae (Metschn.) Sorokin (Hypocreales), a persistent entomopathogenic, and Aspergillus ochraceus G. Wilh. (Eurotiales), an opportunistic insect pathogen. We assessed the mortality rates of three groups of Ac.subterraneus subterraneus workers inoculated with a suspension of one of the two pathogens: external workers (EXT), internal workers with actinomycetes covering the whole body (INB), and internal workers without actinomycetes covering the whole body (INO). We also assessed the encapsulation response, which is a parameter of innate immunity, to determine whether the pathogens affect immune responses in the ant. All bioassays were conducted on workers from seven laboratory colonies of Ac.subterraneus subterraneus. Mortality tests showed that actinomycetes do protect the workers of group INB against M.anisopliae, but not against A.ochraceus. Regarding the encapsulation response, INB ants treated with A.ochraceus conidia exhibited immunosuppression due to this pathogen. All three groups of workers that received M.anisopliae conidia presented some degree of immunosuppression after pathogen exposure. These results indicate that workers without visible bacteria cover, specially the external workers (EXT), are more resistant to entomopathogenic fungi. Nevertheless, bacterial actinomycetes present in young workers are important in protecting them from ubiquitous entomopathogenic fungi.