Associations with symbiotic organisms can serve as a strategy for social insects to resist pathogens. Antibiotics produced by attine ectosymbionts (Actinobacteria) suppress the growth of Escovopsis spp., the specialized parasite of attine fungus gardens. Our objective was to evaluate whether the presence or absence of symbiotic actinobacteria covering the whole ant cuticle is related to differential immunocompetence, respiratory rate and cuticular hydrocarbons (CHs). We evaluated these parameters in three worker groups of Acromyrmex subterraneus subterraneus: External workers (EXT), internal workers with actinobacteria covering the whole body (INB) and internal workers without actinobacteria covering the whole body (INØ). We also eliminated the actinobacteria by antibiotic treatment and examined worker encapsulation response. INB ants showed lower rates of encapsulation and respiration than did the EXT and INØ ants. The lower encapsulation rate did not seem to be a cost imposed by actinomycetes because the elimination of the actinomycetes did not increase the encapsulation rate. Instead, we propose that actinobacteria confer protection to young workers until the maturation of their immune system. Actinobacteria do not seem to change nestmate recognition in these colonies. Although it is known that actinobacteria have a specific action against Escovopsis spp., our studies, along with other independent studies, indicate that actinomycetes may also be important for the individual health of the workers.

Actinomycetes bacteria associated with leafcutter ants produce secondary metabolites with antimicrobial properties against Escovopsis, a fungus specialized in attacking the gardens of fungus-growing ants, which denies the ants their food source. Because previous studies have used fungi isolated from fungus gardens but not from ant integument, the aims of the present study were to isolate actinomycetes associated with the cuticle of the Acromyrmex spp. and to quantify their inhibition abilities against the filamentous fungal species carried by these ants. The results demonstrated that actinomycetes had varied strain-dependent effects on several filamentous fungal species in addition to antagonistic activity against Escovopsis. The strain isolated from Acromyrmex balzani was identified as a Streptomyces species, whereas the remaining isolates were identified as different strains belonging to the genus Pseudonocardia. These findings corroborate the hypothesis that actinomycetes do not act specifically against Escovopsis mycoparasites and may have the ability to inhibit other species of pathogenic fungi.

Metarhizium comprises a phylogenetically diverse genus of entomopathogenic fungi. In Brazil, Metarhizium anisopliae s.str. subclade Mani 2 is predominantly isolated from insects, while M. robertsii and M. brunneum mostly occur in the soil environment. Solar radiation and high temperatures are important abiotic factors that can be detrimental to fungal propagules. We hypothesized that among 12 Brazilian isolates of Metarhizium spp., M. anisopliae Mani 2 (n = 6), being adapted to abiotic conditions of the phylloplane, is more tolerant to UV light and high temperatures than M. robertsii (n = 3) and M. brunneum (n = 3). Inoculum of each isolate was exposed to UV-B for up to 8 h and viability evaluated 48 h later. After 8 h under UV-B, most of the isolates had germination rates below 5%. Discs of mycelia were incubated at different temperatures, and diameter of colonies were recorded for 12 days. Mycelia of M. robertsii isolates grew faster at 33 °C, while M. anisopliae and M. brunneum grew most at 25 °C. Dry conidia were incubated at 20, 25 or 40 °C for 12 days, and then viabilities were examined. At 40 °C, conidia of five M. anisopliae isolates were the most tolerant. In the three experiments, considerable intra- and inter-specific variability was detected. The results indicate that conclusions about tolerance to these abiotic factors should be made only at the isolate level.

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 (INØ). 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.