Dynamic disease management in Trachymyrmex fungus-growing ants (Attini: Formicidae).

Multipartner mutualisms have potentially complex dynamics, with compensatory responses when one partner is lost or relegated to a minor role. Fungus-growing ants (Attini) are mutualistic associates of basidiomycete fungi and antibiotic-producing actinomycete bacteria; the former are attacked by specialized fungi (Escovopsis) and diverse generalist microbes. Ants deploy biochemical defenses from bacteria and metapleural glands (MGs) and express different behaviors to control contaminants. We studied four Trachymyrmex species that differed in relative abundance of actinomycetes to understand interactions among antimicrobial tactics that are contingent on the nature of infection. MG grooming rate and actinomycete abundance were negatively correlated. The two species with high MG grooming rates or abundant actinomycetes made relatively little use of behavioral defenses. Conversely, the two species with relatively modest biochemical defenses relied heavily on behavior. Trade-offs suggest that related species can evolutionarily diverge to rely on different defense mechanisms against the same threat. Neither bacterial symbionts nor MG secretions thus appear to be essential for mounting defenses against the specialized pathogen Escovopsis, but reduced investment in one of these defense modes tends to increase investment in the other.