Fungus-farming ant colonies vary four to five orders of magnitude in size. They
employ compounds from actinomycete bacteria and exocrine glands as antimicrobial
agents. Atta colonies have millions of ants and are particularly
relevant for understanding hygienic strategies as they have abandoned their
ancestors’ prime dependence on antibiotic-based biological control in favour
of using metapleural gland (MG) chemical secretions. Atta MGs are unique in
synthesizing large quantities of phenylacetic acid (PAA), a known but little
investigated antimicrobial agent.We showthat particularly the smallestworkers
greatly reduce germination rates of Escovopsis and Metarhizium spores after
actively applying PAA to experimental infection targets in garden fragments
andtransferring the spores to the ants’ infrabuccal cavities. In vitro assays further
indicated that Escovopsis strains isolated fromevolutionarily derived leaf-cutting
ants are less sensitive to PAA than strains from phylogenetically more basal
fungus-farming ants, consistent with the dynamics of an evolutionary arms
race between virulence and control for Escovopsis, but not Metarhizium. Atta
ants form larger colonies with more extreme caste differentiation relative to
other attines, in societies characterized by an almost complete absence of reproductive
conflicts.We hypothesize that these changes are associatedwith unique
evolutionary innovations in chemical pest management that appear robust
against selection pressure for resistance by specialized mycopathogens.