Chemically armed mercenary ants protect fungus-farming societies

We document the behavioral interactions among three ant species: a fungus-growing host ant, a permanently associated parasitic guest ant, and a raiding agro-predator ant. We show that the presence of guest ants becomes advantageous when host ants are attacked by raider ants, because guest ants use alkaloid venom to defend their host ant colony. Furthermore, detection of the guest ant odors is sufficient to discourage raider scouts from recruiting nestmates to host colonies. Guest ants likely have evolved this protective behavior because they also perish when their host colony dies.