Antimicrobial Properties of Nest Volatiles in Red Imported Fire Ants

In social insects, antimicrobial secretions are often used collectively for the benefit of the whole colony, which is an important component in social immunity.  Many ant species build nests in which air circulation can be controlled.  Volatile antimicrobial agents would be ideal in implementing social immunity for insects with a relatively closed nest system, because volatiles can readily reach pathogens in a nest even before pathogens contact their host.  Vapor phase antimicrobial property of several compounds in termite gland secretions have been demonstrated individually, but at rather arbitrary concentrations.  Antimicrobial properties of nest volatiles have never been demonstrated for any ant species.  Here we report antimicrobial property of nest volatiles produced by red imported fire ants, Solenopsis invicta Buren, against Beauveria bassiana, a common entomopathogenic fungus.  The germination rate of B. bassiana spores were significantly reduced after they were exposed to nest volatiles within an artificial ant nest.  Since the air that contained the same level of O₂ and CO₂ as that in artificial fire ant nests did not suppress the germination rate of B. bassiana, the observed reduction of germination rate under fire ant nest volatiles was not due to the change of O₂ and CO₂ concentrations in the nest caused by ant’s respiration, but the toxicity of nest volatiles.  Nest fumigation may be an important component of the social immune system in S. invicta.