Inhibition of fungal spore germination by Nasutitermes: Evidence for a possible antiseptic role of soldier defensive secretions

The antifungal property of two of the principal components of the terpenoid frontal gland secretions of nasute termite soldiers was studied by incubat ing spore suspensions of the fungus Metarhizium anisopliae with alpha-pinen e and limonene singly or in combination at different concentrations. In vit ro assays showed that these substances reduced spore germination through di rect and indirect (vapor) contact. To determine if the frontal gland secret ions protected termites from fungal infection in vivo, the effect of M. ani sopliae on the time course of survival of Nasutitermes costalis and N. nigr iceps was studied by exposing termites to either a 4.3 x 10(7) spores/ml or a control sporeless suspension. The caste composition of experimental grou ps was manipulated to create mixed-caste subcolonies and monocaste groups. Relative to Coptotermes formosanus, a species that relies on the mechanical defenses of soldiers, N. costalis and N. nigriceps were less susceptible t o fungal infection. Spore-exposed N. costalis and N. nigriceps termites had 1.2 rimes the hazard ratio of death of controls, while the hazard ratio of death of spore-exposed C. formosanus was 11.4 times that of controls. Alth ough the lower susceptibility to infection in Nasutitermes may be explained in part by the antifungal properties of alpha-pinene and limonene, group c omposition also played a major role in the survival of spore-exposed termit es. Mixed-caste and soldier monocaste groups had 3.4 and 4.7 times the haza rd ratio of death, respectively, relative to the worker monocaste treatment . These results suggest that although Nasutitermes terpenoid secretions may have antifungal properties, the caste composition of groups and the social interactions of termites also play a role in determining susceptibility to fungal infection.