Antifungal activity of a termite queen pheromone against egg-mimicking termite ball fungi

Abstract

The sophisticated colony organization of eusocial insects is attributed to their elaborate chemical communication systems. Pheromones mediate most behaviors involved in colony organization including foraging, defense, brood care, and caste regulation. The number of candidate compounds available to regulate multiple systems may be biosynthetically finite and the production of several compounds instead of a single one may be more costly. Therefore, strong selection pressures encourage the use of single natural products for many purposes. Such versatility of signal substances is especially characteristic of queen pheromones in eusocial Hymenoptera. However, little is known about the multifunctionality of the recently identified termite queen pheromone. Here, we demonstrate that volatile compounds in the queen pheromone of a termite, Reticulitermes speratus (Kolbe), have fungistatic properties. Application of the pheromone compounds n-butyl-n-butyrate and 2-methyl-1-butanol significantly reduced the germination rates of the egg-mimicking parasitic termite ball fungus. These pheromone compounds also suppressed mycelial growth of the termite ball fungus and some entomopathogenic fungi. However, the inhibitory activity of each substance differed among fungal strains. Termites likely employ these antimicrobial volatiles to protect eggs and queens, and secondarily as communication agents informing queen fertility. This study supports the notion of evolutionary parsimony, wherein pheromones are originally used as defensive compounds and their communicative function develops secondarily, which is well-documented in social Hymenoptera.