Identification of the Aggregation-sex Pheromone of the Cerambycid Beetle Phymatodes pusillus ssp. pusillus and Evidence of a Synergistic Effect from a Heterospecific Pheromone Component

The longhorn beetle Phymatodes (Poecilium) pusillus ssp. pusillus is a rare, elusive species that is included on Red Lists of threatened species. Previously, 1-hexanol and 1-butanol were reported as putative components of the aggregation-sex pheromone of this species, but behavioral assays to confirm this have not been performed. In this study, we undertook a comprehensive examination of P. p. pusillus to verify the presence of a pheromone. Adult beetles were reared from colonized wood and used for headspace sampling. Analyses by gas chromatography-mass spectrometry revealed that two compounds were present in large quantities in the extracts of males, but absent in extracts from females. Male and female antennae showed repeatable responses to the two compounds in electrophysiological recordings. Using synthetic standards, we were able to identify the compounds as 1-hexanol and 2-methyl-1-butanol. A field bioassay demonstrated that the two compounds were unattractive when applied singly, but elicited significant attraction of female and male beetles when applied in blends of different ratios. We also found that the species exhibited significant attraction to a blend of 3-hydroxy-2-hexanone and 2-methyl-1-butanol, which is the aggregation-sex pheromone of at least two closely related and sympatric species. The presence of the heterospecific component 3-hydroxy-2-hexanone synergized a response to 2-methyl-1-butanol. The pheromone of these species may function as a host cue for P. p. pusillus as the three species have similar phenology and substrate demands. The aggregation-sex pheromone of P. p. pusillus can be used for population monitoring and as a tool to study the general ecology and conservation requirements of this rare species.