Enantiomeric conservation of the male-produced sex pheromone facilitates monitoring of threatened European hermit beetles (<i>Osmoderma</i> spp.)

Abstract:
Hermit beetles of the genus Osmoderma (Coleoptera: Scarabaeidae: Cetoniinae) are known for their fruity odour, which is released in large amounts by males. Two species of the genus occur in Europe, the eastern Osmoderma barnabita (Motschulsky) and the western Osmoderma eremita (Scopoli). Previous studies on Swedish populations of O. eremita showed that the compound responsible for the characteristic scent, gamma-decalactone, functions as a sex pheromone for the attraction of conspecific females. Male O. eremita only release the (R)-enantiomer of the lactone, and both sexes are anosmic to the opposite enantiomer. As the distribution areas of the two hermit beetle species partly overlap, it may be expected that they use different enantiomeric compositions of gamma-decalactone as pheromones to promote species discrimination. This paper reports on the identification of the sex pheromone of O. barnabita. Surprisingly, males from a Polish population produce only the (R)-enantiomer of gamma-decalactone, and conspecific females show equal attraction to the (R)-enantiomer and a racemic mixture of the compound, indicating that O. barnabita is anosmic to the (S)-enantiomer, similarly to what was observed for O. eremita. A mtDNA sequence analysis of the cytochrome oxidase subunit I gene of Polish and Swedish beetles confirmed their taxonomical status as O. barnabita and O. eremita, respectively, with an average sequence divergence of 10.5% between beetles from the two studied areas. Although genetic data suggest that these species diverged several million years ago, they still rely on the same enantiomer of gamma-decalactone for mate finding. Thus, the male-produced pheromone in Osmoderma spp. may be regarded as a territorial signal being exploited by females, rather than a cue for determining species identity. Our data show that the same compound can be used to facilitate monitoring of both beetle species, which are considered indicator species of the species-rich fauna of saproxylic insects in Europe.