Enantiomeric specificity in a pheromone-kairomone system of two threatened saproxylic beetles, Osmoderma eremita and Elater ferrugineus

The scarab beetle Osmoderma eremita and its larval predator, the click beetle Elater ferrugineus, are threatened saproxylic beetles regarded as indicators of the species-richness of insect fauna of hollow deciduous trees. Male O. eremita produce the pheromone (R)-(+)-gamma-decalactone to attract conspecific females, and this compound is also utilized by E. ferrugineus as a kairomone, presumably for detection of tree hollows containing prey. We have investigated enantiomeric specificity to gamma-decalactone in this pheromone-kairomone system by electrophysiological and field trapping experiments. In single-sensillum recordings from male and female O. eremita, which used the (R)-enantiomer and the racemic mixture of gamma-decalactone as odor stimuli, numerous olfactory receptor neurons (ORNs) responding to both stimuli were found. No neurons responded preferentially to the racemic mixture, showing that these beetles seem to lack receptors specific for the (S)-enantiomer. The enantiomeric specificity of ORNs was confirmed by gas chromatography-linked single-sensillum recordings where the two enantiomers in a racemic mixture were separated on a chiral column. Furthermore, in field experiments that used the (R)-enantiomer and the racemic mixture as lures, the attraction of O. eremita females corresponded to the amount of (R)-enantiomer released from lures with the (S)-enantiomer displaying no antagonistic effects. Trap catch data also suggested that the (S)-enantiomer is not a behavioral antagonist for E. ferrugineus. The odor-based system can be highly efficient in attracting the larval predator where trap catch in 1 yr almost equaled the total number of specimens collected in Sweden until 1993. Our study shows that racemic gamma-decalactone could be used for cost-effective monitoring of both beetles.