Semiochemicals related to the aphid Cinara pilicornis and its host, Picea abies: A method to assign nepetalactone diastereomers

Volatiles released by seedlings of Norway spruce infested with the aphid Cinara pilicornis were analyzed using SPME–GC–MS. Among the stress-induced compounds released by the host plant, citronellol, cis–trans-nepetalactone and cis–trans-nepetalactol was found. These compounds originated from the aphids and they were assumed to be pheromone components for this aphid species. To determine the relative stereochemistry of the nepetalactone, a diagnostic method was developed. The method was based on multivariate analysis of tabulated relative intensities of mass fragments of the four nepetalactone diastereomers. In the practical method described, a few pairs of fragments in the mass spectra were compared and, in combination with the Kovat's index, were used to unambiguously identify the relative stereochemistry of the nepetalactone.