Thuja occidentalis: identification of volatiles and electroantennographic response by the invasive cedar bark beetle, Phloeosinus aubei

Recently, the distribution of the Mediterranean cedar bark beetle, Phloeosinus aubei Perris (Coleoptera: Scolytidae), has expanded to Central Europe. Reported mostly on cypress in the Mediterranean area, potential host plants in the invaded range include other scale-leafed conifers, such as cultivars of arborvitae, Thuja occidentalis L. To reveal potential kairomonal cues for P.aubei, volatiles of T.occidentalis were collected and analysed by gas chromatography with electroantennographic detection (GC-EAD). Assignments of chemical structures of antennally active components were carried out by gas chromatography linked to mass spectrometry (GC-MS) using authentic reference samples. Antennal responses to synthetic samples of the identified compounds were studied by electroantennography (EAG), with antennae of female and male P.aubei. GC-EAD analysis of head space volatiles of T.occidentalis revealed 22 antennally active compounds, of which 21 were identified. The most abundant components were - and -thujone, fenchone, camphor, terpinen-4-ol, bornyl acetate and -terpinyl acetate, all of which are oxygenated monoterpenes. When EAG activities of synthetic samples were compared, the most intensive responses from female antennae were elicited by a mixture of - and -thujone, followed by (-)-terpinen-4-ol, (+)-camphor, cis-4-thujanol, (+)-sabina ketone, (+)-terpinen-4-ol, isopulegone, (-)-fenchone, borneol, (3Z)-hexen-1-ol, (-)-1-octen-3-ol and (+)-sulcatol. Male antennae responded the most to (-)-terpinen-4-ol and cis-4-thujanol followed by the mixture of - and -thujone. The next highest responses were elicited by (+)-camphor, borneol, (+)-terpinen-4-ol, (+)-sulcatol and (+)-sabina ketone. Striking differences were found between responses to the enantiomers of fenchone, sulcatol and 1-octen-3-ol, whereas responses to the enantiomers of terpinen-4-ol did not differ significantly from each other. Several antennally active volatiles of T.occidentalis have also been reported from cypress and various other members of the Cupressaceae, suggesting that the sensory apparatus of P.aubei may recognize the shared components, which may enable rapid adaptation to new hosts in the invaded areas.