Bark volatiles from nonhost angiosperm trees of spruce bark beetle, Ips typographus (L.) (Coleoptera: Scolytidae):: Chemical and electrophysiological analysis

Volatiles from bark of three nonhost angiosperm trees of Ips typographus (Coleoptera: Scolytidae) (Betula pendula, B. pubescens, and Populus tremula) were collected by headspace sampling and direct solvent extraction in June 1998, and identified and quantified by coupled gas chromatographic-mass spectrometry (GC-MS). Only small amounts of bark volatiles were detected in the aerations in situ from undamaged stems of the nonhost trees. In headspace volatiles from bark chips the two birch species had a similar odour profile, with two sesquiterpene hydrocarbons, alpha-zingiberene and alpha-santalene, as the most dominant components. Bark of P. tremula emitted significantly less sesquiterpenes and more green leaf volatiles (GLVs) than the two birch species did. Solvent extraction of fresh bark gave similar volatile composition for the two birches compared to the headspace of bark chips, but large differences were found in P. tremula. Coupled gas chromatographic-electroantennographic detection analysis (GC-EAD) of the headspace volatiles from fresh bark chips of the three nonhost species revealed five compounds that consistently elicited antennal responses by I. typographus. The strongest antennal responses were elicited by trans-conophthorin, (SS,7S)-(-)-7-methyl-1,6-dioxaspiro[4. 5]decane with optical purity of ca. 90% (S,S). The other four antennally active bark volatiles were 1-hexanol, (Z)-3-hexen-1-ol, 3-octanol and 1-octen-3-ol, which had similar electroantennogram (EAC) dose-response curves and response thresholds. (+/-)-trans-Conophthorin showed a different, linear EAC dose-response curve, with a 10 times lower response threshold than the other GC-EAD active compounds, similar to the pheromone component, (-)-(4S)-cis-verbenol. These results suggest that not only the green leaf alcohols from leaves of the angiosperm trees but also the nonhost bark volatiles could be used by I. typographus as long-range olfactory cues to discriminate between its conifer host and the nonhosts, birch and aspen.