Odor perception in the bark beetle parasitoid Roptrocerus xylophagorum exposed to host associated volatiles.

Abstract

Y-tube olfactometer bioassays and combined gas chromatography–electroantennographic detector (GC-EAD) analyses were performed to investigate the specific odors utilized as host location cues by the bark beetle parasitoid, Roptrocerus xylophagorum, originating from the southeastern United States. R. xylophagorum parasitizes several economically important holarctic bark beetle species and females oviposit preferentially on late larval stages. Both female and male parasitoids were tested with volatiles derived from host infestations of either Dendroctonus frontalis, the southern pine beetle, or Ips grandicollis, the southern pine engraver. Tested volatiles were steam distillates from the bark of loblolly pine, Pinus taeda, infested with larvae of the respective bark beetle species. Combined gas chromatography–mass spectrometry (GC-MS) was employed for identification and quantification of the compounds in the steam distillates. To confirm the EAD activity of identified compounds, GC-EAD analyses were repeated with a synthetic blend composed predominantly of compounds in the crude extracts that had revealed apparent electrophysiological activity. In Y-olfactometer bioassays, female parasitoids were attracted to both of the above-mentioned distillates. Male parasitoids were tested with one of the distillates but failed to respond. In GC-EAD analyses, the sexes displayed similar relative sensitivities to the components of the blends. Males exhibited generally higher amplitudes of response to the tested compounds than females. Monoterpene hydrocarbons associated with the constitutive resin of the host tree did not elicit significant EAD responses. Compounds known to be associated specifically with the host–tree complex, such as certain oxygenated monoterpenes, generated the greatest EAD responses. Female parasitoids were attracted by a synthetic blend composed of several of the EAD active oxygenated monoterpenes.