Evidence for the importance of odour-perception in the parasitoid Rhopalicus tutela (Walker) (Hym., Pteromalidae)

Possible host location mechanisms in the chalcid wasp Rhopalicus tutela (Wa lker) (Hym., Pteromalidae), a parasitoid of the eight-spined spruce bark be etle, Ips typographus (L.) (Col., Scolytidae), were examined. This was carr ied out in order to repeat and complement former studies on parallel parasi toid-scolytid systems that had contradictory results. Morphological examina tions of the parasitoid antennae were made using both scanning and transmis sion electron microscopy. Possible functions of the sensilla placodea (mult iporous plate sensillum), and other sensilla present on the antennae, have been indicated. For the first time, the placoid sensilla in a pteromalid pa rasitoid have shown porous walls and numerous innervations, which are typic al characteristics for chemoreceptors. Previously the placoid sensilla have been suggested to be an infrared receptor. In order to test the chemorecep tive ability of R. tutela females and males, a synthetic reference blend wa s analysed by combined gas chromatography and electroantennographic detecto r (GC-EAD). Their sensitivity to host-related volatiles (such as certain ph eromone components, and oxygenated monoterpenes) was significantly greater than that for host-tree-related compounds (monoLerpene hydrocarbons), Emplo ying an infrared thermo-scanner, the current study failed to detect 'hot sp ots' associated with susceptible hosts beneath the bark. Results from elect rophysiology and electron microscopy revealed clear odour-perceptive functi ons of the parasitoid antennae. These results strongly support the major im portance of volatiles in host location by the bark beetle parasitoid R. tut ela.