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

Abstract

Possible host location mechanisms in the chalcid wasp Rhopalicus tutela (Walker) (Hym., Pteromalidae), a parasitoid of the eight‐spined spruce bark beetle, Ips typographus (L.) (Col., Scolytidae), were examined. This was carried out in order to repeat and complement former studies on parallel parasitoid–scolytid systems that had contradictory results. Morphological examinations of the parasitoid antennae were made using both scanning and transmission electron microscopy. Possible functions of the sensilla placodea (multiporous plate sensillum), and other sensilla present on the antennae, have been indicated. For the first time, the placoid sensilla in a pteromalid parasitoid have shown porous walls and numerous innervations, which are typical characteristics for chemoreceptors. Previously the placoid sensilla have been suggested to be an infrared receptor. In order to test the chemoreceptive ability of R. tutela females and males, a synthetic reference blend was analysed by combined gas chromatography and electroantennographic detector (GC‐EAD). Their sensitivity to host‐related volatiles (such as certain pheromone components and oxygenated monoterpenes) was significantly greater than that for host‐tree‐related compounds (monoterpene hydrocarbons). Employing an infrared thermo‐scanner, the current study failed to detect ‘hot spots’ associated with susceptible hosts beneath the bark. Results from electrophysiology and electron microscopy revealed clear odour‐perceptive functions of the parasitoid antennae. These results strongly support the major importance of volatiles in host location by the bark beetle parasitoid R. tutela.