Encoding of host and non-host plant odours by receptor neurones in the eucalyptus woodborer, Phoracantha semipunctata (Coleoptera: Cerambycidae)

In the eucalyptus woodborer, Phoracantha semipunctata, responses from single receptor neurotics to host (Eucalyptus globulus) and non-host (Pinus pinaster, Olea europeae) plant odours were examined, using gas chromatography linked with electrophysiological recordings. Thirty-two compounds elicited responses that appeared as increased firing rates correlating with the elution of the active components. A subset of neurones classified as type A (n = 17) responded to one compound, a second subset B (n = 15) responded to two compounds and a third subset C (n = 9) responded to three or more compounds. For each neurone of type B or C, the active chemicals were structurally related. Neurotics responding to monoterpenes, sesquiterpenes and non-terpenoids, respectively, showed non-overlapping response spectra, whereas neurones within one chemical group showed some overlap. The results suggest that most neurotics, which receive information about plant odours, are narrowly rather than broadly tuned, each specialised for the detection of one or two related compounds. Although most neurones responded to compounds released by host and non-host species, a few responded specifically to compounds of only one species. This suggests a chemical "fingerprint" mechanism for discrimination between odour blends of the different plants, where only a few compounds may be specific for a particular species or individual.