Characterization of olfactory sensilla of the olive fly: behavioral and electrophysiological responses to volatile organic compounds from the host plant and bacterial filtrate.

The responses of olive fly (Bactrocera oleae) antennal and palpal olfactory receptors to odors emitted by Pseudomonas putida bacterial filtrate and to volatiles from a host plant were evaluated using electrophysiological and behavioral bioassays. Morphological identification of olfactory receptors was also performed. The third antennal segment (flagellum) bears four types of multiporous sensilla: trichoid, short basiconica, clavate and grooved. Maxillary palps have mechanosensory bristles and multiporous basiconica sensilla. In wind-tunnel bioassays, olive fly responses to volatiles emitted by bacterial filtrate were higher than those to culture medium. Bacterial filtrate was more attractive than ammonium carbonate or a mixture of ethyl acetate and acetic acid in ethanol. GC-MS of bacterial filtrate identified some of the chemicals produced by bacterial activity, including methyl thiolacetate, ammonia, 2-pentanone, 2-heptanone, ethyl tiglate and methyl thiocyanate. Electrophysiological investigations proved that antennal sensilla are responsive to bacterial filtrate odor, methyl thiolacetate, olive leaves and olives, as well as to α-pinene, while acetic acid elicited an inhibitory response. Electropalpgrams recorded a specific response to bacterial filtrate by mated males and females, as well as a dose-dependent response relationship to methyl thiolacetate by mated females. The identification of new active volatile compounds in the semiochemical system of the olive fly is promising for the development of innovative control strategies in area-wide management.