Palpal receptors of the olive fly Bactrocera oleae play a key role in foraging behavior and host finding

The olive fly Bactrocera oleae (Rossi) is a serious pest of olives in several countries in the Mediterranean basin causing important losses in the oil industry. The olive fly is known to respond behaviorally to volatile compounds present in its habitat and, more specifically, to those released from its host plant that play an important role in guiding the oviposition behavior of gravid females. A recent research by Liscia et al. (2013) on a laboratory strain of B. oleae demonstrated that that volatiles released from a bacterial filtrate (obtained culturing Pseudomonas putida) are mainly detected by palpal olfactory receptors rather than the antennal sensilla. On the contrary, α-pinene, a key compound in triggering the oviposition (Scarpati et al., 1993) that is emitted by leaves and half–ripe olives is mainly detected by male insects with the antennal receptors. On these bases, this study has been aimed to further investigate the role of maxillary palps in detecting food and oviposition sites in wild B. oleae adults obtained from pupae collected in different areas of olive orchards in Sardinia. Electrophysiological (EAG and EpG) and behavioral bioassays (Y-tube olfactometer and wind tunnel) were performed to test bacterial filtrate volatiles and some host plant (α-pinene) and food sources (acetic acid) related compounds. Dose-response relationships and differences in sensitivity related to insect sex and physiological condition were identified. Responses were compared to those obtained in lab insects. The results obtained in wild insects confirmed that palpi have a higher sensitivity to bacterial filtrate than the antennae. Otherwise, the EpG recorded in mated females in response to α-pinene and acetic acid, showed a lower threshold and a greater signal amplitude than those recorded in lab insects both in the palpi and the antennae. Electrophysiological results are complementary with the behavioral ones. In conclusion, the maxillary palp olfactory receptors play a primary role in the short-range detection of chemicals cues from host plant and epiphytic bacteria. These findings open new perspectives for improving olive fly control strategies.