Chemosensory basis for feeding acceptance of different host plants in the larvae of the Corsican swallowtail Papilio hospiton.

Papilio hospiton is an oligophagous species, endemic of the islands of Corsica and Sardinia, using various Apiaceae and Rutaceae as host plants, but no plants outside of these families. In Sardinia it uses Ferula communis as an almost exclusive host plant, but when this is unavailable, Ferula arrigonii and Ruta lamarmorae are the plants of choice; in Corsica, it also uses other endemic host plants (Pastinaca latifolia). We previously found that the lateral sensillum has two deterrent GRNs (L and M2), one phagostimulant (M1) and one salt neuron (S), while the medial sensillum has two phagostimulant GRNs (L and M1), one deterrent (M2) and one salt neuron (S). In this work we studied the sensitivity of GRNs to saps of F. communis, F. arrigonii, R. lamarmorae and P. latifolia and evaluated the relationship between taste sensitivity to different host-plants and larval growth rate on each of them. The spike activity was recorded from medial and lateral taste sensilla stimulated with plant saps, and GRN response patterns were cross compared in the light of a different feeding acceptance. The spike responses of GRNs are statistically different. Same neurons are activated by all plant saps, but the phagodeterrent GRNs show a higher activity in response to F. arrigonii and R. lamarmorae than to F. communis and P. latifolia. Behavioral trials show that the time to pupation was significantly longer when larvae were reared on F. arrigonii and R. lamarmorae than on F. communis and P. latifolia. These results suggest that the different activity of the phagodeterrent GRNs can inhibit food acceptance and extend the duration of the larval stage.