Abstract: ObjectiveBehavioral and electrophysiological responses of Pieris rapae larvae to two natural host plants (Brassica oleracea L. var. gemmifera cv. Cyrus L. and Tropaeolum majus L. cv. Glorious Gleam) saps in order to pro-vide reference for host selection mechanism of phytophagous insects. MethodAfter P. rapae larvae were hatched, they were cultured on B. oleracea L. leaves, then the larvae growing into 5th instar for 24-28 h were used for the experiment. The leaves were squeezed to obtain the sap, and double-choice behavior experiment was used to test behavioral responses of the larvae to saps, top recording method was applied to test electrophysiological responses of larvae lateral styloconic sensillum(L-sensillum) and medial styloconic sensillum(M-sensillum) to saps. ResultBehavior experiment results in-dicated that, glass fiber filter paper disc in larvae feeding B. oleracea L. sap was extremely significantly more than those feeding T. majus L. sap (P<0.001). Electrophysiological experiment indicated that electrophysiological responses of L-sensillum to the two saps were significantly stronger than M-sensillum(P<0.01). Moreover, electrophysiological respons-es of L-sensillum to T. majus L. sap was significantly stronger than B. oleracea L. sap(P<0.01). There was no obvious di-fference in electrophysiological responses of M-sensillum to the two saps(P>0.05). Three gustatory neurons in L-sensillum responded to saps, but only two gustatory neurons in M-sensillum responded to saps. Meanwhile, in L-sensillum, response frequencies of gustatory neuron 1 and gustatory neuron 2 to B. oleracea L. sap were extremely significantly stronger than T. majus L. sap(P<0.01 and P<0.001 respectively), the response of gustatory neuron 3 to the two saps showed insignificant difference(P>0.05). In M-sensillum, response frequencies of gustatory neuron 1 and gustatory neuron 2 to B. oleracea L. sap and T. majus L. sap presented insignificant difference(P>0.05). ConclusionP. rapae larvae fed by B. oleracea L. present rejection to T. majus L. sap, and such rejection is related to activation of gustatory neurons in L-sensillum. The re-sponse of the larvae L-sensillum to the two plant leaves are significantly stronger than M-sensillum, which indiates that L-sensillum plays a more important role in detecting host plant.