Several hypotheses have been proposed to explain how herbivorous insects approach plants by sensing plant volatiles. Insect antennae and maxillary palps are believed to have crucial roles in the detection of host plant volatiles. However, few studies have assessed the roles of these olfactory organs in food selection in terms of the effects of individual volatile compounds from plants at various distances. Therefore, we assessed the palp-opening response (POR), biting response, and selection behavior of locust (Locusta migratoria) nymphs in response to volatile compounds from host and non-host plants at various distances. Thirty odorants were identified as the active volatiles to locust by the POR tests. At a distance of 3 m, locusts were attracted to a few common volatiles (1% v/v) of both host and non-host plants, while few components of volatiles acted as repellants at this distance. At a distance of 1 m, locusts responded more readily to volatile compounds. At a distance of 1 cm, locusts mainly used their palps to detect volatiles. However, some components that acted as attractants at long distances had no effect on the biting response at a short distance. Together, the results suggest that plant volatiles generally attract locust nymphs at long distances, but the effects are influenced by distance and concentration. Moreover, there are substantial functional differences in the use of antennae and palps for detecting volatiles at various distances. Overall, the mechanism of food selection by locusts via olfaction can be divided into several continuous ranges according to the sensitivities of the two chemosensory organs and the characteristics of the plant odorants.
Keywords: Chemosensory organs; Distance; Food selection; Herbivorous insect; Locust; Plant volatiles.