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The pathogen Candidatus Liberibacter asiaticus (Las) causes devastating citrus huanglongbing in many parts of Asia. The host selection behavior of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psyllidae), the only known Las vector, is critical to its epidemics. Las-infected citrus plants attract adult psyllids by altering their color and chemical cues. However, it is unclear whether any changes in selection behavior or chemical cues occur in young shoots. Selection behavior bioassays showed the number of adults and eggs on Las-infected shoots was significantly higher than on healthy shoots. Significantly more adults responded to the odor of Las-infected shoots than to that of uninfected shoots. GC-MS analysis of volatile constituents showed that the quantitatively dominant constituents were β-elemene and β-phellandrene in both samples, (+)-epi-bicyclosesquiphellandrene was only detected in Las-infected shoots, but ocimene, germacrene D, δ-cadinene, d-longifolene, and α-sinensal were detected only in healthy shoots. Analysis of the concentrations of surface-soluble carbohydrates indicated that the levels of sucrose, fructose, and glucose were significantly higher on the surfaces of Las-infected shoots than on the surfaces of healthy shoots. Alteration of volatile compounds and soluble surface carbohydrates of young shoots after Las infection could be attributed to the preference of Asian citrus psyllids for Las-infected young shoots. The knowledge obtained from this investigation may contribute to novel control measures for plant diseases transmitted by insect vectors.

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera:Psyllidae), is one of the most important pests of citrus plants worldwide, due to be the dominant vector of the devastating citrus disease Huanglongbing (HLB). Selection of suitable oviposition site is a crucial event of phytophagous insect life. Usually, the chemical and physical characteristics of a host plant were recognized by sensilla on the ovipositor and tarsi of a phytophagous insect after settling on a plant surface. In this study, the morphology, number, and distribution pattern of the ovipositorial and tarsal sensilla of adult female psyllids, were observed by scanning electron microscopy (SEM) techniques. The effect of these sensilla on oviposition was investigated by sensilla deactivation using hydrochloric acid (HCl). The results showed that sensilla trichoid (ST), sensilla chaetica (ChS) and sensilla basiconica (SB) were distributed on ovipositor, while sensilla trichoid (ST) were distributed on the external genitalia. Sensilla chaetica (ChS) and basiconica (SB) were mainly distributed on the end of the ovipositor back plate, and on the base of the sternum. On tarsus, sensilla trichoid (ST), sensilla chaetica (ChS), sensilla triangular (TeS) and sensilla ear ball (SE) were observed. However, whole tarsal segments were covered with Böhm's mane, which was arranged in a circular line. Oviposition assay revealed that a significantly lower number of eggs were laid on the Murraya paniculata seedlings confined by the psyllids with deactivated ovipositorial sensilla followed by protarsal sensilla, mesotarsal sensilla and hindtarsal sensilla. These results suggested that the ovipositorial and tarsal sensilla of citrus psyllid have an important role in the oviposition.