Preliminary research on the mechanism underlying the increased attraction of Diaphorina citri for Citrus reticulate Blanco.cv.Shatangju infected with huanglongbing
[Aim]To determine whether infection of the'Shatangju'variety of Citrus reticulate(Citrus × aurantium L.)with the Asian form of huanglongbing(HLB),a serious disease of citrus associated with the phloem-limited bacterium'Candidatus Liberibacter asiaticus'(CLas),increases the attractiveness of infected plants to the Asian citrus psyllid(ACP),Diaphorina citri Kuwayama.[Methods]Responses of male and female ACP to volatiles(linalool,limonene,α-pinene,and γ-terpinene)emitted by infected,and pathogen-free,seedlings,indicate that those from infected seedlings were not significantly more attractive to females.In a previous study,volatile compounds from young shoots of pathogen-free and'CLas'-infected'Shatangju'mandarins were analyzed.In this study,the attractiveness of several chemical components that were elevated in infected plants,and their mixtures,were tested in both the laboratory and the field.[Results]Males were not attracted to any of the chemicals tested but females were significantly attracted to α-pinene(1 μL/mL),γ-terpinene(0.01 μL/mL)and linalool(0.1 μL/mL),but not to limonene at the concentrations tested.Females were also significantly attracted by 6 mixtures of the above chemicals.Of these mixtures,γ-terpinene(0.2 μL/mL)+α-pinene(0.8 μL/mL)+linalool(0.3 μL/mL)was the most attractive.Males were not attracted to any of the individual chemicals tested,but were significantly attracted by 4 mixtures.Compound monomers and their mixtures were placed in bottle traps in Murraya paniculata(L)Jack trees and the number of insects caught by each lure was compared.The results indicate that 5 of the 15 mixtures were significantly more attractive to D.citri adults than the others,whereas none of the single compounds was significantly more attractive.Three mixtures;y-Terpinene(20 μL/mL)+α-Pinene(80 μL/mL)+Linalool(30 μL/mL),γ-Terpinene(2 μL/mL)+α-Pinene(8 μL/mL)+Linalool(3 μL/mL),and y-Terpinene(20 μL/mL)+Linalool(30 μL/mL),were then tested in a citrus orchard.After 10 days,the average numbers of adults captured in traps using these mixtures was significantly higher than the number captured in control traps.[Conclusion]These findings suggest that the attractiveness of CLas-infected mandarin trees to D.citri is,in part,enhanced by the higher than normal levels of α-pinene and γ-terpinene emitted by infected plants.It may,therefore,be feasible to use these chemicals to trap the D.citri in orchards.
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