Insect Science

AbstractCandidatus Liberibacter asiaticus (CLas) bacterium has been strongly implicated as the causative agent of huanglongbing (HLB), or citrus greening, which is currently the most devastating citrus disease worldwide. HLB is transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), in a persistent manner. We used quantitative-polymerase chain reaction (PCR) to detect CLas in dissected organs of individual D. citri adults infected with HLB in the laboratory or collected from field-infected citrus trees in South Florida. The proportion of infected (CLas-positive) dissected organs was 47–70% for the salivary glands, 72–80% for the alimentary canal, and 79–97.5% for the rest of the insect body. Statistical analysis indicated that, in both field- and laboratory-infected D. citri, the proportion of infected salivary glands was significantly lower than that of other parts in the insect body. With field-collected psyllids, the relative copy number of CLas genomes, compared with psyllid genomic DNA in each sample, was significantly higher in both the salivary gland and alimentary canal compared with that in the rest of the insect body for both males and females. These results provide the first PCR confirmation of CLas in the alimentary canal and salivary glands of D. citri and strongly suggest that the salivary glands constitute an important transmission barrier to CLas in the psyllid vector. Our results also suggest that CLas may replicate or accumulate in both the alimentary canal and salivary glands of D. citri.

Abstract Candidatus Liberibacter asiaticus (Las) has been reported to increase the susceptibility of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), to selected insecticides. Reduced general esterase activity in Las-infected, compared with uninfected, D. citri has been proposed as a possible explanation for this difference in insecticide susceptibility. The current study was conducted to quantify glutathione transferase (GST) and cytochrome P450 (general oxidase) activities in Las-infected D. citri to further explain the possible mechanisms for altered susceptibility to insecticides due to Las infection. GST and cytochrome P450 activities (indirectly through general oxidase levels) were quantified in Las-infected and uninfected D. citri nymphs and adults. Mean (±SEM) GST activity was significantly lower in Las-infected (468.23 ± 26.87 /µmol/min/mg protein) than uninfected (757.63 ± 59.46 µmol/min/mg protein) D. citri adults. Likewise, mean cytochrome P450 activity was significantly lower in Las-infected (0.23 ± 0.02 equivalent units [EU] cytochrome P450/mg protein) than uninfected (0.49 ± 0.05 EU cytochrome P450/mg protein) D. citri adults. Immature stages (second and fifth instars) were characterized by significantly lower GST activity than adults for uninfected D. citri. However, cytochrome P450 activity was significantly higher in second instar nymphs than adults and fifth-instar nymphs for uninfected D. citri. Lower activities of GST and general oxidase in Las-infected D. citri indicate that infection with Las alters D. citri physiology in a manner that could increase insecticide susceptibility. The reduced activities of these detoxifying enzymes due to Las infection may be explained by examining expression levels of associated genes in Las-infected and uninfected D. citri.