The Asian citrus psyllid, Diaphorina citri Kuwayama, is one of the most damaging pests of citrus-producing regions throughout the world. The use of insecticides is the main strategy for controlling psyllid and has increased year by year. In this study, four field populations of D. citri were evaluated for resistance to nine different insecticides using the leaf-dip method. The results showed that the highest level of resistance for D. citri was found in imidacloprid with a resistance ratio of 15.12 in the Zengcheng population compared with the laboratory susceptible population. This was followed by chlorpyriphos (6.47), dinotefuran (6.16), thiamethoxam (6.04), lambda-cyhalothrin (4.78), and bifenthrin (4.16). Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) showed significant synergism on imidacloprid effects in the Zengcheng population (3.84- and 2.46-fold, respectively). Nevertheless, diethyl maleate (DEM) had no synergism on imidacloprid. Biochemical enzyme assays suggested that general esterase, glutathione S-transferase and cytochrome P450 monooxygenase activities were higher in the field-collected populations than in the laboratory susceptible population. However, glutathione S-transferase may play a minor role in the resistance of adult D. citri to insecticides. At the molecular level, resistance of D. citri to imidacloprid is mainly related to the increased expression of CYP4C68 and CYP4G70 (>5-fold).

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BACKGROUND: The Asian citrus psyllid, Diaphorina citri, is one of the major pests in citrus-growing areas around the world. The application of insecticides is the most effective method to reduce the population of D. citri. However, D. citri has developed resistance to multiple classes of insecticides. Understanding resistance mechanisms is crucial to the management of D. citri. In this study, molecular assays were performed to characterize imidacloprid resistance mechanisms.RESULTS: Based on the D. citri transcriptome database and other known insect resistance genes, 16 cytochrome P450, eight glutathione-S-transferase and six esterase genes were selected for cloning and sequencing. The gene expression analysis of 30 detoxification genes demonstrated that the relative expression of CYP4g15, CYP303A1, CYP4C62, CYP6BD5, GSTS1 and EST-6 were moderately high (>5-fold increase) in the imidacloprid-resistant strain. Feeding of double-stranded RNA (dsRNA) reduced the expression of the six genes (46.7%-72.1%) and resulted in significant adult mortality (65.62%-82.76%). We also determined the ability of different insecticides to induce the six selected genes. The expression of CYP4C62 and GSTS1 genes were the most significantly upregulated in adults treated with all insecticides, except for chlorfenapyr. In chlorfenapyr-treated D. citri, expression of CYP4g15 and CYP303A1 were the most highly induced. CONCLUSION: Overexpressed detoxification genes were associated with imidacloprid resistance, as confirmed by RNA interference feeding tests. The induction of the six selected genes when exposed to different insecticides supported the hypothesis that they were involved in the metabolism of the tested insecticides.

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Five compounds (syringic acid, tricin, acacetin, syringoside, and diosmetin) were isolated from the aerial parts of wild oats (Avena fatua L.) using chromatography columns of silica gel and Sephadex LH-20. Their chemical structures were identified by means of electrospray ionization and high-resolution mass spectrometry as well as (1)H and (13)C nuclear magnetic resonance spectroscopic analyses. Bioassays showed that the five compounds had significant allelopathic effects on the germination and seedling growth of wheat (Triticum aestivum L.). The five compounds inhibited fresh wheat as well as the shoot and root growth of wheat by approximately 50% at a concentration of 100 mg/kg, except for tricin and syringoside for shoot growth. The results of activity testing indicated that the aerial parts of wild oats had strong allelopathic potential and could cause different degrees of influence on surrounding plants. Moreover, these compounds could be key allelochemicals in wild-oat-infested wheat fields and interfere with wheat growth via allelopathy.