| During recent years, imidacloprid is the fastest growing pesticide of neonicotinoid insecticides. Imidacloprid selectively acts on insect nicotinic acetylcholine receptors and destructs the normal conduction of the insect central nervous system. Then the insect will be at a state of ecstasy because of disturbed nervous activities and gradual paralysis until death. As soon as imidacloprid was introduced to the market as the first neonicotinoid insecticide in1991, it becomes one of the most common insecticides against Nilaparvala lugens, principally owing to its efficacy and long-lasting effect. However, recent surveys of N. lugens field populations from several Asian countries revealed that the intensity and geographical distribution of imidacloprid resistance in the field has increased substantially since2006. Chinese agricultural sectors suggested the suspending use of imidacloprid singly against N. lugens. However, because of the lower control costs and high control efficiency against other rice planthoppers, imidacloprid will be still an important insecticide against N. lugens.By now, many scholars have studied both biochemical and target insensitivity mechanisms of imidacloprid resistance in N. lugens. The previous studies in our laboratory on lab-selected imidacloprid resistance in N. lugens have identified a single point mutation (Y151S) within the extracellular agonist-binding domain of two nAChR a subunits. As to biochemical mechanisms, both in laboratory resistant strain and field populations, the increased cytochrome P450monooxygenase activity is reported as the important or major mechanism for the imidacloprid resistance in N. lugens. However, the P450gene(s) responsible for imidacloprid resistance has not been identified in N. lugens until now. In order to find out the biochemical mechanisms of imidacloprid resistance in N. lugens, we detected the expression of different P450genes at mRNA levels and compared between Res and Sus strains. In addition, in order to utilize imidacloprid with the highest effection and reduce the field dosage of imidacloprid, we screened out two optimum mixture ratios with imidacloprid and pymetrozine, and field trials confirmed the synergistic effects of these two mixtures. Belong to neonicotinoid insecticides, dinotefuran is one of alternative compounds of imidacloprid in the future. We also have screened out two high efficient insecticide mixtures of dinotefuran and pymetrozine, and the field trials will be soon performed to confirm the control effects in fields..1. Biochemical mechanisms of imidacloprid resistance in Nilaparvata lugensWe analyzed mRNA levels of14P450monooxygenases by qRT-PCR with gene specific primers and compared between Sus and Res strains. Significant higher mRNA levels of six P450genes were found in Res strain when compared to Sus strain. For the biggest difference (17.94-fold) in mRNA levels between Res and Sus strain, CYP6AY1was selected to perform functional study in vitro and in vivo. By expressing in E. coli cells, CYP6AY1was found to metabolize imidacloprid with high activity, with the peak activity of37.6μg/mg protein/min. When CYP6AY1mRNA levels in Res strain was reduced by RNA interference, imidacloprid susceptivity was recovered. In four field populations with different resistance levels, high levels of CYP6AY1transcript were also found. It is the first time that the biochemical mechanisms of imidacloprid resistance in N. lugens were detected at genetic level.2. Screening of high efficient insecticide mixtures of imidacloprid and pymetrozinePymetrozine is an important insecticide used to control N. lugens recently, when imidacloprid resistance becomes more serious. Pymetrozine is with the unique action mode, with good sustained effects, but without knockdown activity, which shows that the bioassay on pymetrozine needs long treating time. In order to find out a suitable bioassay method for pymetrozine against N. lugens, the traditional rice seedlings impregnation method was improved to meet the requirements of pymetrozine. The improved method can determine the toxicity of pymetrozine stably and accurately.Joint-toxicity of imidacloprid and pymetrozine against N. lugens has been tested by the above methods. The result indicated that the mixture of imidacloprid and pymetrozine showed the significant synergistic effects. Two ratios have co-toxicity coefficient of410.38and616.17with the mixture ratios of3:2and1:3for imidacloprid and pymetrozine. Subsequently, field trials confirmed the synergistic effects of these two ratios. The control efficiency of these two ratios against adults and nymphs in field were better than the single insecticide at the same dose. The ratios were also good at the quick effects and sustained effects.3. Screening of high efficient insecticide mixtures of dinotefuran and pymetrozineDinotefuran has been marketed in many countries against the hemipteran insect pests as a new neonicotinoid insecticide, including N. lugens. Although dinotefuran has not been registered in China, it is believed that dinotefuran will be released soon in Chinese market as the alternative of imidacloprid and play important roles in N. lugens control. In order to use dinotefuran rationally and avoid the quick resistance development in N. lugens and other rice planthoppers, we try to screen the possible insecticide mixtures with dinotefuran. Because of the quantity of the tested insects, we tested the joint-toxicity of dinotefuran and pymetrozine on Sogatella fureifera, a rice planthoppers normally being together with N. lugens in rice filed. The results showed that the mixtures of dinotefuran and pymetrozine showed significant synergistic effect s. Two best ratios have co-toxicity coefficient of3712.62and3206.61, with the ratios of1:7and4:175for dinotefuran and pymetrozine. These efficient mixtures of dinotefuran and pymetrozine could reduce the usage of dinotefuran in field, lower the production const and delay the resistance development in rice planthoppers. |
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