| In order to expound the mechanism of chlorfenapyr and the resistance risk of beet armyworm (Spodoptera exigua) to chlorfenapyr, the toxicity of chlorfenapyr to beet armyworm larvae (S. exigua (Hübner)) and the synergism of DEM, TPP and PBO to chlorfenapyr were studied by topical application method. The influence of chlorfenapyr on respiration, endogenous enzymes of protective system (SOD, CAT, and POD) and the detoxifying enzymes (CarE, GST, and cytochrome P450) in beet armyworm larvae were determined by biochemical measurement at the dose of LC25 and LC50. The S.exigua strain with low level resistance to chlorfenapyr was obtained after selection of 12 generations in laboratory by topical application method. The coupling sensitiveness of oxidative phosphorylation, activities of endogenous enzymes of protective system and the detoxifying enzymes and fitness were compared between the susceptible strain and chlorfenapyr-resiatance strain of S.exigua. The Cross-resistance to other insecticides in chlorfenapyr-selected population of S. exigua was also studied. The results were as follow.1. Toxicity of chlorfenapyr to beet armyworm larvae S. exigua (Hübner) and the synergism of TPP, PBO, and DEM to chlorfenapyr were detected by topical application method. The results indicated that the LD50 of chlorfenapyr to 3rd, 4th and 5th instar beet armyworm larvae were 7.9369μg/g, 9.1730μg/g and 8.1467μg/g, respectively. For the susceptibale strains, 24h after treatment, DEM and TPP had either synergism or antagonism and the synergism ratio were 1.15and 0.92, while PBO had distinct synergism and the the synergism ratio was 0.21. Compared to 24h after treated with chlorfenayr, there were no changes on the effect of DEM and TPP, but the toxicity of chlorfenapyr to S.exigua went up. For the resistance strains, DEM had synergism and the synergism ratio was 1.43, while PBO and TPP had slight antagonism and the the synergism ratio were 0.94 and 0.83, respectively.2. The influence of chlorfenapyr on respiration of beet armyworm was studied in this paper. The results indicated that there were same trend of respiration rate among two treatment groups and one control, the respiration rate of the treatment groups, which were higher than the control significantly, started to go up 1h after treating and reached the resperation peak, 1.5 and 1.8 folds higher than control, and then went down.3. The influence of chlorfenapyr on endogenous enzymes of protective system and the detoxifying enzymes of beet armyworm larvae were determined by biochemical measurement. The result suggested that SOD activity was induced by chlorfenapyr while CAT activity, POD activity, CarE activity were inhibited first and then resumed to the normal level. The activity of GST was induced significantly from 6h to 24h after treating and the content of cytochrome P450 was also induced by chlorfenapyr.4. After selection for 12 generation in the laboratory with chlorfenapyr at the dose of LD50, S.exigua showed low level resistance (4.7-fold) to chlorfenapyr.5. Biochemical study showed that there were no significant changes on the coupling sensitiveness of oxidative phosphorylation between susceptible and resistance strains of S. exigua. The specific activity of GST and superoxide dismutase, content of P450 were higher in the resistant strain than those in the susceptible strain, the ratio of enzymatic of GST, P450 and superoxide dismutase to susceptible strain was 2.01, 1.36 and 1.1 respectively, and the difference was significant; the ratio of enzymatic of CarE to susceptible was 0.6; the ratio of enzymatic of peroxidase and catalase to susceptible was 0.98 and 0.95 respectively,there were no significant differences.6. Relative fitness experiments for the populations of susceptible and resistance strains of S. exigua, showed that relative finesses of resistance strain was 0.73, in comparison with that of susceptible strain. There were significant different between susceptible strain and resistance strain of S. exigua on the pupation rate which were (83.33±2.03) % and (75.00±1.53) %, respectively. There were no significant differents on the growth stage of larval, survival rate, pupal stage, adult stage, eclosion rate, sex ratio, eggs per female and Egg hatchability,which were 11.92±0.22days and 12.42±0.36 days, (80.35±1.25)% and (78.64±2.38)%,8.00±0.28 days and 8.58±0.30 days, 11.17±0.60 days and 11.80±0.36 days,(83.83±1.69)% and (80.00±2.08)%, (51.75±3.64)% and (50.24±2.62)%, 691.6±279.5eggs and 635.0±244.6 eggs and (83.00±2.51)% and (81.33±1.85)%, respectively.7. The resistance strain showed no cross-resistance to tebufenozide, emamectin-benzoate, spinosad, chlorpyrifos, methomyl and alpha-cypermethrin, the resistance ratio were 1.09, 1.27, 0.99, 1.12, 1.03 and 1.05, respectively.
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