| Beet armyworm, Spodoptera exigua (Hubner), is a worldwide dsitributed polyphagous pest on various cultivated crops, including vegetables, cotton, fruit trees, ornamentals, and so on. In past two decades S. exigua has emerged as a serious pest in many countries.Resistance to 8 insecticides in six field populations of beet armyworm, Spodoptera exigua (Hubner), was evaluated in laboratory. The results determined by topical application showed that 3rd instar larvae of these populations collected from Shenzhen City, Guangdong Province, Hengyang City, Hunan Province, Shanghai City, Changshu and Nanjing City, Jiangsu Province developed high levels of resistance to high effect cypermethrin with 172.1-408.7 folds, moderate-high levels to cyfluthrin with 42.7- 109.8 folds and to chlorpyrifos with 21.0-50.3 fold, suseptibile - moderate levels to methomyl with 2.3-21.0 folds and to phoxim with 3.0-15.1 folds. The population from Longrao County, Hebei Province had only low level of resistance to high effect cypermethrin with 5.1 folds, but susceptible to other insecticides. The results tested by leaf-dipping method showed that first instar larvae of all tested populations was susceptible to chlorfuazuron(0.7-3.4 folds), thoidicarb(1.5 - 3.9 folds) and tebufenozide (1.0-3.1 folds) except that the population from Shenzhen area had low level of resistance to chlorfuazuron(6.4 folds) .Resistance to cyhalothrin in beet armyworm, Spodoptera exigua, collected from Jiangpu County, Nanjing, Jiangsu Province was determined by three bioassay methods. The results indicated that the sequence of resistance levels was topical application for 3rd and 5th instar larvae (5499.5- and 3973.2-fold)>leave-dipped method for 3rd instar larvae (1041.6-fold) > leave-sandwich-poison method for 5th instar larvae (24.7-fold), ratio of their resistance levels was 222.7-160.9:42.2:1.The results above demonstrated that resistance levels caused by contact toxicity through cuticular penetration higher than that by stomach and contact toxicity through mouth and cuticular penetration, even much higher than that by stomach toxicity through mouth. The rate of cuticular penetration of I4C-cyhalothrin in 5th instar larvae of the resistant and susceptible strains was determed. The results indicated that the rate of cuticular penetration in resistant strain was nearly the same in the susceptible strain, but after 2h treatment the rat (8.5%) in the resistant strain was slower than that (13.2%) in the susceptible strain, and after 8h treatment the rat (22.1%) in the resistant strain was much slower than that (39.8%) in the susceptible strain, only about 55.5% of penetration rate in the susceptible strain. It is safe to say that delayed cuticular penetration was one of the important mechanism responsible for this resistance.Biochemical mechanism of resistance in beet armyworm to cyhalothrin has been investigated by enzyme synergists (PBO, SVi, DBF and DEM) and enzyme activities assays of esterase > glutathione S-transferase and microsomal O-demethylase. The synergism ratios of PBO, SVi, DEF and DEM to cyhalothrin were 2.3, 1.3, 2.9 and 1.5 folds in susceptible strain, respectively; 32.5-,19.3-,6.6- and 3.5-fold in resistant strain, respectively. Ratios between resistant and susceptible strain were 14.4, 14.8, 2.3 and 2.3 folds, respectively. The result shows that synergism to cyhalothrin by PBO was the most distinct. This implied that mixed-function oxidase was involved in the resistance to cyhalothrin. Synergism to cyhalothrin by DEF and DEM was not much significant. The ratios of enzymatic activity of esterase, glutathione S-transferase and microsomal O-demethylase in susceptible and resistant strains were 1.05-, 0.91- and 1.05-fold, respectively, no significant difference. All these indicated that detoxification enzymes such as esterase, glutathione S-transferase and microsomal O-demethylase contributed little to the resistance of beet armyworm to cyhalothrin.The relative fitness between resistant and susceptible strains to cyhalothrin in beet armywor...
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