| Beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera:Noctuidae), and common cutworm, Spodoptera litura (Fabricius), are major pests of numerous cultivated crops. Frequent use of chemical insecticides controlling S. exigua and S. litura had led to the development of insecticide resistance. Chlorantraniliprole is the first commercialized ryanodine receptor insecticide from anthranilic diamides class and has exceptional insecticidal activity on a range of Lepidopteran pests.In this paper, the susceptibility of S. exigua and S. litura to chlorantraniliprole, resistance risk in S. exigua to chlorantraniliprole, the effects of chlorantraniliprole on development and reproduction of S. exigua and detoxifying enzyme activities were studied. The main results are as follows:1. Baseline susceptibility of S. exigua and S. litura to chlorantraniliproleBaseline susceptibility of S. exigua showed that the susceptible strain had a very high susceptibility to chlorantraniliprole (LC50=0.014 mg AI/L), and the susceptibility of S. exigua of 18 field populations was relatively high and with wide variation (LC50 0.039-0.240 mg AI/L). Among these populations, Dongguan, Shenzhen, Huizhou, Zhangzhou, Shanghai, Hefei and Tai'an populations developed moderate levels of resistance to chlorantraniliprole; Changsha, Chaohu, Xinxiang and four Nanjing populations were low levels of resistance, whereas Nanning and Wuhan populations were still susceptible to chlorantraniliprole. Baseline susceptibility of S. litura to chlorantraniliprole was determined using diet incorporation method. The results showed that LC50 value of the susceptible strain to chlorantraniliprole was 0.004 mg AI/L, and the 13 field populations represented 6.8-24.4 folds of resistance compared with the susceptible population. Shenzhen, Chaohu, Dongguan, Huizhou, Zhangzhou, Hefei and Shanghai populations developed moderate levels of resistance to chlorantraniliprole, and other populations were low levels of resistance to chlorantraniliprole. 2. Assessment of resistance risk in S. exigua to chlorantraniliproleA field-population of S. exigua collected from Shanghai was selected after repeated exposure to chlorantraniliprole to determine the risk of resistance evolution. After 22 generations of selection, there was a 12.0-fold increase in LC50. The realized heritability (h2) of resistance was estimated as 0.1082 by using threshold trait analysis. The projected rate of resistance evolution indicated that if h2=0.1082 and 70%-90% of the population was killed at each generation, and then a 10-fold increase in LC50 would be expected in 14.4-21.7 generations for chlorantraniliprole. These results show that the risk of resistance development exists in S. exigua to chlorantraniliprole after continuous application.3. The resistance biochemic mechanism in the geographical and selected populationsThe results of the detoxification enzyme activities showed that almost all the detoxification enzyme activities of the field-collected populations of S. exigua were high than that of the susceptible population, there were few positive correlations between MFO, EST and GST activities and toxicities of chlorantraniliprole on S. exigua. Artificial selection of S. exigua in laboratory with chlorantraniliprole was carried out,23 generations of continuous selections resulted in 3.0-fold and 3.7-fold increases in mixed function oxidase and esterase, respectively. There were different variations of the detoxification enzyme activities in seven field populations of S. litura, however, the correlations between detoxification enzyme activities and LC50 values of S. litura showed that GST, EST and MFO might not be involved in the tolerance of S. litura to chlorantraniliprole. In the synergism assays, PBO showed 1.70- to 2.23-fold synergism ratios (SR) on chlorantraniliprole in the four populations, indicating MFO may involve in the detoxification of S. exigua to chlorantraniliprole. DEF (SR,1.00- to 1.28-fold), DEM (SR, 1.06- to 1.78-fold) and TPP (SR,0.94- to 1.33-fold) did not show significant synergism in all three strains, these suggested that detoxification metabolism mediated by glutathione S-transferase and esterases might be unimportant in the observed tolerance to chlorantraniliprole in S. exigua. There may some other metabolisms involve in the the detoxification of S. exigua to chlorantraniliprole.4 Effects of chlorantraniliprole on development and reproduction of beet armywormIn this study the toxicity and lethal effects of chlorantraniliprole on Spodoptera exigua were evaluated. The LC30 and LC50 values of S. exigua to chlorantraniliprole were 0.007 mg AI L-1 and 0.014 mg AI L-1 respectively. After exposing neonate of beet armyworm to LC30 and LC50 concentration of chlorantraniliprole for 3 days, the larval developmental durations were extended for 22.5% and 28.6% respectively, and the pupation rates (74.75% and 54.68%, respectively) were also reduced. Percentage of individuals with high pupal weight increased in chlorantraniliprole treatment, and heavy pupa over 150 mg showed up in chlorantraniliprole exposed treatments compared with control, especially the mean of pupal weight in LC30 concentration treatment was significantly greater than that in control. There was significant decrease on egg hatch rate when using LC50 concentration treatment, but no significant differences in pupa duration, emergence rate, sex ratio, egg number per female and longevity of adults were observed among treatments in this study. |
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