Risk Assessment and Genetic Analysis of Fipronil Resistance and Insecticide Susceptibility in the Small Brown Planthopper,Laodelphax striatellus(Homoptera:Delphacidae)

Risk assessment and genetic analysis on fipronil resistance and insecticide susceptibility in the small brown planthopper were conducted in laboratory by rice stem dipping method.After 41-generation selection with fipronil in 43 generations,the resistance level of the small brown planthopper(a population of L.striatellus from the wheat fields in Wuxi City,Jiangsu Province in 2005) to fipronil increased from 8.4-to 230.5-fold compared with the susceptible strain,and the realized heritability(h2) of resistance at different selection stages according to the method described by Tabashnik was estimated as 0.0388(F1 to F31),0.2636(F32 to F43) and 0.1113(F1 to F43),respectively.It is suggested that the L.striatellus had the definite risk of resistance to fipronil.The susceptibilities to 15 insecticides were evaluated in two field populations of L.striatellus,collected from Wuxi City,Jiangsu Province and Changxing County,Zhejiang Province in 2007 and 2008 by the rice stem dipping method.The results showed that the highest toxic insecticides to the insect was a phenyl-pyrazole class of insecticides including fipronil,butylene-fipronil and ethiprole(LC50: 0.2-1.7 mg/L),followed by thiamethoxam,nitenpyram,pymetrozine and chlorpyrifos(LC50: 1.7-9.7 mg/L),and toxicity of the others was lower.According to a year-to-year variation in susceptibility,it was noted that susceptibilities to fipronil and ethiprole in L.striatellus population from Wuxi City in 2008 were reduced with a reducing rate of 1.1-fold,compared with those to the two insecticides in 2007.Fipronil was used widely for controlling this insect in recent years,but ethiprole wasn′t.Therefore,it seems that the L.striatellus population whose susceptibility to fipronil was reduced had cross-resistance to ethiprole.Inheritance characteristic of resistance to fipronil in L.striatellus was studied through assaying dose-response data of the resistant(R) and susceptible(S) parents,reciprocal crosses(F1,F1′),self-bred(F2) and backcross(BC) progenies to fipronil by the rice stem dipping method,and the results showed that the resistance to fipronil in L.striatellus was found to be polygenic,autosomal and inherited as a partial dominance trait [D(F1) =0.20,D(F1′) =0.38].Resistance management in L.striatellus was also discussed.