Four strains of the Q biotype and one of the B biotype of the whitefly Bemisia tabaci collected from China were characterised for resistance to four neonicotinoid insecticides and pymetrozine.Q biotype strains showed moderate to strong resistance to imidacloprid,thiamethoxam and acetamiprid,but little or no cross-resistance to dinotefuron.Resistance to neonicotinoids was consistently associated with resistance to pymetrozine,despite the latter having a distinct(though unresolved)mode of action.The single B biotype strain proved largely susceptible to all the insecticides investigated.Resistance in the Q biotype strains was associated with over-expression of a cytochrome P450 monooxygenase gene,CYP6CM1,whose substrate specificity presumably accounts for the observed cross-resistance profiles. Four strains of the Q biotype and one of the B biotype of the whitefly Bemisia tabaci collected from China were characterized for resistance to four neonicotinoid insecticides and pymetrozine. Q biotype strains demonstrated strongly to strong resistance to imidacloprid, thiamethoxam and acetamiprid, but little or no cross-resistance to dinotefuron. Resistance to neonicotinoids was consistently associated with resistance to pymetrozine, despite the latter having a distinct (though unresolved) mode of action. The single B biotype strain substantially permit to all the insecticides investigated. Resistance in the Q biotype Was associated with over-expression of a cytochrome P450 monooxygenase gene, CYP6CM1, whose substrate specificity presumably accounts for the observed cross-resistance profiles.