Abstract: The field-collected NJ strain of B-type B. tabaci has 266-fold resistance to alpha-cypermethrin compared with the SUD-S strain （non B-type）. The NJ-R1 （811-fold） and NJ-R2 （2 634-fold） strains were derived from the NJ strain by using mass selection and single pair family selection with alpha-cypermethrin, respectively. The activities of esterase （EST）, mixed function oxidase （MFO） and glutathione S-transferase （GST） were similar among the NJ, NJ-R1 and NJR2 strains,while the frequency of L925I mutation of para-homologous sodium channel gene （kdr mutation） was 55%, 80.6% and 100% in the NJ, NJ-R1 and NJ-R2 strains, respectively. This suggested that the detoxifying metabolism did not increase during selection with alpha-cypermethrin, and the enhanced resistance to alpha-cypermethrin in the NJ-R2 strain of B-type B. tabaci  was mainly due to the increased frequency of kdr mutation. Piperonyl butoxide （PBO） is a MFO and EST inhibitor. The synergism ratios of PBO to alpha-cypermethrin in the NJ, NJ-R1 and NJ-R2 strains were all about 20-fold, but no synergism in the SUD-S strain was found. The synergism ratio of triphenyl phosphate （TPP） to alpha-cypermethrin in the NJ strain was 12-fold, implying that the synergism of PBO in the B-type whitefly is mainly due to inhibition of esterase activity. It is concluded that B-type B. tabaci has about 20-fold inherent resistance to alpha-cypermethrin （mainly conferred by EST）compared with the non-B type SUD-S strain, and the determinant factor for alpha-cypermethrin resistance in B-type B. tabaci is kdr mutation frequency.

Key words: Bemisia tabaci B-type, alpha-cypermethrin, resistance, detoxifying metabolism, kdr mutation