Abstract: Larvae of the beet armyworm, Spodoptera exigua (Hübner), originally collected from the field of sugar beet, were reared on artificial diet and evaluated for resistance to lambda-cyhalothrin for 43 generations under laboratory condition without exposure to any insecticide (under non-selection). Lambda-cyhalothrin showed LD₅₀ value of 0.9672 μg/larva at F₁ generation and decreased gradually in onward generations,  resulting in LD₅₀ value of 0.0325 μg/larva at F₄₃ generation by using topical application, and the resistance level of the non-selected strain decreased from 4 836.0 fold to 162.5-fold compared with the susceptible strain. In leaf dipping bioassay, lambda-cyhalothrin exhibited LC₅₀ value of 185.6 mg/L at F₁ generation and decreased gradually to onward generations with LC₅₀ value of 9.2 mg/L at F₄₃ generation. Correspondingly, the resistance level of the non-selected strain decreased from 964.7-fold to 47.8-fold compared with the susceptible strain. Though decreased 29.8-fold (topical application) or 20.2-fold (leaf dipping bioassay) from F₁ to F₄₃ generation, the resistance levels of F₄₃ generation was still quite high,  and this indicated that it was very difficult for the beet armyworm to recover the sensitivity to lambda-cyhalothrin. Activities of four monoxygenases,  i.e., methoxyresorufin O-demethylase (MROD), ethoxyresorufin O-deethylase (EROD),  arylhydrocarbon hydroxylase (AHH) and aldrin epoxidase (AE), of F₂, F₂₀ and F₄₁ generation were compared with that of the susceptible strain, respectively. Compared with the susceptible strain, the activities of MROD and AE in midguts in the 5th instar larvae of F₂ generation was significantly higher, but those of F₂₀ and F₄₁ generations showed no significant difference. Similarly,  the activities of EROD and AHH of F₂,  F₂₀ and F₄₁ generation were also significantly higher compared with the susceptible strain. The results suggested that the resistance of S. exigua to lambda-cyhalothrin was closely correlated with multi-function oxidase which may play different roles at different resistant levels.

Key words: Spodoptera exigua, lambda-cyhalothrin, resistance reduction, multi-function oxidases, resistance mechanism