Abstract: The mechanism of resistance to fenitrothion in the rice stem borerChilo suppressalit Walker is comprehensively investigated. Resistant strain ?, initiated from a field population, was selected successively by fenitrothion to the 7th generation and the resistant ratio was 14.1 as compared with that of the susceptible strain (S), which was obtained by selection with single-egg mass method. Synergism experiments showed that the synergistic ratio of DEF and PBO to fenitrothion in R strain were 1.3X and 34.8X, respectively. It suggested that resistance to fenitrothion was partially associated with multiple function oxidase (MFO). Furthermore, the activities of non-specific esterase, carboxylesterase (CarE) and MFO were determined in vitro. The results indicated that the activities of non-specific esterase in R and S strains showed no significant difference when using substrates of a-naphthyl acetate and β-naphthyl acetate. But there was difference in the activity to Care between R and S strains. The activities of Care from the larvae of R strainwith above-mentioned substrates (with 10^-4 esterin) were 2.42-and 2.92-fold higher than that of S strain, respectively. The ratio of 0-demethylation activities of MFO extracts from R strain to that of S strain was found to be 1.43. The acetylcholinesterase (ACHE) activities and the kinetic parameter, K_m and I₅₀ of paraoxon were also measured. The ratios of activities, K_m and I₅₀ of R strain to that of S strain were 1.6, 1.7 and 2.0, respectively. It was apparent that the Ache in R strain have altered. From the results obtained, it is concluded that the onset of resistance to fenitrothion in the rice stem borer is due to multi-factor including high activities ofCarE, MFO, enhanced O-demethylation of MFO and insensitivities of ACHE.

Key words: Chilo suppressalis, fenitrothion., resistant mechanism