Abstract: The high-level resistant strain (Cyh-R) of Musca domestica to cyhalothrin was selected by using deltamethrin and pirimiphos-methyl to study the effects of several insecticides on Cyh-R housefly strain and explore the methods of housefly resistance management. The knockdown resistance (kdr) allele frequencies of these housefly strains were detected with PCR amplification of specific allele （PASA）technology in order to study the relationship of kdr allele frequency and resistance level. The results showed that the toxicity of cyhalothrin to Cyh-R strain after selection with pirimiphos-methyl tended to decline. The LD₅₀ value dropped from 2.8434 μg/housefly in F₀ to 0.4404 μg/housefly in F₈. However, the toxicity of cyhalothrin to Cyh-R strain after selected with deltamethrin tended to rise. The LD₅₀ value increased from 2.8434 μg/housefly in F₀ to 24.3249 μg/housefly in F₁₆. These results suggest that the high-level resistance of housefly strain to cyhalothrin could decline with the application of organophosphate pesticide or without insecticides, while increase with the application of pyrethroids. The results of PASA showed that kdr allele frequency of Cyh-R strain was 88.8%. The kdr allele frequency decreased to 69.7% without insecticide selection and 78.8% after pirimiphos-methyl selection, respectively, while increased greatly after deltamethrin selection, amounting to 98.9%. The results further indicate that there is a positive correlation between kdr allele frequency and LD₅₀ value of cyhalothrin.

Key words: Musca domestica, lamda-cyhalothrin, insecticide selection, resistance management, kdr allele frequency