Abstract: The genetics of resistance to avermectins (AVMs) and the relative fitness between resistant and susceptible strains to AVMs in diamondback moth (DBM), Plutella xylostella, were investigated during 1997～1999. The methods of cross and backcross between AVMsselected resistance strain and isogenic susceptible strain were used for exploring the inheritance mode of resistance to AVMs in DBM. The results showed that the degree of dominance of FRS and FSR were -0.64 and -052, respectively. It suggested that the resistance of DBM to AVMs was autosomal, incompletely recessive and probably polygenic confirmed by χ² test. The activities of acetylcholinesterase, carboxylesterase and glutathione S-transferases of F₁ hybrids were lower than that of resistance parental strain, and continuously declined in F₂ and BC. The resistant strain possessed 0.84 of fitness value relative to the isogenic susceptible strain.

Key words: Plutella xylostella, avermectins, resistance inheritance, population fitness