Abstract
The stability of OP resistance due to amplification of esterase B1 or B2 was investigated in unselected strains. We show that resistance and amplification are both lost when strains are not homozygous for the presence of the amplified genes, due probably to fitness differences. In homozygous strains, both esterases remained amplified and expressed, and resistance was stable during at least 60 generations, indicating that events generating lower gene copy number are uncommon. Results are discussed with respect to the evolution of insecticide resistance.