Company amplification of estalpha2/estbeta2 and correlation between esterase gene copy number and resistance to insecticides in the field Culex pipiens pallens strains collected from Beijing, China.

Seven field strains of Culex pipiens pallens were evaluated in the summer of 2004 for their susceptibility to three insecticides. Larval bioassays were carried out in accordance with WHO mosquito susceptibility tests. Field samples showed resistance to dichlorvos and propoxur. Low levels of resistance to phoxim were detected compared with the laboratory-susceptible strain. Quantitative analysis showed that co-amplification of estalpha2 and estbeta2 occurred in the seven field Cx. pipiens pallens. However, the levels of transcription of estalpha2/estbeta2 genes in the field strains indicated a 15.9:1 ratio of estbeta32 cDNA over estalpha2 cDNA. Such observed differential transcription of the estalpha2/estbeta2 genes showed that the promoters regulating their expression are independent and of different strengths. Regression analysis of expression level of estalpha2/estbeta2 gene and LC50 value to insecticides showed a significant correlation between expression level ofestalpha2 gene and LC50 value to dichlorvos. However, correlation of estalpha2 gene and LC50 value to propoxur and phoxim, or estbeta2 gene and three kinds of insecticides, was not significant.