| Summary: | Propoxur-sel strains of <i>Culex pipiens quinquefasciatus</i> were derived from a lab-bred strain following 16 generations of propoxur exposure under sublethal concentrations of LC<sub>25</sub> (lethal concentration of 25%) and LC<sub>50</sub> (lethal concentration of 50%), respectively. This resulted in resistance development in F16 with ratios of 8.8× and 6.3×, respectively, compared with F0. The fecundity, longevity, sex ratio (F/M), and hatchability of the propoxur-exposed <i>Cx. quinquefasciatus</i> adult survivors and their offspring were decreased, with no effect on the emergence ratio and pupa survival rate. In addition, the intrinsic rates of increase (r), the net reproduction (R<sub>0</sub>), and the finite rate of increase (λ) of the <i>Cx. quinquefasciatus</i> offspring generations were also decreased significantly compared to F0. Correspondingly, the mean generation time (T) and the population double time (DT) in propoxur-sels were increased. Enhanced activities of cytochrome P450 monooxygenase and esterase were also observed in propoxur-sels, indicating that a detoxification mechanism might be responsible for resistance development in <i>Cx. quinquefasciatus</i>. Except for the three genes <i>cyp4d42v1</i>, <i>cyp4c52v1</i>, and <i>cyp6aa9</i> which displayed a coincidence in some degree in different treatments, induction by different doses of propoxur and constitutive expression in different generations of propoxur-sel strains resulted in an inconsistent identification of the P450 genes probably related with resistance.
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