Comparison of Pyrethroid Resistance in Adults and Larvae of Culex pipiens pallens (Diptera: Culicidae) from Four Field Populations in China

The susceptibility of adults and larvae from four field populations of Culex pipiens pallens (Coguillett) (Diptera: Culicidae) in China to deltamethrin, beta-cypermethrin, and permethrin was investigated using the World Health Organization standard susceptibility test methods. One to 2 d old nonblood fed female mosquitoes emerged from pupae resulting from wild-caught larvae (F₁ laboratory reared adults) were exposed to various doses of the pyrethroids. The larval bioassay was conducted using a range of concentrations to determine the LC₅₀ values. Using allele-specific polymerase chain reaction, the frequency of the kdr resistance gene was determined in each population. The bioassay data indicated that deltamethrin was the most active larvicide in all four populations with resistance ratios (RR) of 1.6–3.9-fold. This was followed by beta-cypermethrin with RR of 1.1–3.3-fold, while larvae from three mosquito populations from Beijing, Jinan, and Kaifeng had developed resistance to permethrin with RR of 12.6-, 24.0-, and 18.8-fold, respectively. The overall RR of larvae for all insecticides in ascending order was Changchun < Beijing < Kaifeng < Jinan; and ranking of the adult mortality was Changchun < Jinan = Kaifeng < Beijing. For the three pyrethroids tested, the correlation coefficient among the three test methods on the four populations and one lab strain was quite weak. The R value was 0.113–0.320 when we compared kdr detection for kdr% with LC₅₀, -0.565 to -0.793 when kdr% versus adult mortality test, and -0.750 to 0.505 for LC₅₀ versus adult mortality. Permethrin showed the best correlation (R = -0.793) between kdr frequency and adult mortality. This indicates that levels of susceptibility changes to an insecticide should not be based on a single method of assessment.