Resistance to pyrethroid insecticides in Culex pipiens pipiens (Diptera: Culicidae) from Tunisia

Resistance toward 2 pyrethroïd insecticides (permethrin and deltamethrin), and an organochloric one (DDT) was analyzed in 18 samples of Culex pipiens pipiens common mosquitoes collected from different Tunisian areas between March 2002 and November 2005. Bioassays were performed over different larvae samples. The recorded mortalities, after 24 h exposure to increased doses of insecticides, were compared to those obtained on a susceptible reference strain (S-LAB). All samples were resistant to permethrin. However, a large variation in the tolerance to this insecticide was observed between samples. Resistance ratio levels at LC50 (RR50) for the analyzed samples were ranged from 1.5 to 9092 folds. For deltamethrin, 14 among 17 studied samples were resistant. RR50, in the resistant samples, were ranged from 3 to 453 folds. Resistance to the two tested pyrethroids and DDT were correlated. The use of synergists showed that the cytochrome P450-dependent monooxygenases had a major contribution to the permethrin and deltamethrin resistance and that the esterases (and/or glutathione-S-transferases) had only a minor contribution. Results were discussed in relation to resistance mechanisms and mosquito's control.