Pyrethroid resistance was investigated in thirty-three samples of Culex quinquefasciatus Say from twenty-five cities in Côte d'Ivoire and Burkina Faso. Permethrin resistance ratios at LC50 ranged from 9.5- to 82-fold in Côte d'Ivoire and from 17- to 49-fold in Burkina Faso. For deltamethrin, resistance ratios were lower and ranged from nine to thirty-eight in both countries. A strain was selected with permethrin to investigate resistance mechanisms. After forty-two generations of selection, permethrin resistance level reached 3750-fold, but deltamethrin resistance remained unexpectedly unchanged. This indicated that a specific mechanism was involved in permethrin resistance. Synergist assays and biochemical tests indicated that resistance was partly due to P450-dependent oxidases. A target site insensitivity (kdr) was also involved, associated with DDT cross resistance and a dramatic loss of permethrin knockdown effect on adults. This resistance should be taken into consideration when planning the use of pyrethroid-impregnated materials in urban areas, as Culex is by far the main source of nuisance. Any failure in nuisance control due to resistance is likely to demotivate people in using impregnated materials.

ABSTRACï Resistance mechanisms of a strain (MSE) of Culex pipiens L., collected in sauthern France in 1979 and highiy resistant to chlorpynfos. were investigated by comparing the resistance characteristics to vanous organophasphates and carbamates in the absence or presence of synergists and determining the biochemical characteristics of four enzymes (esterases. glutathione-S-transferases, mixed function oxidases, and acetylcholinesterase) compareci with a susceptible strain and a chlorpyrifas-resistant strain (S54) collected in the same area in 1960 and 1974, respectively. Cblorpyrifos resistance in S54 was due to a detoxifying esterase as previously dexribed, whereas resistance in MSE was associateci with an acetylcholinesterase insensitive to the inhibition by chlorpyrifoxon and some carbamates (propoxur, carbosulfan. and carbaryl), and with an increase of oxidative metabolism.

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