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Susceptibility of Larval Stage of Mosquito Culex pipiens against Chlorpyrifos Insecticide in Southern Tunisia

Jaber Daaboub, Ahmed Tabbabi, Raja Ben Cheikh, Ali Lamari, Ben Jha Ibtissem and Hassen Ben Cheikh

Three populations of mosquito Culex pipiens were used to study the susceptibility of larval stage against organophosphate chlorpyrifos and carbamate propoxur insecticides in Southern Tunisia. All samples were resistant to chlorpyrifos (RR>1, p<0.05) and the tolerance to this insecticide was varied between 1.8 and 1318. The sample # 3 was the most susceptible and disclosed a little difference when compared to S-Lab (RR50=1.9). The highest resistance levels (>1,000-fold) was recorded in samples # 2 (South East). The level of resistance was lower, not exceeding 2-fold in samples # 3 (Far South). Total esterase activity of field populations detected one or several esterases in all the studied samples, with the exception of the sample # 3 despite the addition of DEF showed that the increased detoxification by EST and/or GST was not responsible for chlorpyrifos resistance in these samples. Oxidative metabolism accounted for only a small part of the observed resistances because chlorpyrifos resistance ratios remained high in the presence of the Pb. The mortality due to propoxur was significantly correlated with the LC50 of chlorpyrifos and indicated an insensitive AChE in samples # 2 which showed the highest resistance levels to chlorpyrifos insecticide (1%). Results were discussed in relation to resistance mechanisms and Culex pipiens control.