Chlorpyrifos Resistance Characteristics of Culex pipiens (Diptera: Culicidae) from Northern Tunisia

We collected four field populations of Culex pipiens larvae from Northern Tunisia to study their chlorpyrifos resistance characteristics. Assays were performed using ethanol solutions of chlorpyrifos and data were analyzed using a log-probit program. All samples were resistant to chlorpyrifos (RR>1, p<0.05) and a large variation in the tolerance to this insecticide was observed. The sample 3 (Northeast) was the most resistant with a level higher than 10,000-fold (RR50=43,174). The level of resistance was lower, not exceeding 5-fold in samples 1 (North West). CYP450, EST and/or GST enzymes accounted for only a small part of the observed resistances. Synergists tests confirmed biochemical study where four esterases were detected in studied samples with low and average frequencies (0.03%-0.68%). Moreover, the mortality due to propoxur was significantly correlated with the LC50 of chlorpyrifos (P<0.01) indicated an insensitive AChE 1.