Characterization of resistance to pirimiphos-methyl (an organophosphate insecticide) in Culex pipiens pipiens (Diptera: Culicidae) from Northern and Southern Tunisia

Despite the public health importance of Culex pipiens pipiens, their resistance to pirimiphos-methyl insecticides has not been explored enough. Late third and early fourth larvae of Culex pipiens pipiens were collected from three localities between 2003 and 2005 in Northern and Southern Tunisia. All bioassays were carried out using pirimiphosmethyl and propoxur insecticides. Populations of Culex pipiens pipiens were susceptible, moderate and resistant to pirimiphos-methyl insecticide. Resistance to this compound ranged from 2.62 in sample # 2 to 19.9 in sample # 1. The moderate resistance (5.25) was recorded in sample # 3. Synergist's tests showed that the resistance to pirimiphos-methyl was not affected by detoxification enzymes. However, biochemical assays showed the involvement of both metabolic (esterases) and target site (insensitive acetylcholinesterase) resistance mechanisms. The highest frequencies of the resistant phenotypes ([RS] and [RR]) (<0.74) were detected in the most resistant samples (#1). Four esterases enzymes including C1 encoded by the Est-1 locus and three esterases encoded by the Ester super locus: A2-B2, A4-B4 (or A5-B5, which has the same electrophoretic mobility) and B12 were detected. The highest (0.61) and the lowest (0.22) frequencies of these esterases were recorded in samples # 1 (Sidi Hcine) and # 2 (El Fahs) which recorded the highest and the lowest level of resistance, respectively. Monitoring of insecticide resistance should be evaluated regularly for management of vector control.