Selection of cypermethrin resistance in mosquito Culex pipiens larvae in Assiut, Egypt

To induce cypermethrinresistance in Culex pipiens larvae, about 20000 4th instar larvae of Cx. pipiens were collected from Assiut field and exposed to cypermethrin selection pressure for fifteen generations using dipping method. This study revealed that the resistance ratio (RR50) of the parent strain (G0) was 14.49-fold as compared with susceptible strain. This RR50 value increased gradually from each generation to the next one as a result of selection pressure with cypermethrin. The regression line for cypermethrin against the parent field strain showed slope value 2.38 which indicate a moderate level of heterogeneity of parent strain to cypermethrin. The first five generations during the course of cypermethrin selection showed that resistance ratio (RR50) was increased gradually from one generation to the next (The RR50 increased from 14.49-fold in G0 to 135.40-fold resistance in G5). By the 6th generation, the rate of resistance development to cypermethrin was fairly rapid where; the RR50 to cypermethrin increased from 135.40-fold in G6 to 272.00-fold resistance in G11. With the beginning of the 12th generation, the development of resistance to cypermethrin was increased very slowly during the last four generations from 279.92-fold in G12 to 295.52-fold resistance in G15 (plateau phase). Finally, the slope values were volatile until the eleventh generation (2.38 – 3.63) and then relatively proven in the last four generations (4.55 – 4.52). This is evidence of increasing homogeneity of selected strain in the last five generations.