Development of malathion resistance in Culex quinquefasciatus Say (Diptera: Culicidae)

A malathion resistant colony of C. quinquefasciatus was developed in the laboratory. LC50 and LC90 for larvae were calculated at every generation and the values were 0.3 ppm and 1.13 ppm for first generation and 61.09 ppm, 136.3 ppm for 25th generation respectively. The fold increase in LC50 and LC90 were 2036 and 2726 folds respectively. Cross resistance against propoxur and chlorpyrifos showed 6.64 and 6.52 fold and 600 and 720 fold increase in their LC50 and LC90 values respectively. Triphenyl phosphate (TPP) and piperonyl butoxide (PB) were used as synergists and TPP indicated proportional decrease in LC50 and LC90 values while not much change was observed with PB. No change in biotic potential (larval hatchability, adult emergence and male and female ratio) between susceptible and malathion resistant colonies was observed.