[Progressive acquisition of resistance to chlorpyrifos in Culex pipiens (L.) larvae in the South of France].

A high resistance level has been detected in certain Culex pipiens populations after seven years of chlorpyrifos larval treatments in the Montpellier area (Southern France). The highest rate recorded in 1973 was x 15, in 1974 x 33,3, in 1975 x 47 and in 1976 x 73. The resistance level depends on the application frequency of this larvicide and the various biotope types. "Closed populations" are subjected to a higher selection pressure than "open populations". It is now necessary to find a replacement compound for chlorpyrifos as there is a quick reinfestation and thus the treatment frequency had to be increased these last two years.