Exposure time versus concentration in the WHO standard test for mosquito resistance to chlorohydrocarbon insecticides.

In order that the WHO standard test for mosquitos may yield a median lethal dose value (LC(50)) in the case of highly tolerant species and resistant strains, it is necessary to increase the exposure period. The experiments described in this paper, done with a DDT-tolerant strain of Culex pipiens fatigans, show that doubling the exposure period has the same effect as doubling the DDT concentration, since it doubles the uptake of DDT into the mosquitos, and has the result of halving the LC(50) obtained.