Insecticide resistance in populations of Anopheles gambiae in West Africa.

A method published by Elliott in 1958 for investigation of insecticide resistance in Anopheles larvae has been applied to strains of A. gambiae from various parts of West Africa, and the results analysed to provide information as to the proportions of the three possible genotypes in the different populations.At Kano, the gene conferring resistance to dieldrin and gamma-BHC appears to be increasing in the wild population, being present in about 50% and 42% of the population in the homozygous (RR) and heterozygous (RS) states respectively. Theoretical considerations suggest that in a situation where the male heterozygote is sterile, the gene in the minority will tend to become extinct in the absence of any specific selection. In a colony maintained in captivity for six generations without intentional selection the reverse was seen to happen, the S gene increasing its representation at the expense of the R gene.In Sokoto Province, some figures of the same type were obtained; these are analysed and found to produce results compatible with those from Kano, especially as regards the LC(50) level derived for the effect of gamma-BHC on the RS fraction of the population.At Freetown, a similar analysis suggests that the factor conferring resistance to gamma-BHC has also appeared. About 66% of the sample examined appear to carry the R gene.At Ibadan, samples of the population showed some tolerance to dieldrin, but the presence of the specific type of resistance seen elsewhere was not established. The tolerance appears to be non-specific, as some relative tolerance to DDT appeared to be present also.