Insecticide resistance in Anopheles gambiae in Sokoto Province

Abstract

The distribution of resistance to insecticides in A. gambiae, in the area of the Western Sokoto Malaria Control Pilot Project and Mass Campaign, was investigated by exposing 4th-instar larvae from eight different towns in the area to aqueous dilutions of BHC and DDT, using a short exposure period of 1 hour. The degree to which the normal population has been replaced by a population bearing the gene conferring resistance to dieldrin and BHC varied according to the different insecticidal regimens that had been applied. Where BHC only had been applied for 2 years, the population appeared to be homozygous for this gene, and also to possess an additional minor resistance factor specific for BHC. Where dieldrin only had been used, or dieldrin followed by a short period with BHC, a mixed population was found, consisting of homozygous and heterozygous resistant material, with little indication of the presence of homozygous susceptible individuals. Where BHC had been used once or twice only, the process of replacement had not proceeded so far, and pure susceptible material was still present. Where no insecticides had yet been used, a marked over-spill of the resistance factor was seen to have already taken place, reaching at least 14 miles from the nearest sprayed area. No sign of cross resistance to DDT could be detected.