In Culex pipiens mosquitoes, A2 esterase alleles are co-amplified with B2 esterase alleles in response to selection with organophosphate insecticides. In this study the amplified A2 and B2 sequences were compared between twelve strains from four continents by restriction mapping. The restriction maps were almost identical in each strain throughout 22 kb surrounding the genes, suggesting that this represents a constant core sequence. A polymorphism was found in two strains collected from Egypt and Kenya in the mid 1980s. This polymorphism was present in all copies of the amplicon, which suggests that a mechanism of sequence homogenization was operating, i.e. concerted evolution. These two strains were almost certainly descendants from the same population and migration probably occurred along the River Nile. Although the maps were almost identical in each strain, dot blotting demonstrated that amplification levels differed by up to 13-fold between strains. Thus the presence of the A2-B2 haplotype cannot be used to indicate the level of amplification or any particular degree of resistance.

For many decades, Botswana has been engaged in various malaria control activities that involved programmes that focused on the elimination of the malaria vector Anopheles arabiensis, by using DDT and pyrethroids. Despite the numerous and continuous application of these insecticides, studies have shown that there is susceptibility of this vector to DDT and pyrethroids in Botswana. Natural insecticides such as Bacillus thuringiensis and Spinosad, as alternatives to the use of chemicals, have shown to be effective against the eggs and larvae of DBM. Insect-resistant crop varieties were also found as alternatives in order to minimise insecticide resistance through the application of insecticides on insect infesting crops. The appearance of esterases B1 and A2-B2 in the Gaborone and Molepolole strains of Culex, respectively, indicates dispersion of these esterases through human migration.