Pesticide selection pressure on Anopheles subpictus in Sri Lanka: comparison with two other Sri Lankan anophelines.

Adult Anopheles subpictus from Sri Lanka show a broad spectrum of resistance towards organophosphate insecticides but not to carbamates in contrast to the broad resistance to organophosphates and carbamates reported earlier for An. nigerrimus. In both species the frequency of resistance to malathion and fenitrothion increased between 1980 and 1987, despite the ban on agricultural use of these two compounds and the restriction of malathion to indoor residual spraying in malaria control since 1977. In contrast, An. culicifacies shows only low level specific resistance to malathion at a very low frequency. As An. subpictus breeds to a large extent in paddy fields which are highly contaminated by agricultural pesticides, and is highly endophilic, selection for resistance theoretically could occur through both agricultural and anti-malarial pesticide use. However, the anti-malarial use of malathion may have been less important, taking into consideration the low level of resistance of An. culicifacies which is also highly endophilic but breeds to a negligible extent in paddy fields.