DDT resistance in Anopheles culicifacies Giles and A. subpictus Grassi (Diptera: Culicidae) from Sri Lanka: a field study on the mechanisms and changes in gene frequency after cessation of DDT spraying

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Abstract
The frequency of DDT resistance in Anopheles culicifacies Giles in Sri Lanka remained relatively constant from the cessation of DDT use for malaria control in 1975 until 1982. From 1983 to 1986, the frequency of resistance decreased slowly but significantly. In contrast, the resistance gene frequency in A. subpictus Grassi declined between 1980 and 1983, after which the trend was reversed, and there was a significant increase in DDT resistance in 1984–86. This rise in DDT resistance cannot be attributed to positive DDT selection pressure as there has been little or no agricultural use of DDT during this period. It is also unlikely that the rise in resistance is due to pyrethroid selection as the quantity of pyrethroids used on the island is low and there is no evidence of pyrethroid resistance from WHO susceptibility tests. Studies with the synergists FDMC (4-chloro-α-(4-chlorophenyl)-α-(trifluoromethyl)benzenemethanol) and piperonyl butoxide indicated the involvement of DDT-dehydrochlorinase rather than oxidase in the resistance. This was confirmed by metabolism studies, as DDT-resistant adults of both A. culicifacies and A. subpictus showed an increase in the rate of DDE production, which was also correlated with an increase in glutathione transferase activity.