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

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 1975control in until 1982control in . From 1983control in 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-a-(4-chlorophenyl)-a-(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.

Variations in susceptibility to common insecticides and resistance mechanisms among morphologically identified sibling species of the malaria vector Anopheles subpictus in Sri Lanka

“…GSTs are important in organophosphate resistance. Since high GST levels were further selected by the organophosphates, that were introduced after the cessation of DDT use in 1975/77, the vector populations are still resistant to DDT in the absence of DDT use for nearly three and a half decades as also earlier reported by Herath et al [21]. Metabolic resistance to organophosphates in insects can be due to mutated enzymes that metabolize the insecticide more rapidly [17,18].…”

Insecticide susceptibility status in individual species of the Anopheles gambiae complex (Diptera: Culicidae) in an area of The Gambia where pyrethroid impregnated bednets are used extensively for malaria control

“…Hence its detection by bioassay at low frequencies is impractical. This type of resistance appears to be relatively common in Aedes aegypti (Linnaeus), but there is no well documented case in Anopheles despite reports of pyrethroid resistance in this genus (Rongsriyam & Busvine, 1975;Prassitisuk & Busvine, 1977;Herath et al, 1988;Hemingway et al, 1989). The difference between the two mosquito groups may be due to differential exposure to DDT or pyrethroids, basic biological differences between the groups, or simply because the majority of studies of resistance in the anophelines rely on standard WHO bioassays with DDT alone and give no data on resistance to other insecticides or on the underlying resistance mechanisms.…”

Susceptibility of rice insect pests and their natural enemies to commonly used insecticides

“…DDT and y -BHC dust were heavily used in paddy cultivation until the mid 1970s 7 . DDT was introduced in 1957 and later it was replaced by malathion in 1975 8 . Usage of organophosphates such as malathion and fenitrothion in agriculture was legally restricted because of their important role in malaria control.…”