Studies on insecticide resistance in Anophelene Mosquitoes

The resistance spectra of different populations of a number of anopheline vectors of malaria were studied using some organochlorine, organophosphate, carbamate and pyrethroid insecticides. The species tested were Anopheles culicifacies from Sri Lanka, Pakistan and India, A. stePhensi from India and Iran, A. sacharovi from Greece and Turkey, A. maculipennis and A. superpictus f'rom Greece, A. hyrcanus from Turkey, A. albimanus from Panama and El Salvador. There was a diversity in the response to diff'erent insecticides both within species as well as betteen different species. With the exception of A. superpictus all other species showed either one or more populations with already developed multiple resistance or the potential for such development. Of signifiicance may be the resistances shown by a number of these populations towards some OPs considered as potential alternatives to DDT in malaria control. Most of these variations were considered a result of the nature of the selection pressures exerted on the relevant populations. However, the differences observed in the response to malathion, in A. culicifacies and A. stephensi from that in the species A. sacharovi, A. maculipennis and A. hyrcanus, all f'rom the Mediterranean region were attributed to a possible difference in the predominance of this resistance factor/factors. The malathion resistance in the former was common whereas it was rare in the latter group of species. The high malathion resistance in a multiple resistant population of A. culicifacies from India was shown to be of an almost completely dominant nature. At least two genetic factors, one controlling the specific carboxyesterase mechanism and the other possibly a more generalised mechanism were suggested to be involved in this resistance, the latter in addition conferring cross resistance to fenitrothion. The resistance to this insecticide in the Iranian population of [...] A. stenhensi was of an incompletely dominant nature and a single gene involvement was suggested. The malathion selected (laboratory) Iranian population in which only the carboxyesterase mechanism could be demonstrated, showed no cross resistance to fenitrothion, and other OPs lacking the carboxyester bonds. Continued selection of the same population with fenitrothion, however, showed increasing trends in the tolerances to OP's, chlorphoxim, pirimiphos methyl and phoxim, suggesting a possible relationship between these resistances, In A, albimanus from El Salvador, evidence from use of synergists, only, had suggested possible involvement of carboxyesterases, and mfo 's in the malathion resistance. In A. sacharovi from Turkey, both fenitrothion and iodofenphos resistances were attributed to hydrolytic esterases as well as mixed function oxidases. While the DDT resistance in A. culicifacies. and A. stephensi was attributed to the specific DDT-ase mechanism, that in the populations of A. sacharovi from Turkey was suggested to involve DDT2.ss. mfo's and a third factor, possibly in the nature of a knockdown resistance mechanism. At least one or more of these mechanism were considered to have imparted cross reeistance to the pyrethroid insecticides.