Factors affecting the rotational use of insecticides for the management of resistance in mosquitoes

The rotation of the use of chemically unrelated insecticides has been advocated to delay the build up of resistance. To examine this concept in the laboratory, Anopheles albimanus and Culex guinquefasciatus were subjected to two kinds of rotational selection which may be referred to as short term pre-planned rotation and "opportunistic" rotation. No difference was observed between these two methods in term of the time for each resistance level to reach 5Q%. The effectiveness of selection for resistance depends on the protection conferred by resistance genes and this was tested by laboratory releases of the DDT resistant and susceptible homozygotes and heterozygotes of An. gambiae into DDT sprayed and unsprayed miniature huts. All the genotypes were killed with freshly applied DDT but survivors were observed from month 2, 3 and 5 onwards for RR, RS and SS genotypes, respectively. Persistence of DDT on the sprayed wall and roof of the mud hut was studied by bioassays and biochemical assays on scrapings from the wall and roof. Linkage between resistance genes could effect the rotational use of insecticides. No linkage was found between propoxur and dieldrin resistance genes by combining bioassay and biochemical methods. As a preparation for a field studies in Malaysian field collected Aedes aegypti and Culex guinguefasciatus larvae were selected with temephos and Bti. Cx guinguefasciatus responded to temephos selection but Ae. aegypti did not respond to temephos and neither species responded to Bti selection. ~ Caged adults were exposed to thermal fogging in the field. The partially resistant Cx guinguefasciatus strain hardly survived any better than susceptible strains. Resistant and susceptible larvae were exposed to water samples from containers which had been treated with temephos sand granules. 100% mortality was obtained for all the strains up to week 6. Resistant Culex started to survive at week 7 but susceptibles did not do so until week ten. The prospects for the various proposed strategies for resistance management are discussed.