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The rotation of insecticides used by the Onchocerciasis Control Programme in West Africa is reviewed and the motives for this rotation are shown to be not only management of temephos resistance in the Simulium vectors but also constraints on what compounds are usable at particular seasons. A computer model indicates that without these seasonal constraints there is unlikely to be an advantage in a pre-planned rotation of insecticides, as compared with the prompt switching of compounds as dictated by detection of build up of resistance and switching back to the original compound if the regression of resistance is found to give the opportunity to do so. The latter sequence of events can hardly be called a 'strategy' for resistance management, but is what any well-managed pest control programme would be expected to do. The use of an insecticide mixture is different in principle from the use of rotation and depends on the idea that if the mixture is used from the outset, when resistance to both components of the mixture is likely to be rare, the double resistance combination would be so rare as to be dwarfed in numbers by those insects which avoid exposure altogether. The prospects for successful use of a mixture depend on each component killing a very high percentage of the exposed insects which are genetically susceptible to it. Whether this condition is met could be tested, for example, in the case of exposure of mosquitoes to insecticide-treated bednets.

A microtitre-plate assay which distinguishes propoxur-resistant from susceptibles Anopheles albimanus Weidemann was used to test for linkage between the genes for propoxur- and dieldrin-resistance. The adult progeny of a backcross between a doubly-resistant colony and a fully susceptible colony were exposed in conventional test kits to the standard discriminating dose of dieldrin, and kept in the insectary overnight. Both live and dead insects were then assayed individually for propoxur-resistance. The results showed that heterozygotes for propoxur-resistance could be reliably distinguished from susceptibles whether or not they had been killed up to 24 h previously by dieldrin treatment. In this way all the backcross progeny could be scored at both resistance loci, and all four genotypic classes identified. Resistant and susceptible alleles at the two loci were inherited independently, demonstrating the absence of linkage. The usual method of testing for linkage between resistance genes is inefficient and open to bias, because insects have to be exposed to each insecticide in turn, and only half of them can be scored at both loci. The method shown here avoids these drawbacks.

Cultivated plants of Iraq and their importance , Cultivated plants of Iraq and their importance , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی