The effective dominance of resistance genes in relation to the evolution of resistance

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Abstract
Field selection for a rare insecticide resistance (R) gene can be substantially delayed if R is rendered effectively recessive [if all or most heterozygotes ( R + ) are killed by the field dosages] and if a representative sample (+ +, R +, RR) of the population is permitted to escape exposure. The relationship between heterozygote mortality and escape in influencing selection rates has been considered at various frequencies of R. The cases when mating precedes selection, and when selection precedes mating, have been calculated using computer simulations. It is concluded that when R genes are rare it is advantageous (a) to apply the highest possible doses of insecticide in such a form that all exposed insects pick up an amount sufficient to kill all or most heterozygotes, (b) to apply the insecticide to adults after they have mated, rather than to younger adults or to larvae, and (c) to ensure that the proportion of insects given the opportunity to escape contact is as high as practical considerations allow. Once resistance is obvious (when samples are brought into the laboratory for tests), the R gene frequency will have risen to a level which makes the suggested strategy no longer valuable. Alternative strategies are briefly discussed.