Selection for malathion resistance in Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae): fitness values of resistant and susceptible phenotypes and their inclusion in a general model describing the spread of resistance

Abstract

A malathion-resistant strain of Oryzaephilus surinamensis (L.) was subjected to selection with malathion for ten generations at three doses, 65, 260 and 390 mg/m². The relative fitness of the resistant homozygote, the heterozygote and the susceptible homozygote were estimated at each dose and in an untreated control population by comparing the change in frequency of the susceptible homozygote from generation to generation. In the absence of the insecticide, the resistant genotypes had a fitness coefficient of 0·82 compared to the susceptibles, whereas following selection at the highest dose of malathion, the fitness coefficient of the susceptible homozygote was 0 and that of the heterozygote 0·4, compared to the resistant homozygote. With selection at the highest dose of malathion, the frequency of the resistance gene rose from 0·5 to around 1·0 during the experiment, and this was accompanied by a 2·3-fold increase in the resistance factor at the ED50. A model is provided to show how the data collected in these experiments can be used to predict the increase in resistance under various treatment regimes. The model shows that although resistance would be delayed longest at the lowest dose, such a dose would fail to control the pest population. Using the data from these experiments, the model predicts that the best combination of control and delay of the spread of resistance would be achieved by using the highest dose of malathion while allowing the maximum acceptable proportion of beetles to remain untreated.