The effect of increasing pest resistance to insecticides on the optimal control of a pest population is investigated by constructing a single-pest, single-crop management model and analyzing the resulting optimality conditions. Use of insecticides under these conditions results in both monetary costs and user costs. It is suggested that growers do not generally consider these user costs and therefore do not obtain maximum profits. The dynamic formulation of the model results in an extension of the literature dealing with the "economic-threshold," which under reasonable conditions is shown to be increasing during the course of the season.

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