Dean Pennsylvania State University University Park, Pennsylvania
Models and experiments of pesticide mixtures for delaying resistance, dating back to the 1980s, indicate that a key feature of the efficacy of pesticide mixtures in delaying resistance is redundant killing, i.e., that exposure to each pesticide if used alone Is sufficient to kill nearly 100% of the susceptible pests. If pesticide mixtures are to be used, formulated premixed insecticides probably increase the likelihood that each insect will receive a lethal dose of both pesticides at initial application, i.e., there is a high spatial correlation of exposure. In some cases, there may even be synergism between the two pesticides, such that redundant killing is even more likely. There may be cases where it's virtually impossible for the pest to evolve resistance to at least one of the pesticides, but if that proves to be true, there would be less cost and environmental impact if only the resistance proof pesticide was used.
However, mixtures cannot offset the decay of residues, which at some time after application, may allow some exposed susceptible pests to survive, providing the opportunity for selection for resistance to the mixture. In some specialized cases, such as insecticide-treated bed nets, or in GM crops, excessive decay of residues may be prevented. Another challenge with pesticide mixtures in an IPM context is that although many pesticides are selective and have little impact on natural enemies, it's less likely that both components of a mixture will be so benign, increasing the chance of pest resurgences and increased pesticide dependency
