Invasion of drug and pesticide resistance is determined by a trade-off between treatment efficacy and relative fitness.

Drug and pesticide resistance are among the most pressing problems facing public, animal and plant health today. In order to design effective resistance management strategies it is imperative to identify criteria for the invasion of resistant forms. Two key determinants of the ability of a resistant pest or pathogen to invade are any inherent fitness costs to the resistant subpopulation, and the effect of treatment on the sensitive and resistant subpopulations. For two generic classes of model which encompass many of the standard models in this field, we summarize relative fitness and treatment efficacy via two simple parameters, and demonstrate that invasion of resistance depends critically on a trade-off between them. Thresholds for invasion are derived when the effect of treatment is a constant reduction in the life-history parameters of the pathogen, and when treatment efficacy varies periodically with the repeated application and subsequent decay of the chemical.