A revolving dose strategy to delay the evolution of both quantitative vs major monogene resistances to pesticides and drugs

The evolution of pesticide and drug resistance presents a daunting problem as the pests often evolve resistance after a short time. Debate once focused on whether resistance stems primarily from single major gene sources or sequential accumulation of quantitative traits. Such debates became moot, as examples of both are known, even within the same species and population. The type of resistance has important implications for determining the dosages that will delay the appearance of resistant populations. Successive high doses of a toxin (i.e. those well above the LD) can only select for monogenic 99 resistance, while the weaker selection pressure of low (near lethal) doses can also result in an incremental increase in quantitative changes leading to increasingly higher levels of resistance. We propose and use a model to mathematically test whether a revolving rotation of a series of low doses periodically alternated with a higher dose could delay the evolution of resistance longer than by applying a constant dose (either low or high) in every treatment. We suggestthis dosage rotation strategy for situations in which crops and alternative pesticides or drugs cannot be rotated, and suggestthatitbe used in conjunction with other methods of control (biocontrol, IPM) to further delay resistance. We model the evolution of resistance under truncation selection (more typical in the laboratory), as well as under a smooth selection regime that is more likely to describe selection in the field where pests develop more asynchronously, and application is less uniform. In addition, we consider the level of population control imposed by the toxin, since, for economic and health reasons, both the size of the pest population as well as the frequency of resistant pests must be considered. We also examine how the degree of dominance of the major resistance gene and changes in heritability with repeated episodes of selection affect the success of dosage rotation compared with constant doses. Modelled dosage rotations could delay resistance and suppress pest populations longer and require less pesticide (or drug) than successive constant doses. Constant doses may suffice to delay resistance over the short run if heritability is low, but revolving doses are essential over longer periods. The modelled results seem worthy of field validation, as they predict a longer resistance-free period at the lowest possible doses.