Natural enemies and the evolution of resistance to transgenic insecticidal crops by pest insects: The role of egg mortality

We explore the influence of egg mortality dynamics on the rate at which target pests evolve resistance to high-dose transgenic insecticidal crops. We develop a two-patch deterministic population genetic model in which pests can develop in either toxic or nontoxic (refuge) fields, and their eggs are subject to varying levels and forms of egg mortality. The three standard forms of egg mortality are studied: density independence (DI), positive density dependence (PDD), and inverse density dependence (IDD). Resistance is modeled as a single locus with a fully recessive allele that confers complete resistance with no fitness cost. Insect movement and oviposition is modeled as follows: males move panmictically before mating and females may either stay in their natal patch to oviposit or move after mating before oviposition. While our simulations show that both the magnitude and form of egg mortality can influence the rate of resistance evolution, important caveats apply. Higher levels of DI egg mortality can lead to substantial delays in resistance evolution, but this effect is dependent on the presence of intraspecific competition among larvae. The rate of resistance evolution is affected by the form of density dependence (DI versus PDD versus IDD), but these effects are dependent on at least some females ovipositing in their natal field. If this condition is met, the rate of resistance evolution is fastest when eggs are subject to PDD mortality and slowest when eggs are subject to IDD egg mortality. DI egg mortality produces intermediate rates of resistance evolution.