Although it is safe to say that most biological control workers prefer to solve their pest problem in ecological time, the potential influence of evolutionary factors on long-term success has long been appreciated. Disadvantageous evolutionary changes in weed control programs have been a particular concern. For example, the agent may undergo a host-plant shift following introduction. Even so, formal theory has only recently addressed such concerns, and the application of theory to specific problems has been somewhat limited. This is likely to change in the near future, as the tools of molecular biology and the underlying genetic theory that supports them make the genetic manipulation of agents feasible and affordable.

A biological control agent achieving sustained control of a target potentially exerts strong selection pressures on that target to evolve resistance to the agent, analogous to the evolution of resistance to pesticides. This is a problem that is often mentioned in the biological control literature, but is not well studied. Holt et al. (Chapter 13) use a simple, population-genetic model to address it. Their focus is on spatial heterogeneity as measured by differences in the susceptibility of hosts across habitats of varying quality, echoing the theme of the previous section. They also stress the importance of non-equilibrium population dynamics in delaying or preventing the evolution of resistance to parasitoids. Although their discussion is theoretically driven, it sheds light on the applied problem of the breakdown of biological control over longer time periods.