WHEN IS BIOLOGICAL-CONTROL EVOLUTIONARILY STABLE (OR IS IT)

The evolution of resistance by insect and weed pests to chemical pesti cides is a problem of increasing importance in applied ecology. It is striking that the evolution of resistance by target pest species in bi ological control is much less frequently reported, particularly in con trol involving parasitoids and predators, rather than pathogens. Altho ugh it is conceivable that this reflects biases in reporting or freque ncy of application, we suggest that there is a puzzle here worthy of s crutiny, and we outline several potential underlying causes. In order of discussion (not necessarily of importance), these are: (1) lack of genetic variation; (2) genetic constraints on selection; (3) weak sele ction; (4) temporally varying selection; and (5) coevolutionary dynami cs. We, in particular, focus on the potential for weak selection on th e host for increased resistance, despite effective control. The very s patial mechanisms (e.g., refuges, metapopulation dynamics) believed to facilitate the persistence of many natural enemy-victim systems with strong biological control may also incidentally provide an environment where selection is weak on target pests to evolve improved resistance to control agents, thereby biasing coevolution toward the enemy. The basic insight is that in a spatially heterogeneous environment, a stro ng limiting factor on a population can be a weak selective factor. The hypotheses presented here provide ingredients needed to predict which biological control systems might be evolutionarily stable, and which not. Our aim in this thought piece is to stimulate more attention to t he evolutionary dimension of biological control systems.