Population response to resource separation in conservation biological control

Enhancement of resources to improve the impact of natural enemies in agroecosystems is an important component of conservation biological control. As many organisms depend on more than one resource throughout their life cycle it is necessary to take into account the distance separating vital resources and how it may affect natural enemy population development. It is also important to examine possible interactions with the fourth trophic level. In this study life history values for the parasitoid Diaeretiella rapae are incorporated into a mathematical model to explore these issues. In particular, a matrix model is employed to explore the impact of distance of separation between food (nectar in flowering plants) and reproductive (hosts found in the crop field) resources on the primary parasitoid population growth rate. Furthermore, the effects of spatial dissociation of resources on hyperparasitoids are considered. The results suggest that primary parasitoid population dynamics are influenced mainly by limitations in reproduction and by the response of parasitoids to separation of floral resources and host patches. Furthermore, hyperparasitism may cause primary parasitoid populations to decline and undermine biological control, depending on the extent to which hyperparasitoids systematically search for patches of primary parasitoids. The approach provides an initial theoretical framework to natural enemy use of multiple resources.