Parasitism of the European corn borer by Macrocentrus grandii: Linking individual behavior to spatial patterns of parasitism

A great deal of scientific attention has been given to the diverse factors that influence parasitoid foraging, but these individual behavior characteristics have rarely been connected to spatial outcomes for the host. The purpose of this dissertation is to link individual behavior of the specialist parasitoid Macrocentrus grandii to the spatial patterns of parasitism experienced by its host, the European corn borer (Ostrinia nubilalis). Chapter 2 documents that density-dependent parasitism by M. grandii is modified by spatial proximity effects: small aggregations of hosts receive lower levels of parasitism than larger aggregations, but only when sufficiently isolated from larger aggregations. Chapter 3 shifts focus to inter-specific proximity effects, showing that parasitism of European corn borer by M. grandii is also influenced by an indirect interaction with a non-host herbivore. M. grandii avoids open habitats such as that created by the corn rootworm, resulting in reduced parasitism (associational resistance) for co-occurring European corn borer. Chapter 4 is a theoretical consideration of the empirical data described in Chapter 2 that suggests that simultaneous patterns of positive density dependence and spatial proximity effects (spillover) can only occur when foragers have an immigrative response: more parasitoids must arrive at large aggregations than small aggregations of hosts. Chapter 5 develops an optimal foraging model for intermittently-refuged prey that predicts M. grandii behavior within different sized host aggregations should also vary as a function of host density: parasitoids should invest more time waiting for hosts to emerge from their refuges when perceived host density is low. Observation of M. grandii behavior in the field qualitatively confirmed this prediction, indicating that M. grandii can adjust its foraging behavior to efficiently exploit local host density conditions. However, Chapter 6 shows that previous experience can have a persistent effect on M. grandii patch-leaving behavior, indicating that its behavioral response takes into account more than the immediate spatial and temporal environs, and can potentially generate spatial patterns of positive density dependence. This dissertation therefore presents plausible behavioral mechanisms that are consistent with empirical patterns of parasitism, and substantially improves our ability to connect individual behavior to population processes in this system.