Dynamical Effects of Host Size- and Parasitoid State-Dependent Attacks by Parasitoids

Abstract

1. Immature insect stages that are hosts for parasitoids usually increase in size as they age. Parasitoid responses include feeding on the smallest stages and laying progressively more female-biased and larger clutches as host size increases. Among koinobiotic parasitoids, eggs laid in older hosts tend to develop faster and survive better as larvae. Additionally, the parasitoid's decision concerning host feeding and sex allocation can also depend on her egg load. Except for the pure development rate effect in koinobionts, these processes all result in a larger gain to the future female parasitoid population from attacks on older hosts. 2. We introduce a framework for evaluating the dynamical consequences of these processes by summarizing existing models describing an invulnerable adult host stage, host size-dependent sex allocation and host feeding, and parasitoid state-dependent attacks when the host has no stage structure. 3. We introduce new models of host size-dependent clutch size, a combination of that and host size-dependent sex allocation and host feeding, and a combination of the latter and parasitoid behaviour that depends on her egg load. A model in which the parasitoid responds to the mean egg load of the population is a good approximation to one that follows each egg load class individually. Finally, we incorporate the two aspects of koinobiotic parasitoids. All of the behaviours examined, except for faster parasitoid development on older hosts, have the same qualitative effect on dynamics: they introduce or increase the strength of delayed density-dependence in the parasitoid population. This arises because when the parasitoid attacks younger hosts, which produce a relatively small gain to the future female parasitoid population, these hosts are not available later, when they would have produced a larger gain. 4. We discuss how the results provide a unified framework that brings together an apparently disparate set of processes in parasitoid-host interactions and thus simplifies and generalizes the theory. We incorporate these with related modelling results on stage-structured parasitoid-host interactions into a general framework.