Population dynamic consequences of parasitised-larval competition in stage-structured host parasitoid systems

Parasitism can influence many aspects of the host’s behaviour and physiology, which in turn can have a profound impact on their population and evolutionary ecology. In many host parasite interactions there is often a time lag between infection and the death of the host, yet little is known, experimentally or theoretically, about the effects that intra-class competition between parasitised and unparasitised hosts have on the host parasite population dynamics. In this article we address this gap in our understanding using a stage-structured mathematical model for a host parasitoid interaction, which has been parameterised for the Plodia Venturia experimental system. In the case where parasitised larvae do not compete and do not cannibalise unparasitised larvae, our model predicts a wide range of host parasitoid dynamics, ranging from host parasitoid generation cycles, to host generation cycles with parasitoid half-generation cycles, to host parasitoid equilibria, to host generation cycles with parasitoid extinction. However, when parasitised larvae can compete with their unparasitised larvae counterparts, the host parasitoid population dynamics can dramatically change. In particular, we show that high levels of competition exerted by unparasitised larvae upon parasitised larvae is more likely to lead to parasitoid extinction. In addition, we demonstrate that unparasitised host larvae that are sufficiently susceptible to intra-class competition, or parasitised host larvae that are sufficiently strong competitors, can have a stabilising effect on the host parasitoid population dynamics. The implications of these theoretical results are discussed in light of our understanding of host parasitoid interactions and host parasite systems in general.