The Dynamics of Insect-Pathogen Interactions in Stage-Structured Populations

Few insects are susceptible to pathogens at all stages of their life cycle. The population dynamic consequences of introducing age and stage structure are examined through the use of models of insect-pathogen interactions in which hosts are assumed to contract the disease by ingesting infectious particles in the environment. The models are phrased as delay-differential equations that assume constant demographic parameters within age or stage classes. The first and simplest model assumes that only the adult insect is susceptible to infection, while the second model assumes a juvenile susceptible stage. Two further variants of the second model are considered: one in which the disease does not have a fixed incubation period and one that assumes that a reservoir for pathogen particles exists in the environment. Both linear and nonlinear transmission processes are examined. The introduction of explicit time delays has little effect on model equilibria but strongly influences dynamics. Multigenerational host-pathogen cycles are predicted by all models, although the parameter space in which they occur is markedly model specific. A type of population dynamics previously observed in host-parasitoid models, but not in insect-pathogen models, was also found: cycles with a period of one host generation or of a fraction of a host generation. This dynamic behavior arises through the interaction of unequal time delays in the two populations. The absence of a fixed incubation period and the presence of a pathogen reservoir both tend to be stabilizing. The models developed in this article are most applicable to insect-pathogen interactions in tropical or subtropical environments, where populations are not synchronized by a severe winter.