HOST-PATHOGEN POPULATION-CYCLES IN FOREST INSECTS - LESSONS FROM SIMPLE-MODELS RECONSIDERED

A host-pathogen model with free-living infective stages and host self-regulation has been investigated with a view to clarifying the possible role of pathogens in the cyclic dynamics of forest insect pests generally, and of a granulosis virus in the dynamics of the larch budmoth in particular. Contrary to previous related analyses, lacking self-regulation. no support is provided for the view that host-pathogen interactions alone are capable of generating cyclic dynamics of the type observed in forest Lepidoptera. Overall, cycles are less likely here than in the absence of simple self-regulation, and parameters from field estimates usually fail to generate cycles. Even where cycles are generated, and the cycle period corresponds with field data, disease prevalence tends to peak at too high a value, and host density peaks at a very much lower value than field data suggest. Notwithstanding, the model emphasises that host-pathogen interactions may still be important in generating observed cycles, even though they are not solely responsible for them.