The Dynamics of Host-Parasitoid-Pathogen Interactions

Abstract

The growing importance of pathogens in biological control warrants an investigation into the likely effects of disease on the widely successful class of biological-control agents, the parasitoid. Our models consider a parasitoid that can exhibit a range of attack behaviors ranging from clumped to random. The pathogen is assumed to be transmitted by external stages that can span periods when the host is rare or absent. When these two natural enemies occur simultaneously in a given host, the result can be the elimination of one enemy by the competitor in all coinfected hosts or it can be some intermediate outcome. We show through an analysis of invasion that both natural enemies may coexist with constant, cyclic, or chaotic populations. Other outcomes are also possible. One enemy may exclude its competitor either unilaterally or on the basis of the initial densities of the host and the competing natural enemies. The particular outcome depends on four main factors: (1) the finite rate of increase of the host, (2) the clumping of parasitoid (and pathogen) attacks, (3) the relative extrinsic potentials of the competitors, and (4) the relative intrinsic potentials of the competitors. We conclude that although situations may arise in which the host is best controlled by only one natural enemy, multiple introductions are likely to be a sound strategy for a wide variety of systems.