THE POPULATION-DYNAMICS OF PATHOGENS THAT CONTROL INSECT OUTBREAKS

The population dynamics of an insect pathogen with a free-living infec tious stage are considered. Most of the time, the insect host is regul ated at an equilibrium density below that at which the pathogen can ma intain itself. At regular or irregular intervals, the host escapes its normal regulatory agencies resulting in an outbreak that is eventuall y brought under control by the pathogen. Models are developed to explo re the factors that influence the persistence of the pathogen in such an interaction. If all outbreaks are assumed to be of identical form, and if the occurrence of outbreaks is controlled by a Poisson process, analytical estimates of the mean and variance of the persistence time can be obtained. Such estimates provide good approximations to the nu merical predictions of more realistic models in which the dynamics dur ing an outbreak are represented explicitly so that outbreaks vary in s ize. In the latter models, outbreak size is predicted to be linearly r elated to the time since the last outbreak. It is argued that the type of model developed here may describe many naturally occurring insect- pathogen interactions, and that it can assist in the choice of pathoge ns for use in biological control and in the design of genetically mani pulated pathogens. (C) 1995 Academic Press Limited