A host-host-pathogen model with free-living infective stages, applicable to microbial pest control

A model has been investigated of the dynamics of the interaction between two hosts which are both attacked by a common pathogen, where the pathogen has free-living infective stages the population size of which must itself be modelled explicitly, and where the host species do not interact with one another except through their shared pathogen. If either host interacted with the pathogen alone, three broad classes of dynamics would be possible: host regulation, pathogen persistence and pathogen extinction. Here, all possible types of combinations of hosts are examined: regulation-regulation (both hosts would be regulated if they interacted with the pathogen alone), regulation-persistence, regulation-extinction, persistence-persistence persistence-extinction and extinction-extinction. A wide range of dynamics is generated, including a number of patterns quite unlike those found in the one-host pathogen case (e.g. persistence in one host, elimination of the other host) and behaviour contingent on initial densities in the system. For clarity and pertinence, attention is focused on the case where one host is a pest, the pathogen is a potential microbial control agent, and the other host is a non-target species which it is undesirable to harm. The model suggests, broadly, that non-targets are unlikely to be seriously threatened in such cases, and also that non-targets, far from undermining pest control, are quite likely to contribute to its efficacy.