Host-host-pathogen models and microbial pest control: the effect of host self regulation

A model has been investigated of the dynamics of the interaction between two hosts that are both attacked by a common pathogen with free-living infective stages, where the hosts are also subject to self-regulation. If either host interacted with the pathogen alone, two types of dynamics would be possible: an uninfected state where the host settles at its carrying capacity, and an infected state where the host settles at, or cycles around, a density lower than the carrying capacity. The three possible combination of two hosts have been investigated: uninfected-uninfected (both hosts uninfected if alone with the pathogen), infected-uninfected and infected-infected. A range of dynamics is generated, depending on parameter values, including infected co-existence of the two hosts (arrived at by a variety of routes), uninfected co-existence of the two hosts, exclusion of one host by the other which remains in an infected state, and a number of outcomes contingent on the initial densities in the system. Free-living infective stages make uninfected co-existence more likely and introduce additional contingency into the dynamics. The implications for microbial pest control are into the dynamics. The implications for microbial pest control are markedly different from those derived from related models without host self-regulation. There appears to be little chance of a non-target host undermining pest control, relatively little chance of the non-target enhancing pest control and a small but non-negligible threat to non-targets when parameter values are appropriate. The application of the results is commended but great caution is urged.