Sublethal infection and the population dynamics of host-microparasite interactions

Summary 1. A large group of parasites, including many of the larval pathogens of insects, cause an infection from which infectious hosts are unable to recover. In addition, a proportion of those individuals that do not develop the lethal disease on exposure to the parasite may still be harmed by it. 2. We examined the role of these sublethal effects on host-population dynamics. Specifically we considered the case where there are three distinct classes of individuals: (i) susceptibles; (ii) infected and infectious individuals that will not reproduce and cannot recover; and (iii) sublethally infected individuals. 3. Parasites with sublethal effects are less likely to persist and control their host population. This is a consequence of the sublethally infected individuals not being infective. Less intuitively, the sublethal infection is highly destabilizing, increasing the risk of cyclic behaviour in host–parasite population densities. 4. Because sublethal infection acts as a destabilizing force in these host–parasite interactions, parasites with pronounced sublethal effects may be less effective as classical biological control agents.