The impact of diseases and pathogens on insect population dynamics

Parasitism is a distinct and diverse life-history mode that is prevalent in all major taxa. Understanding how parasitism affects the distribution and abundance of species is one of the central themes of population ecology. However, the study of the population ecology of insect host–pathogen interactions is not as well developed as the study of other insect enemy– victim interactions, such as predator–prey or host–parasitoid systems. In some ways, this is surprising given the economic impact insects have, the potential for the development of microbial insecticides, the impact novel diseases can have on sensitive populations and the effects pathogens have on the structure of species assemblages. However, unlike predator– prey interactions, a major disadvantage in the study of insect disease is the problem of taxonomy and identification. Although modern molecular methods provide one approach for clear accurate disease diagnosis, this collective group of organisms still provide challenging taxonomic problems. Notwithstanding, the study of insect host–pathogen dynamics has a number of distinct advantages over other trophic interactions in species assemblages (Fuxa & Tanada, 1987) The ecology of host–pathogen interactions is diverse: parasites can affect host behaviour, can enhance or prevent species invasions, promote genetic variation, affect mate choice and impact on the conservation of endangered species. Not only does understanding the consequences of parasitism require novel and detailed empirical investigations, but also a theoretical framework is essential to posit hypotheses and make predictions. The initial development of appropriate mathematical models for insect host–pathogen interactions examined relatively unrealistic models that captured the abstract generality of these trophic interactions (Anderson &May, 1980, 1981). More recently, more realistic and system-specific models have been developed to make predictions and generate testable hypotheses (Onstad & Carruthers, 1990; Briggs & Godfray, 1995, 1996; Bonsall et al., 1999). However, the role of appropriate theoretical frameworks and epidemic models for insect host–pathogen interactions remains relatively undeveloped. In this article, the role of pathogens on the population dynamics of insects is explored. Throughout, theoretical models are developed to illustrate the role of particular disease mediated processes in insect host–pathogen interactions. First, the processes that affect pathogen invasion and persistence are examined. In particular, the concepts of a disease threshold, the basic reproductive ratio and disease transmission are discussed as key determinants in the success of the spread of a disease through a susceptible insect population. Second, statistical and mechanistic approaches to the population dynamic consequences of parasitism are considered. Time series of forest insects are presented and the mechanisms by which parasites may be responsible for the observed fluctuations are discussed. Finally, the role of parasites on the structure of insect assemblages is explored. The article concludes with a brief overview and future direction in the study of insect host–pathogen population dynamics.