41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

Viral disease is a major component of the cyclic population dynamics of some Lepidoptera including western tent caterpillars. Epizootics of nucleopolyhedrovirus and host population subdivision provide an arena in which selection on virulence of virus and resistance of hosts could act. Theory predicts that epizootics should select for host resistance and that viral isolates should respond to this change on a population-by-population basis. Experiments provide evidence that these interactions are occurring but that patterns are weak as compared to other factors that determine the cyclic population dynamics. In addition there is no evidence for induced immunity or selection within a generation of tent caterpillars. The factors that promote the rapid development of NPV epizootics remain a mystery and are the topic of future research.

Invertebrate and other animal populations harbour genetic variation for immune capacity, which may seem paradoxical given the importance of immune performance to fitness. Why is functional variation in immune capacity not purged by natural selection? Why are susceptibility alleles not eliminated? Accumulating evidence suggests that environmental heterogeneity may retard the long term efficiency of natural selection and even maintain polymorphism, provided alternate host genotypes are favoured under different environmental conditions. “Environment” in this context may refer to abiotic factors such as temperature or food availability, or the genetic diversity of pathogens. These factors are controlled in many laboratory experiments measuring pathogen resistance, and yet they may be overwhelmingly important in the evolution of resistance, virulence, and, ultimately, coevolution. In this talk, I will discuss how the abiotic environment interacts with host and parasite genotypes to shape the evolutionary interactions between the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa.

Many recent emerging infectious diseases in humans, such as HIV and Ebola resulted from host shifts, are maintained within wildlife populations, and pose a substantial health risk. Most research has focused on controlling epidemics, however, using model systems can formulate predictions about the factors that lead to successful disease emergence. Here, I employ an insect-virus system to test the conditions that lead transient infections to become self-sustaining diseases. The Indian meal moth (Plodia interpunctella) and the Almond moth (Ephestia cautella) are worldwide pests of stored food products, and due to their tractability in laboratory experiments have been used to study host-parasite dynamics. EcNPV, a nucleopolyhedrovirus, is largely host specific on Ephestia, but can be transmitted to the new host, Plodia; demonstrating altered disease expression. PiGV, a granulosis virus, is host specific on Plodia, with little evidence of transmission to Ephestia. Here, I measure how infection route affects the infectivity of each virus on both hosts; testing the standard oral inoculation route versus direct intrahaemocoelic injections of the inclusion-bodied virus on important epidemiological parameters (infectivity, disease induced mortality, sub-lethal effects, covert infection). These findings will elucidate the important components of host-pathogen dynamics that can lead to long-term sustainability of emerging diseases.

The factors affecting the evolutionary ecology and dynamics of the interaction of Bt (Bacillus thuringiensis) with its lepidopteran host, the diamondback moth (DBM) will be discussed. The evolution of host resistance to Bt threatens the sustainable use of this bacteria to modern agriculture and our understanding of this host-pathogen interaction provides a fantastic system in which to explore ideas about the evolutionary ecology of pathogen virulence, pathogen transmission and host resistance. We will focus on three aspects of our research from within-host to field dynamics. First, from a detailed study of the within-host mechanisms of infection, we will discuss how the presence of alternative (non-toxin) genes (or Bt-related bacteria that express these non-toxin genes) are essential to Bt infectivity and transmission. Second, selection experiments have revealed how DBM resistance evolves in relation to Bt strain diversity and host population density and this work will be considered in conjunction with evolutionary theory on pathogen virulence and host resistance. Finally, from field experiments, we will illustrate how the diversity and population structure of native Bt floras (and related bacteria) are affected by the presence of pest insects (e.g., DBM) and/or Bt-based insecticides (e.g., DiPel).