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

Parasites can affect the outcome of biological invasions in different ways. Outbreaks of parasites may lead to host population crashes and resultant community change. But parasites do not only act on host population density. We present studies of short-term, behavioural effects of parasites and their effect on invasions. We focus on parasite regulation of crustacean invasions. Using empirical studies and mathematical modelling, we show that two parasites play keystone roles in UK amphipod invasions. Firstly, the microsporidian Pleistophora mulleri may facilitate invasion by two smaller species of amphipod; it has no direct effect on the survival of the native G. d. celticus, but infected animals are less likely to prey on the two smaller invaders. Secondly, the acanthocephalan Echinorynchus truttae may promote coexistence, as infection of the invading species Gammarus pulex reduces its predation on native G. d. celticus. Microsporidia may also drive crayfish invasions. We provide evidence from sequence data that the invading signal crayfish has acquired Thelohania contejeani (porcelain disease) from the native. However, whilst the invader may suffer little from the infection, transmission to the native can cause reduced activity and mortality and so increase the rate of extinction of this species.

Most studies on insect pathogens are within the context of insect pest control and there has, in comparison, been little research into the role that pathogens may play in regulating natural populations of insects. Studies of pathogens in natural populations present a number of methodological and sampling challenges. For example, the host range of a pathogen within a natural insect population may be difficult to define as groups of unrelated hosts may be infected. In comparison to agroecosystems there are generally a greater number of species in natural habitats making it necessary to precisely define the particular habitat a host may occupy. Host density may also be low and therefore pathogen epizootics may not occur regularly making direct observations of pathogens difficult. Sampling the habitat in these cases may be more useful in assessing the prevalence of particular pathogen groups. Sampling strategies also need to account for host phenology as pathogens may occur as low level, covert infections present in different host life stages and at different frequencies during host development. I refer to examples of methods being used in a project to assess the prevalence and distribution of UK Lepidoptera pathogens and draw on work from other research groups.

Lepidoptera are attacked by numerous virus strains, but in many cases do not show obvious signs of infection. Molecular techniques now allow the monitoring of viruses in wild populations without overt disease, and this has revealed a surprising diversity of pathogens. Baculoviruses were traditionally known for their lethal impact on hosts but are now known to also form persistent, almost symptomless infections, first detected in Mamestra brassicae. Such hidden infections may be vertically transmitted over many generations, be vectored by pathogens, have major to minimal impacts on host fitness and may interact with other invading pathogens. We report the detection of covert infections caused by baculoviruses and cypoviruses in a range of species, and explore their ecological significance.

Entomopathogenic microsporidia produce chronic infections that often do not produce obvious symptoms.This group of primary pathogens is, therefore, best known in managed insects or in well-studied pest populations. Microsporidiosis of domesticated insects such as honey bees and silkworms are known to cause serious effects on colony health and productivity. In the field situation, however, microsporidian disease is more difficult to observe and the effects on non-pest wild insects have rarely been studied. Nosema bombi, a microsporidian pathogen of bumble bees (Bombus spp.) was implicated in the decimation of commercially produced Bombus occidentalis in the early 1990’s in California, but the effects of this pathogen on natural Bombus populations has only recently been addressed. Other issues involve the use of exotic insects in classical biological control programs that may be infected with microsporidia. This presentation will address both the current situation concerning microsporidiosis in Bombus spp. in North America, and that of a microsporidium infecting a coleopteran predator, Sasajischymnus tsugae, of the hemlock woolly adelgid, Adelges tsugae.

Entomopathogenic fungi infect a wide array of insects from most orders and they are among the natural enemies that contribute to the regulation of insect populations. However, only a limited number of studies have focused on the impact of fungal pathogens on populations of non-pest insects. Effects of entomopathogenic fungi on non-pest host populations should receive more attention based on the increasing interest in conservation biological control. In this strategy, founded on competition theory, non-pest host populations adjacent to cropping systems will in principle affect pest populations through shared natural enemies. We present examples of selected non-pest host-fungus systems from temperate ecosystems that are relevant for the expected ecosystem service provided by entomopathogenic fungi. Predators are among the non-pest hosts that are infected by fungi. Recent advances in our understanding of the effect of pathogens on the behaviour of predators may shed light on the significance of entomopathogenic fungi for the regulation of predator populations. We discuss what we can learn about host-pathogen interactions from behavioural ecology and which life history parameters in the host that may be important for the impacts of fungal pathogens on their host populations.