A host-virus interaction in a pasture habitat

Host Range Expansion of an Endemic Insect Herbivore is Associated With High Nitrogen and Low Fibre Content in Exotic Pasture Plants

Endemic moth species of the genus Wiseana spp. (Hepialidae) have become serious pests of introduced pasture plants in New Zealand. The original native host plants of these moths have not been confirmed. This study investigated the performance (survival, development time, weight gain) of three Wiseana species on seven putative host plants: five native and two exotic species. The aim was to identify native hosts for the three Wiseana species and to compare their performance on native plants and exotic pasture plants. The chemical composition of the seven putative host plants was investigated to compare native and exotic plant chemistries, and to test for associations between plant characteristics and performance of selected Wiseana species. Carbon, nitrogen, silica and fibre contents were measured for each plant species; primary metabolite composition was determined by gas chromatography-mass spectrometry. For the three moth species, increased survival and weight gain were significantly associated with high nitrogen and low fibre contents in one exotic host plant, white clover (Trifolium repens), although one species, W. umbraculata, did not complete development to adult on any of the plants tested, including clover. Two exotic plants (T. repens, Lolium perenne × Lolium multiflorum), and two native plants (Aciphylla squarrosa and Festuca actae) supported W. copularis development to the adult stage, but only one exotic (T. repens) and one native (F. actae) species supported complete development of W. cervinata. Exotic and native plant species had distinct metabolite profiles, but there was no significant association between metabolite composition and Wiseana performance. We conclude that W. copularis and W. cervinata, but not W. umbraculata, have expanded their host range, because of their ability to use both native and new hosts. No evidence was found for a host shift, i.e., a loss of performance on the ancestral host compared with the new host.

Theoretical basis for the use of pathogens as biological control agents of pest species

The population dynamics of insect–pathogen interactions are examined with the aid of simple mathematical models. Three concepts of central importance to the interpretation of population behaviour are discussed, namely the ability of the pathogen to persist within its host population, the ability to regulate and depress host population abundance, and the ability to induce non-seasonal cyclic changes in host density. The selection of pathogen species or strains to depress pest population growth is discussed and the optimal characteristics are shown to be intermediate pathogencity combined with an ability to reduce infected host reproduction, high transmission efficiency, including elements of vertical as well as horizontal transmission stages. When the pathogen plays a significant role in the regulation of host population growth, it is argued that many insect–pathogen interactions will exhibit non-seasonal oscilations in host and pathogen abundance. Mathematical models are used to explore the patterns of population behaviour that result from the continual introduction of a pathogen into a target pest population. It is shown that there exists a critical introdution rate, above which the eradication of the pest is theoretically possible. Significant reductions in pest population abundance will not occur until the introduction rate approaches this critical value, whereupon the oscillatory behaviour of the interaction between host and pathogen population will be suppressed.

A general dicussion is given of the problems arising from the combined use of chemical agents and pathogens for the control of pest species, and the evolutionary pressures acting on host and pathogen populations.

A rapid bioassay screen for quantifying nucleopolyhedroviruses (Baculoviridae) in the environment

A quantitative bioassay method for the detection of Helicoverpa armigera (Lepidoptera: Noctuidae) singly encapsulated nucleopolyhedroviruses (HaSNPVs) in soil is described. Calibration curves used for estimating soil virus titres in environmental samples were generated by incorporating a sterilised soil into a semi-synthetic insect diet then inoculating known concentrations of an HaSNPV into the soil-diet mixture. Calibration curves were constructed for soil diets containing varying proportions of soil: 0, 1, 5, 10 and 25% soil (w/v). Their accuracy was assessed in a series of blind tests in which the actual soil virus concentration fell within the estimated mean 95% confidence region for each of three samples. The five soil-diet incorporation rates were compared in terms of larval survivorship and growth rate. There was no significant difference in larval survivorship after 10 days (i.e. for the duration of the bioassay period). The stage structure of bioassay larvae at 10 days and pupal weight at 20 days was significantly different for individuals reared on 25% soil-diet in terms of both a slower growth rate and a lower mean pupal weight compared to individuals reared on 10, 5 and 1% soil diets. This did not, however, appear to lead to greater variability in bioassay response at the high soil rate at 10 days. The level of sensitivity of virus detection achieved using this method was extremely good with the LC10 value for mid-first instar H. armigera larvae reared on the 25% soil-diet estimated at 26 polyhedral inclusion bodies (PIBs) per gram of soil. The suitability of using this approach for quantifying Helicoverpa NPVs in Australian soils was assessed by comparing percent bioassay infection across a range of five isolates known to be present in Australia. The effect of soil pH and soil management (cultivated versus non-cultivated) on percent bioassay infection was also examined. In both cases, no significant differences were observed. Finally, percent idopathic mortality, percent NPV infection and estimates of Helicoverpa SNPV concentration in a selection of samples from the Australian environment are presented.

Ecological considerations for the environmental impact evaluation of recombinant baculovirus insecticides

The history of baculoviruses in insect control and the current status of recombinant baculovirus (recBV) insecticides in the laboratory and the field are briefly outlined. A conceptual model for impact evaluation is described that distinguishes between scientific impact evaluation and regulatory risk assessment. Its components are identified and reviewed in the light of existing ecological theory and experimental study under the categories of impact identification, exposure identification, and impact evaluation. Impact identification aims to identify species and populations sensitive to direct or indirect impacts by a recBV. Exposure identification examines how susceptible populations may be exposed to a recBV. Impact evaluation combines these data to predict the potential for recBV impacts in the environment.

The potential role of pathogens in biological control

It is now well established that pathogens such as viruses, fungi bacteria and protozoans can have profound effects on the dynamics of their invertebrate host populations. Theoretical models of invertebrate host-pathogen interactions which assume uniform structure of the pathogen population may reasonably explain the oscillatory behaviour observed in some systems, but do not adequately describe the existence of more constant populations found in other host-pathogen interactions. An examination of the literature relating to these relatively stable systems suggests that the common thread is the eventual transmission of some of the more protected, longer-lived stages of the pathogen occurring in reservoirs, such as the soil, host cadavers on trees, or the live host itself. In this letter, I propose a new theoretical model which incorporates this population structure and accounts for the range of dynamics observed in natural systems. In particular, I show that host populations may be regulated to low and relatively constant densities if sufficient numbers of pathogens are trans-located from pathogen reservoirs to habitats where transmission can occur. An understanding of pathogen reservoirs may be of value in the design of biological control programmes and may greatly increase the effectiveness of pathogens as biological control agents.