Browsing PhD e-theses by Subjects
Now showing items 1-2 of 2
Ecology and physiology of the aphid pathogenic fungus Erynia neoaphidisErynia neoaphidis Remaudiere and Hennebert (Zygomycetes: Entomophthorales) is an obligate pathogen of invertebrates, especially aphids, and has therefore been studied as a possible biological control agent for a number of years. However, a number of important physiological and ecological questions regarding optimal conditions for conidial production and transmission 0 f the fungus through an aphid population had to be answered. This thesis investigated some of these aspects. Solid and liquid media were used to culture the fungus, and E. neoaphidis was cultured on a fully defined medium for the first time. A sporulation monitor and digital image analysis was used to quantify conidial production from E. neoaphidis biomass produced in vivo and in vitro. This was a completely novel method and is useful for gathering data on large numbers of conidia, 50 that size distributions can be constructed and the physiological status of the conidia inferred from this. E. neoaphidis infected aphid cadavers produced more, smaller conidia when grown in vitro. Biomass harvested from exponential growth phase in fed batch culture produced significantly more conidia than biomass harvested from any other growth phase although further work on the nutritional requirements of E. neoaphidis in vitro is required. The duration of the conidial discharge was also greatest from biomass harvested at the exponential phase and therefore. biomass harvested from the exponential phase should be used if the fungus is to be applied as a control agent. E. neoaphidis biomass kept at low humidity during simulated winter conditions produced infective conidia after 24 weeks, indicating that mycosed cadavers may act as a reservoir to infect the next season's hosts. Pesticides adversely affected the growth and production of conidia by E. neoaphidis, with herbicides having the least deleterious effects, and therefore being most compatible in an integrated pest management program. Laboratory and field studies were used to assess the transmission of E. neoaphidis through aphid populations. Position of the inoculum on the host plant affected the primary transmission of the fungus through aphid populations in the laboratory and in the field, and secondary transmission of the fungus in the laboratory. It is therefore important to apply the fungus to where it will maximally spread. There was some evidence for effects of host and inoculum density on the transmission of the fungus, especially in the laboratory, indicating that, in practice, the fungus is unlikely to spread rapidly through low densities of aphids and therefore to achieve control of such populations, a high inoculum density may be required. There was also very Iittle transmission of the fungus via aphid vectors to susceptible aphid populations on different host, although as a general observation, vectoring of conidia by the wind may be very important. The smaller conidia produced by in vivo biomass may be vectored more easily by wind than the large conidia produced in vitro.
Resistance to conventional and novel insecticides in the glasshouse whitefly, Trialeurodes vaporariorumThe incidence, influencing factors and mechanisms of resistance to insecticides from a range of chemical groups were examined in UK and European populations of the glasshouse whitefly, Trialeurodes vaporariorum (Westwood). Toxicological assessments of populations from a range of plant production glasshouses and comparisons with the responses of a laboratory susceptible strain disclosed levels of resistance to pyrethroid, organophosphate, insect growth regulator (IGR) and neonicotinoid insecticides. Responses to conventional compounds indicated varying levels of resistance, potentially reflecting disparate usage between collection sites. All strains examined possessed resistance to the IGR, buprofezin; some populations were virtually immune to this commonly used control agent. Selection experiments demonstrated reciprocal crossresistance between buprofezin and a further IGR, teflubenzuron, both of which are frequently incorporated into integrated pest management (IPM) programmes for this species. Results for the leading neonicotinoid, imidacloprid, revealed resistance in both UK and European strains, representing the first documented cases of neonicotinoid resistance in this species worldwide, and the first in any insect species within the UK. The lethal effects of vapour emitted by applications of buprofezin and the anti-feedant effects of imidacloprid were demonstrated in T. vaporariorum for the first time. The potential consequences of these factors for both the control and selection of resistance were highlighted. Mechanistic studies using electrophoresis and kinetic spectrophotometer readings showed that neither non-specific esterases nor modified acetylcholinesterases were involved with resistance to either pyrethroid or specific organophosphate insecticides.