Ecology and physiology of the aphid pathogenic fungus Erynia neoaphidis
Erynia 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.