|dc.description.abstract||Two of the species of hover fly native to New Zealand, Melangyna novaezelandiae (Maquart) and Melanostoma fasciatum (Maquart) produce larvae which are aphidophagous and are potentially important in arable crops as bio-control agents. Unfortunately, numbers of hover flies are often insufficient to effectively reduce insect pest numbers and therefore most agricultural systems rely on pesticides to manage pest numbers. To compensate for low local populations of hover flies, floral diversity can be restored which may enhance their numbers and thereby indirectly improve levels of predation, reducing reliance on pesticides. Habitat manipulation for Syrphidae has revealed local increases in adult fly numbers near floral resources, and in some instances, has shown a consequent effect on oviposition rates and prey number. Before the full potential for biological control is understood, key aspects of the ecology of the predator need to researched. These include factors limiting the dispersal of the predator, how the provision of non-floral foods may influence seasonal abundance and the natural phenological patterns of the predator.
In New Zealand, hover fly ecology is poorly understood and research into the ecology and efficiency of this group as predators is needed if their potential for biological control is to be realised. This thesis outlines experiments which investigate the potential for habitat-manipulation techniques aimed at increasing hover fly numbers to reduce aphid density. A crop of oilseed rape (Brassica napus (L.)), was used to demonstrate that aphid numbers could be reduced in areas adjacent to planted strips of Tansy Leaf Phacelia tanacetifolia (Benth.) (Hydrophyllaceae). Although, this result was difficult to attribute exclusively to predation by hover flies, as their numbers were not significantly enhanced adjacent to the floral resources. The movement of hover flies in arable land was also investigated and the results indicated that a fence of 1 m in height impeded their dispersal and that the sex and reproductive status of the flies influenced the height at which they intercepted the fence. The provision of supplementary, non-floral food resources in autumn as a method of enhancing local populations of hover flies was investigated. The results from this study were inconclusive, although it appears that the provision of supplementary food when numbers are naturally low is unlikely to enhance hover fly numbers. A two year study of natural phenology indicated that both hover fly species produced two generations each year, one peaking in December and the other smaller generation peaking in March. This study also revealed a greater natural abundance of M. novaezelandiae over M. fasciatum which contradicts other studies on these species. The possible influences of trap colour and the provision of a blue flower source (Phacelia) on the relative abundances of the two species are also addressed. In conclusion, these studies have investigated and discussed the limitations of the existing techniques used to quantify the effectiveness of hover flies as predators of aphids and investigated some of the factors that contribute to their biological control potential.||en