Anesthesia methods and the agroecology of Scaptomyza flava (Drosophilidae), a Brassicacae pest in New Zealand and associated parasitoid, Asobara nr. persimilis (Braconidae)

A key pest of brassicas worldwide is the leaf-mining fly, Scaptomyza flava, the larvae of which can cause cosmetic damage leading to crop rejection by supermarkets/consumers. In climates where leafy salad brassicas are harvested all year, the flies are almost always present. This necessitates control throughout the entire harvesting season and this usually consists of the prophylactic application of insecticides. One way of potentially ameliorating the negative environmental impacts of this approach is to enhance the effectiveness of biological control by providing alternative food sources for natural enemies. Planting of selected flowering plants can be useful in this respect, as many parasitoids and other beneficial insects feed on nectars. This in turn, can improve their efficacy by increasing fecundity, longevity and other aspects of their biology, contributing to increased ‘fitness’ and efficacy. This PhD aimed to find selective flowering plants that provided more benefit to the natural enemy when compared with the pest. This work was conducted through a series of laboratory and field experiments. Before the selection of flowering plants occurred, it was clear that the common practices facilitating handling drosophilid flies (carbon dioxide and chilling) were not appropriate for this work as it had been found that those methods can impact on the longevity of some insects. So, a series of experiments was conducted to compare alternative handling methodologies. Triethylamine (TEA) was compared with chilling and carbon dioxide because it had been shown in the literature to have some success but was still poorly studied and had not been used specifically to handle insects for ecological studies. Carbon dioxide and chilling were common in the literature as most studies that handled insects used one of these two. It was found in this work that triethylamine does not affect longevity and has a long anesthetic effect on the flies. So, TEA was used to handle the insects for the bioassays with flowering plants. Once this aspect of the work had been completed, female S. flava and one of its natural enemies, the braconid parasitoid wasp Asobara sp. (nr. persimilis) were used to evaluate six cultivars of alyssum (only the fly was used for these cultivars) and four flowering plant species (alyssum, buckwheat, phacelia and Leptinella dioica, Hook.f.). The latter is an endemic New Zealand plant in the Rosaceae. Cultivars differed from each other in their effect on the fly and on A. sp. (nr. persimilis). Buckwheat appeared to enhance longevity of the fly and the natural enemy to the greatest degree. The crop was Brassica juncea L. ‘Mizuna’. Problems associated with the frequency distribution of the data meant that although Cox’s proportional hazard model was initially used to compare survival times on different flowers, monotone likelihood made it difficult to ascribe statistical significance to the comparisons. Rankings, however, were consistent for the pest and Asobara and conclusions were drawn with appropriate caution. Because of the somewhat equivocal nature of the data and results, an analysis of variance was conducted to provide more robustness to the conclusions drawn. It seemed that none of the flower species evaluated was more beneficial to the parasitoid than to the pest. However, models have shown that as a parasitoid’s attack rate is the most important parameter in that natural enemy’s biocontrol potential, no differences between the effects of the ‘best’ flower (buckwheat (Fagopyrum esculentum) between the pest and the parasitoid) do not mean that other parameters which could have been recorded for that and other plants do not differ in their relative contributions to potential ‘fitness’ of the parasitoid and its host in the studied system. Attack rate a sensu Bailey et. al. has been shown to be the most important variable in this respect and can improve when buckwheat nectar is provided to some parasitoids. In the field, insect sampling targeting S. flava and its parasitoid occurred before and after buckwheat was sown to obtain information on a wide range of potential natural enemies of Scaptomyza and how they are affected by the buckwheat. A marking technology (rubidium chloride) was used to investigate patterns of potential natural enemy numbers in relation to that plant’s flowers. Analysis was also carried out on the invertebrate natural enemy communities in that area. Overall, the work was the first time that these research questions had been applied to S. flava and useful ideas for future biological control work on this pest were demonstrated.