Floral resource subsidies for the enhancement of the biological control of aphids in oilseed rape crops

Abstract

Food production is achieved by the interaction of man-made infrastructures with natural ecosystems, the latter providing soil, light, and regulating services, including biological control. However, such natural capital has been put increasingly at risk by modern agricultural practices. For example, the use of insecticide compounds can be harmful to organisms in the soil, the water and the vegetation, including beneficial insects. This thesis investigated how the ecological management of a conventional oilseed rape (OSR) cropping system can enhance the biological control of insect pests by their natural enemies, which could alleviate the use of insecticides. OSR hosts three aphid species, namely, Brevicoryne brassicae (L.), Myzus persicae (Sulzer), and Lipaphis erysimi (Kaltenbach). In New Zealand, these three species are attacked by the parasitic wasp Diaeretiella rapae (McIntosh) [Hymenoptera: Braconidae], which completes its larval development inside an aphid body, and is a free-living organism when adult. In that stage, the wasp only feeds on carbohydrate-rich fluids, e.g. floral nectars and honeydew. Floral resource subsidies consist in the addition of nectar-providing vegetation in the habitat of parasitoids, to enhance their reproductive output, which in turn cascades into decreased pest density. This approach has known successes and failures, and its potential could be increased by a better understanding of its ecological functioning. In the introduction, this thesis lists current knowledge gaps in the ecology of floral subsidies targeted at enhancing the control of pests by parasitoids. In the second chapter, this thesis reports how nectar feeding affects the behaviour of D. rapae. It was observed that feeding on buckwheat (Fagopyrum esculentum Moench) enhanced ca. 40-fold the time spent searching for hosts and greatly reduced the time spent stationary. The consequences of this for the reproduction of the parasitoid, and biocontrol, are discussed. The third chapter addresses the potential competition between pollinators and parasitoids for nectar, when the latter is provided as a floral subsidies. This question is crucial because the potential effect of floral subsidies on biocontrol could be negated by if the provided nectar is consumed by pollinators. A manipulative field experiment indicated that this negative interaction is not existent or weak, although the power of the test was low. A laboratory trial presented in the fourth chapter showed that the longevity of D. rapae fed on OSR or buckwheat nectar was enhanced ca. 3-fold compared to unfed conspecifics. Feeding on M. persicae honeydew and nectar from two candidate floral subsidies enhanced longevity ca. 2-fold, indicating a lower nutritional quality. Two other plants did not cause any longevity enhancement. The value of these results with regard to the understanding of the nutritional ecology of D. rapae is discussed. The food-web of aphids, parasitoids and hyperparasitoids (fourth trophic level) living in OSR crops in New Zealand has not been documented. Understanding the composition and structure of the food-web is important to guide the implementation of floral subsidies. The fifth chapter presents a protocol for the reconstruction of food-webs, based on the molecular analysis of aphid mummies. The further use of this tool for the construction of aphid-based food-webs in general is discussed. The thesis findings are discussed in the context of OSR as an ephemeral, multi-species, spatially complex and dynamic habitat. The concept of “foodscape” is adapted to parasitoids and biological control. In its last section, the discussion integrates ecological and agricultural considerations to suggest the intercropping of a flowering plant in OSR crops.