Insect pollination of oilseed rape

Ecological intensification of agriculture is suggested as a way to reach higher crop yields without increasing inputs that may degrade the environment. Increased insect pollination in crops, such as oilseed rape, Brassica napus, has been suggested to increase yields, but is rarely integrated in crop management. To determine the value of enhanced crop pollination as a means of ecological intensification, reliable estimates of how yield is affected by insect pollination are needed. Further, little is known about interactions between insect pollination and other crop management factors such as cultivar, nitrogen fertilization, pest control, and irrigation. Finally, it needs to be assessed how increasing crop pollination by adding honey bees to crops impact the wild fauna of flower-visiting insects.I addressed these issues in two sets of experiments. First, I performed a landscape-scale experiment with replicated whole fields of winter oilseed rape. I manipulated the pollinator community by adding honey bee hives or controlled the surroundings for absence of honey bees. I chose fields such that they were embedded in either intensive-ly cropped landscapes or heterogeneous landscapes with more semi-natural pastures, expecting greater diversity of wild pollinators in the latter. In two cultivar types, I examined how honey bee addition affected crop yield and the wild pollinator community. Second, I performed two field plot experiments, in winter and in spring oilseed rape, to assess how insect pollination and the crop management factors cultivar, nitrogen fertili-sation, pest control, and irrigation interactively shaped crop yield.Insect pollination increased winter oilseed rape yield, but only in open-pollinated cultivars. Cultivars of open-pollinated type gave higher yields than cultivars of hybrid type. Thus, phasing out open-pollinated cultivars from the market emerges a missed opportunity for increased yields.I show that the crop’s access to water, nitrogen, and herbivory affect pollinator behaviour and potentially crop pollination. Interestingly, insect pollination tended to increase yields when no nitrogen was applied, indicating higher nutrient use efficiency in plants with access to insect pollination. Adding managed honey bee hives had nega-tive effects on the densities of wild flying- and flower-visiting insects, with potential negative effects on crop pollination and biodiversity conservation. Overall, my thesis demonstrates the importance of including both agronomic and environmental perspectives when developing crop production systems that are productive and sustainable