Insects are considered to be responsible for 80-85% of all pollination, with honeybees being well known for their pollination services for many crops. The effect of managed honeybee pollination on sunflower seed yield and quality (germination percentage and oil content) was investigated at the University of Pretoria experimental farm and in commercial sunflower fields at Settlers. This was done through pollinator exclusion and pollinator surveys on sunflower field plots located at different distances from managed honeybee colonies. Observations on the foraging behaviour and activity of honeybees throughout the day were also made. The data presented in this thesis reveal that seed quantity and quality of sunflower increased significantly as a result of insect visits. Insect pollination improved the mass of 100 seeds (by 38%), as well as their germination percentage (by 38%) and oil content (by 36%). Moreover, visitation frequency, seed yield and quality were negatively correlated with distance to the honeybee hives, suggesting that the distribution of honeybee colonies is not enough to maintain an adequate pollination service throughout the large sunflower fields at Settlers. Honeybee foraging activity varied throughout the day, the highest activity being from 9h00 until 16h00, and activity was correlated with temperature. The results of this study suggest that the use of managed honeybees in sunflower crop production can effectively increase the seed quality and quantity, but additional management measures should be considered to improve production in large monocropping farms that are currently isolated from pollinator sources. Additional provision of honeybee colonies is needed in sunflower production areas during the flowering period. Furthermore, as our results show that in the presence of pollinators other than honeybees seed yield was also improved, management measures that promote the biodiversity of sunflower visitors may also have an important contribution. / Dissertation (MScAgric)--University of Pretoria, 2010. / Zoology and Entomology / unrestricted

Several aspects of plant-pollinator interactions are presented in the thesis. It contains a review on the open questions of plant-pollinator interactions from single species to complex networks. The following sections document novel results. Firstly, the conservation of complex pollination networks is addressed through the hierarchy of species' importance. In addition, the habitat requirements and interactions of a threatened rare pollinator species are explored. In the following chapters, the results from manipulative approaches applied in the field to plant-pollinator interactions are presented. The effect of pollinator's population decline on pollinators' foraging for pollen is investigated. Moreover, the way plant species loss impact several aspects of pollinator visitation is presented. Lastly, the impact of species removal on plant-pollinator network topology and on species ability of establishing new interactions is investigated.

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Les abeilles sauvages (20 000 espèces à travers le monde) participent à la pollinisation de 87,5% des espèces de plantes. Le récent déclin des abeilles sauvages en Europe met en péril la viabilité du service de pollinisation. Parmi les causes de ce déclin, la diminution des surfaces d’habitats semi-naturels (e.g. les prairies permanentes), fournissant aux abeilles sauvages des ressources floristiques diversifiées, joue un rôle particulièrement important. En parallèle de cette diminution des habitats semi-naturels qui s’est opérée depuis la seconde moitié du 20ème siècle, on observe sur la même période une forte expansion de la surface de colza en Europe. Cette culture offre une ressource abondante de nectar et de pollen pendant sa période de floraison. L’objectif de cette thèse est d’évaluer quelles sont les influences de l’organisation spatiale des habitats semi-naturels et du colza sur l’abondance des abeilles sauvages, leur diversité et le service de pollinisation en prairie permanente. Pour répondre à cet objectif, plusieurs expérimentations ont été mises en œuvre dans des prairies permanentes du Parc Naturel Régional de Lorraine, pendant et après la floraison du colza. Dans un premier temps, nous avons montré que les surfaces de colza en fleur et d’habitats semi-naturels influencent l’abondance et la richesse spécifique des abeilles en prairie permanente (effets différents en fonction de leur socialité) ainsi que la distribution intra-spécifique des tailles du corps d’Andrena cineraria. Les effets du colza s’estompent quelques semaines après la floraison du colza. Dans un second temps, nous avons trouvé que l’abondance en prairies d’Andrena cineraria (une espèce dont le pic d’émergence et le nourrissage des larves coïncident avec la floraison du colza) pendant une année N est reliée positivement à la surface occupée par le colza lors de l’année N-1. Enfin, une dernière expérimentation a été menée pour comprendre l’effet du colza sur le service de pollinisation en prairie permanente. Nos résultats montrent que l’augmentation de la surface de colza dans le paysage engendre une diminution du succès reproducteur chez Cardamine pratensis, une plante sauvage fleurissant à la même période que le colza. Ce travail de thèse souligne l’influence du colza et des habitats semi-naturels sur plusieurs niveaux trophiques (pollinisateurs, plantes sauvages) et niveaux d’organisation du vivant (communauté et population) en prairie permanente / Wild bees (20 000 species worldwide) contribute to the pollination of 87.5% of angiosperm species. The decline of wild bees in Europe jeopardizes the sustainability of the pollination service. Among the causes of this decline, the diminution of the surface of semi-natural habitats (e.g. permanent grasslands), providing diverse floristic resources, plays an important role. While the surface of semi-natural habitats has reduced since the second half of the 20th century, the surface of oilseed rape has considerably increased in Europe. Oilseed rape offers a large amount of nectar and pollen during its flowering. The main objective of this study was to evaluate the influence of the spatial organization of semi-natural habitats and oilseed rape fields on the abundance of wild bees, their diversity and the pollination service in permanent grasslands. To achieve this objective, we implemented several experiments in permanent grasslands of the “Parc Naturel Régional de Lorraine”, during and after oilseed rape flowering. Firstly, we showed that the areas covered by flowering oilseed rape fields and semi-natural habitats influence the abundance and species richness of bees in permanent grasslands (different influences according to their sociality) as well as the body size intra-specific distribution of Andrena cineraria. Effects of oilseed rape disappear few weeks after oilseed rape flowering. Secondly, we found that the abundance in grasslands of Andrena cineraria (a species whose peak activity and larval feeding coincide with oilseed rape flowering period), during year N is positively related to the area covered by oilseed rape during the year N-1. Finally, a last experiment was conducted to detect potential effects of flowering oilseed rape on the pollination service in permanent grasslands. Our results showed that increasing areas of flowering oilseed rape in the landscape lead to a reduction of the reproductive success of Cardamine pratensis, a co-flowering wild plant in permanent grasslands. This study highlights the influence of oilseed rape on several trophic levels (pollinators and wild plants) and levels of organization of life (community and population) in permanent grasslands