Bee pollination of strawberries on different spatial scales – from crop varieties and fields to landscapes

The global majority of our crops is dependent on pollination. Hence, pollination contributes to one third of global crop production and is an ecosystem service of high commercial and social importance. Bees are the most important crop pollinators, but they are endangered by several anthropogenic impacts, in particular agricultural intensification. Facing rising global demands for food and energy in the background of a growing world population, the prevention of an impending pollinator crisis attracts increasing interest from the public, policymakers and scientists. However, despite new reports on crop pollination are frequently arising, pollination research seems still at the beginning and our knowledge on crop pollination by bees at various scales is scarce. This work aims to explore so far unknown benefits of bee pollination to highlight its overall importance. It is also focused on the main drivers of crop pollination by bees at different spatial scales, using strawberries as a study organism. The first part (chapter 2) explores the benefits of bee pollination on commercial value and post-harvest quality of strawberry fruits from different varieties. Exclusion experiments with bee, wind and self pollination treatments were conducted on nine strawberry varieties at an experimental strawberry field. Bee pollination strongly increased the commercial value of strawberry fruits across all varieties by producing well shaped fruits with higher weight. It further elongated the shelf live of strawberries from most varieties, which was calculated from firmness values. Bee pollinated fruits had a more intense red colour and lower sugar-acid-ratios in most varieties. In general, effects differed between varieties, but with mostly similar directions. These results give a positive reply on the questions wether bee polination benefits commercial value and post-harvest quality of strawberries and the differences between varieties. The second part of this work (chapter 3) focused on the relationship between bee pollination and shelf life of strawberries in detail. From the first part (chapter 2), the number of fertilized achenes from fruits of the variety Yamaska was related to pollination treatments and commercial grades. In 2012, further fruits were harvested on a commercial strawberry field and divided into commercial grades in relation to malformations from insufficient pollination. The process of firmness, fruit weight and decay during storage was analysed in relation to commercial grades. During storage time, firmness and fruit weight were decreasing while the proportion of decayed fruits was increasing, independent from commercial grades. Commercial grades had a general effect on fruit degradation, with improved values in higher grades. Thus higher commercial grades resulting from improved pollination elongated the shelf life of strawberries. Differences between varieties in part one (chapter 2) lead to the question wether strawberry varieties differ in their attraction to bees with consequences on their visitation rate (chapter 4). The influence of different volatile emissions between strawberry varieties, the bees’ responses on distinct compounds and resulting visitation rates on a commercial strawberry field were analysed. Three strawberry varieties were grown in a greenhouse and the flower volatile emissions from each variety as well as anntenal responses of Osmia bicornis L. females on each compound were tested. Further the bees’ visitation rates on two varieties were counted at a commercial strawberry field. All strawberry varieties emitted the same flower volatile compounds, but differed in the quantities of most of them. Anntenal reactions of O. bicornis females differed between most compounds and were higher than responses on controls. Under field conditions, the varietey that produced higher quantities of the most attractive compounds was visited much more frequently. Hence, the question about volatile emissions affecting the responses and visitation of solitary wild bees could be positively answered. The last part of this work (chapter 5) was focused on the main drivers of various bee diversity parameters that explain the community composition of bees and their influence on strawberry fruit weight. As bee diversity is known to change related to landscape complexity, the study was conducted on commercial strawberry fields located in landscapes ranging in a gradient from homogenous to more heterogenous, to get ranging values of the bee diversity parameters. Bees were collected via sweep netting and strawberries were harvested at four field positions, that were located from field edge to center. In general, strawberry weight was strongly influenced by Evenness and to a lower level by Shannon-diversity of bee pollinators, whereas results from species richness and bee abundance were inconsistent. Higher Evenness and Shannon-diversity of the bee community improved strawberry weight at the field edge, whereas higher proportions of honeybees improved fruit weight at the field center, seemingly due to higher conspecific pollen loads. Evenness was negatively correlated to the proportion of honeybees. The research questions could be answered with Evenness being most influential on strawberry fruit weight, with contrasting effects between field locations, driven by the proportion of honeybees. In conclusion, the commercial value and post-harvest quality of strawberry fruits was strongly dependent on bee pollination, which was generally conducted by both honeybees and wild bees, but further affected by diverse scale-dependent effects. The strong relationship between fruit quantity and quality emphasizes, that pollination countervails to increasing global demands on high quality food. However, against the background of serious pollinator declines due to increasing anthropogenic impacts on various scales, pollination appears to be an ecosystem service that is strongly endangered, which may have extensive impacts on world food security. Hence, the results of the current work emphasize, that international conservation strategies will become more efficient by focusing on a broad scale of facets affecting crop pollination to maintain this highly important ecosystem service for the future.