Bees increase oilseed rape yield under real field conditions

Oilseed rape (OSR, Brassica napus L.) is a common crop found in many European agricultural landscapes. It is pollinated by a wide variety of insects, but the reported contribution of pollinators to yield varies widely between studies (from 0 to 50%). Moreover, such a contribution has seldom been estimated at the field scale in real farming conditions. We analysed OSR yields in response to insect pollination; over four years, at two different scales: farm fields and individual plants. We used both empirical and experimental approaches along a gradient of pollinator diversity and abundance. The empirical approach was based on farm surveys (151 fields) while the experimental approach used various pollination exclusion methods (570 plants in 101 fields) to estimate the relative contributions of insect, wind, and self-pollination. The OSR yields were positively correlated to total bee abundance and bee genera diversity, through improved fruiting success and plant seed mass (after adjusting for plant biomass). Hoverfly diversity and abundance, and bumblebee abundance did not have any effect. The main OSR pollinators in our study were honeybees (Apis mellifera) and wild bees (Lasioglossum spp.). Yields were increased, on average, by up to 37.5% (27.7% - 47.5%) at field scale when bee genera diversity increased from a single genus to more than 10 genera (pan-trap data). Insect pollination contributed about 30% of plant yield. Self-pollination and wind pollination accounted for the remaining 70%, with self-pollination being the major contributor. Our study demonstrates that pollinator diversity and abundance, at least at very high levels, have a major effect on OSR yields. This suggests that establishing a monetary value for pollination services in OSR farming systems could be used to balance the cost of managing semi-natural habitats or meadows to maintain bees and other pollinators.