Pollination services provided to small and large highbush blueberry fields by wild and managed bees

P>1. Plantings of pollinator-dependent crops vary from large, intensively managed fields to small fields that are managed less intensively, yet there is relatively little information on how pollinator populations and their contribution to crop productivity vary across this gradient. 2. We determined the relative importance of wild bees and managed honey bees Apis mellifera L. for crop pollination in the blueberry Vaccinium corymbosum L. system of Michigan, USA, by comparing bee communities in small, isolated blueberry fields with those in large blueberry fields (stocked with managed honey bee hives) and measuring the difference in yields of open-pollinated and pollinator-excluded clusters of flowers. We combined these assessments to calculate the contribution of bees to crop production in this system. 3. Wild bees were the dominant pollinators in small fields, comprising 58% of flower-visiting bees, whereas 97% of bees in large fields were honey bees. Large fields had four times as many bees visiting flowers than small fields, but only one-tenth as many bumble bees as small fields. 4. Levels of fruit set exceeded 85% in all fields and were similar between field sizes. Berry weight increased with flower exposure to pollinators and was positively correlated with bee abundance. Berry weights from open-pollinated flowers were twice as high in large fields compared with small fields. 5. By combining berry weight increases attributable to honey bees and wild bees in blueberry fields with measurements of the bee community adjusted for pollinator efficiency, we calculated the relative contributions of honey bees and wild bees to pollination of Michigan blueberries. We estimate that wild bees provide 82% of the pollination in small fields but only 12% of the total pollination services across this system, mostly through their secondary role in large fields. 6. Synthesis and applications. Wild bees are the primary pollinators of small blueberry fields, but these insects are at low abundance in large fields, perhaps due to a lack of nesting resources or competition for resources with honey bees. Our findings highlight the dependence of commercial fruit producers on honey bees and suggest that increasing the pollination contribution of other bees, particularly bumble bees, in large fields will require that growers adopt wild bee conservation strategies or stock their fields with managed colonies. Quantifying the contributions of managed and wild pollinating bees across the range of crop production scenarios will help to direct development of integrated crop pollination strategies to minimize the risk of pollination deficits affecting food production.