A diversity of wild bee species inhabit agricultural lands and may be more effective pollinators than the widely employed European honey bee (Apis mellifera). High species richness within crop pollinator assemblages has been linked to enhanced fruit and seed yields through spatial and temporal complementarity in pollen deposition, but the individual influences of each flower visitor on pollination function have received limited attention. This lack of mechanistic understanding of the contributions of individual pollinators limits our ability to predict how yield will be affected by changes in pollinator community composition. We investigated the effects of bee diversity and species identity on pollen deposition and crop yield in strawberry through a field experiment that independently manipulated diversity and abundance of flower visitors. We used a new pollen deposition measurement technique to determine the contribution of each bee visit to the total pollen load per flower. With this data we compared the pollinator performance of wild bee species versus managed honey bees in strawberry, and calculated the influence of species richness and number of visits on fruit mass, using the fruit that developed from each sampled flower.
We found no significant influence of bee species richness or flower visit frequency on strawberry mass. Visit number had a positive effect on pollen deposition, but species richness did not. Most pollen was deposited by the first three flower visitors, and subsequent visits contributed minimally to the total pollen load. Thus, pollination in this system was accomplished by the first few bees to visit a flower, and the majority of visits thereafter were functionally redundant. The bee community at the farm site was dominated by managed Apis mellifera colonies, wild Andrenids and Halictids. There were no significant differences in the amount of pollen deposited between flowers visited by wild bees and those visited by honey bees. However, berries from flowers visited by wild bees were equal in mass to those produced by pollen-supplemented flowers, whereas at least three visits by honey bees were required to produce a berry of the same mass. This suggests that wild bees may deposit higher quality pollen on a per-visit basis than honey bees in strawberry crops. If wild bees consistently deposit higher quality pollen than honey bees, incorporating crop management practices that preserve and enhance wild pollinator populations has the potential to increase efficiency of pollination and crop production.