Host specialization in a wild bee community: Implications for pollinator effectiveness
Recent research suggests that plant-pollinator communities are highly generalized. However, these studies are often based on observations of pollinators visiting flowers. They are therefore plant-centric and overestimate generality because bees will visit many flowers for nectar even if they only collect specific types of pollen to feed their offspring. We assess the composition of pollen carried by bees collected on apple blossoms over five years and characterize the community of plants which these bees utilize as pollen resources as well as the variation within and between bee species. Based on these pollen surveys, we identify oligolectic (pollen specialist) and polylectic (pollen generalist) species. Specialist bees carrying purely conspecific pollen are putatively more effective as pollinators than those carrying mixed pollen loads. Thus, the degree of host-plant preference for apple should be a factor that contributes to the pollinator effectiveness of wild bee species. By combining data on bee preference for apple based on pollen loads plus bee abundance, we quantify the relative contribution of honey bees and native bees to overall pollination in apple.
Overall, we found the degree of specialization was much higher when based on pollen composition than when based on floral visitation. We found that specialization varied greatly both within and between bee species. Some species specialized almost completely on the family Rosaceae (especially apple), while others carried pollen from a wide array of plant species. Within a given bee species, individuals often carried different pollen loads: from nearly pure apple pollen to more than 10 different pollen types. The family Andrenidae, known for its oligolecty, accounted for nearly a third of the bee species collected and more than 40% of the abundance. Within this family, we found a high degree of specialization; individuals of many andrenid species carried pure pollen loads. We identify the bee species with a high abundance and consistently high proportion of apple pollen as potentially effective crop pollinators. Given the abundance of these wild bees, and the purity of the pollen loads they carry, we suggest that they could be managed as alternative pollinators for apples.