Very Few Honeybees Carry Cross-Pollen in a Self-Sterile Tree-Crop Orchard

Fruit production in many crops depends on pollen transfer by animals, and many self-sterile crops rely on long-distance pollen transfer by animals between different genotypes, i.e., between different cultivars. Foragers such as honeybees may need to visit and carry the pollen of more than one cultivar to be effective pollinators, but we currently have little understanding of how many foragers are carrying more than one cultivar of pollen. We determined the number of cultivars carried by honeybees returning to their hive with pollen at two locations in an orchard of a predominantly self-sterile tree crop, macadamia. The locations were either (i) close to only one macadamia cultivar or (ii) between two macadamia cultivars. We sampled honeybees early in the flowering period, when the floral resource availability was lower, and at peak flowering when the resource availability was higher. We identified the cultivars carried by individual honeybees using a SABER-MassARRAY method that distinguishes cultivar-specific SNPs in pollen DNA. We found that most honeybees carried pollen from only one identified macadamia cultivar, regardless of the hive location and time within the flowering period. Only 15% of the honeybees were carrying pollen from more than one identified macadamia cultivar. This suggests that most honeybee foraging visits to flowers in the orchard were unlikely to have resulted in cross-pollination. Pollenizer trees could be interplanted among the main cultivars in macadamia orchards to increase the number of honeybees carrying cross-pollen and increase their pollination efficiency.