Global synthesis of apple pollination research highlights general pollen limitation and positive contributions of wild bees compared to honeybees

Apple is one of the most important pollinator-dependent fruit crops worldwide. To secure high-quality yields, it is crucial to know which, and to what extent, pollinating insects contribute to its pollination success as measured by fruit set, fruit weight and seed set. We perform a meta-analysis of field studies conducted across multiple orchards on insect-mediated pollination in apple cultivation, using raw data from 29 studies, totalling 532 orchard replicates. We assessed the extent of pollen limitation on different pollination outcomes and assessed the contribution of honeybees, wild bees and bee species richness to apple pollination. Across all studies, we detected strong evidence of pollen limitation for fruit set and seed set, but not for fruit weight. Honeybees were the most abundant flower visitors (average relative visitation of 71.9%) compared to wild bees; but when correcting for their pollination efficiency, the relative pollination contribution of honeybees was lower compared to their relative visitation (vice versa for solitary bees). We conclude that honeybee visitation rate did not influence fruit or seed set; yet increasing honeybee visitation had a small, negative effect on fruit weight. Fruit set was not influenced by wild bee visitation rate, whereas wild bee visitation had a small, but clear positive effect on fruit weight and seed set. Bee species richness had a small, positive effect on seed set; whereas it did not affect fruit set and fruit weight. Syntheses and applications. Our study highlights that pollen limitation is common in this global crop. While managed honeybees are dominant pollinators, a diverse community of wild bees contributes significantly to apple pollination and high-quality yield. The positive effect of wild bees and species richness on fruit weight and seed set demonstrates that wild bee pollination results in better-quality fruit production (increased weight & seed set). Therefore, our synthesis highlights the importance of conserving pollinator diversity to maintain pollination services. The absence of a clear effect of honeybee visitation rate on fruit and seed set, coupled with its negative impact on fruit weight, suggests a need for further optimisation of honeybee management to improve the cost-efficiency of pollination management.