Per-visit effectiveness of native bees in apple pollination

Recent declines in honey bee health and increasing demand for pollination services highlight a need for enhancing wild pollinators in agriculture.  One case in point is apple, an economically important crop in the U.S. that requires cross pollination by bees to set fruit. Though commercial growers rent increasingly expensive honey bees to insure adequate pollination, apple is visited by a diverse and abundant community of native spring bees, most notably solitary Andrena bees in the subgenus Melandrena. To quantify the importance of these native bees in apple pollination, we compared their pollinator effectiveness with that of honey bees (Apis mellifera).  We recorded number of pollen grains deposited in a single visit to a virgin, emasculated apple flower; visit duration; approach (front or side-working) and foraging type (land only, pollen or nectar foraging). Bee, approach and foraging types had significant main effects on pollen deposition. Melandrena deposited more pollen grains than Apis.  Side-working bees deposited fewer grains, regardless of taxon.  Apis disproportionately approached flowers from the side, which may limit their effectiveness as pollinators.  In our study, Melandrena was a more effective pollinator than Apis, on a per-visit basis. The fact that Melandrena are often abundant in apple orchards, that they tend to show a low tendency to side-work flowers, and that they deposit more grains per-visit than honey bees indicates that Melandrena species are likely contributing significantly to commercial apple production. We will discuss Melandrena’s potential to be an alternative pollinator at the orchard level.