Pollen removal and deposition by honeybee and bumblebee visitors to apple and almond flowers

1. To help evaluate the worth of alternative pollinators in agriculture, we present a theoretical framework for comparing the effectiveness of two or more pollinators by measuring pollen removal and deposition. 2. We report pollen removal and deposition data by Apis mellifera and Bombus spp. during single visits to four cultivars of apples (Golden Delicious, Starkrimson Delicious, Empire/Maclntosh and Rome.) and Mission almond. 3. Apis and Bombus removed similar amounts of pollen from apple flowers but Bombus deposited more pollen on stigmas. Pollen-collecting bees removed more pollen from apple anthers than nectar-collecting bees. Apis that approached nectaries laterally deposited substantially less pollen than other visitors. 4. Apis and Bombus removed and deposited similar amounts of pollen on almond flowers. Apis tended to remove more during pollen-collecting visits than nectar-collecting visits. The type of resource sought did not significantly influence deposition. 5. Based on removal and deposition data, additions of Bombus may increase pollen delivery in apple orchards but reduce pollen delivery in almond orchards if Apis already serve as primary pollinators. Additional data on inter-tree and inter-row flights would be necessary to know how much these changes in pollen transfer might affect fertilization. 6. Measures of pollen-transfer effectiveness do not provide a complete assessment of pollination value, but can serve as a general, inexpensive tool for pre-screening possible alternative pollinators.