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 Bombu s 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 mo re pollen from apple anthers than nectar-collecting bees. Apis that approac hed nectaries laterally deposited substantially less pollen than other visi tors. 4. Apis and Bombus removed and deposited similar amounts of pollen on almon d 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 p ollen delivery in apple orchards but reduce pollen delivery in almond orcha rds if Apis already serve as primary pollinators. Additional data on inter- tree and inter-row flights would be necessary to know how much these change s in pollen transfer might affect fertilization. 6. Measures of pollen-transfer effectiveness do not provide a complete asse ssment of pollination value, but can serve as a general, inexpensive tool f or pre-screening possible alternative pollinators.