Assessing pollinator efficiency in blueberry: From bumblebee buzzing to ant foraging

Highbush blueberry (Vaccinium corymbosum) depends on biotic pollination to produce marketable fruits. A wide range of insects, from managed honey bees (Apis mellifera) to wild bees, hoverflies and other insects, visit blueberry flowers, but their efficiency as pollinators varies greatly. This variation is partially related to blueberry flower morphology, which limits access to nectar and pollen, and to the presence of poricidal anthers that require buzz pollination for more efficient pollen release. We assessed the pollination efficiency of the main floral visitors of blueberry in Portugal. Previously bagged flowers of cultivar Duke were exposed to individual visitors, and pollen loads on stigmas were quantified after each visit. Additionally, we monitored plant-pollinator interactions throughout the day and assessed pollen deposition on the stigmas from flowers exposed to open pollination. Surprisingly, most visits were made by the ant Formica rufibarbis, closely followed by the bumblebee Bombus terrestris. These were also the only taxa recorded visiting flowers within the pollinator efficiency experiment. Bombus terrestris deposited significantly more pollen tetrads, and flowers visited by this species consequently developed more pollen tubes than those visited by F. rufibarbis. However, open pollinated flowers exhibited greater pollen deposition and higher pollen tube numbers than flowers visited by either B. terrestris or F. rufibarbis. Our results indicate that B. terrestris is a more efficient pollinator than F. rufibarbis and can deliver sufficient pollen tetrads to achieve full pollination in a single visit, depending on its foraging behaviour. Despite their low efficiency, ants may still contribute to blueberry pollination under conditions that limit the activity of flying pollinators.