Do Buzz-Pollinating Bumble Bees Facilitate Honey Bee Pollination in Southern Highbush Blueberry Through Increasing Pollen Release?

Facilitation between species in diverse communities can enhance ecosystem services like pollination, a crucial service for southern highbush blueberry (SHB). SHB growers rely heavily on managed honey bees, but often experience insufficient pollination, possibly because blueberries require buzz pollination for optimal pollen release, which honey bees are incapable of providing. Buzz-pollinating bumble bees could help to facilitate pollination services provided by honey bees through increasing pollen release, though this has never been explicitly tested. To test whether bumble bees facilitate pollination by honey bees, we caged SHB bushes with only honey bees (HB only) and with honey bees plus bumble bees (HB + BB). Across treatments, we assessed foraging honey bee pollen loads on their bodies to see if this increased when buzz-pollinating bumble bees were present; as well as bee visitation rates, direct interactions between bees that could enhance cross pollination, and berry weight. We found no evidence that pollen loads (incidental or corbiculate) on honey bees were higher when bumble bees were present. Instead, we found that honey bees could release and collect blueberry pollen under the constraint of a caged environment with no alternative forage, even when buzz pollinators were absent. We did find evidence for other forms of facilitation and complementarity such as three-fold more direct interactions in the HB + BB treatment and differential foraging across temperatures by honey bees and bumble bees. Finally, berry weight did not vary between treatments likely due to the high pollinator density in both treatments. Our experiment provides clear evidence that honey bees can collect blueberry pollen despite the inability to buzz pollinate; alternatively, it did not support our hypothesis that bumble bees would facilitate pollen release and transfer by honey bees. However, we found evidence that bumble bees and honey bees complement and facilitate each other in additional ways, suggesting that using both could improve pollination.