Mitigation of pollen limitation in the lowbush blueberry agroecosystem: effect of augmenting natural pollinators

Growers of small fruit crops often supplement the natural pollinator community by introducing pollinators into commercial orchards and fields, but there are relatively few studies that test the extent to which such interventions increase fruit yield. To test whether plants are limited by pollen availability, inflorescences in 78 commercial lowbush blueberry fields during three years were handpollinated either with supplemental outcross pollen, or marked and left as controls (open-pollination). Maximum fruit set with supplemental pollination was in the range 50–65%, whereas with openpollination, it was slightly, but significantly lower, in the range of 47–57%, suggesting that pollen limitation can affect fruit set. In a two-year experiment, we augmented native pollinators with introduced honey bees, bumble bees, and leaf cutter bees and all combinations in 48 fields. Stigmatic pollen loads were influenced by the total numbers of managed bees in a field in 2010 but not 2011. The presence of leaf cutter bees had a small but positive effect on seed set in 2010, and honey bees had a small but negative influence on seed set in 2011. There was a strong correlation between fruit set reproductive output of supplementally pollinated and open-pollinated plants, suggesting that plant health or plant resources influence reproductive success. Temperature variation among regional groupings of fields was minimal and is unlikely to have accounted for differences among fields in fruit set. We propose several reasons why, despite pollen-limitation of fruit and seed set of blueberry plants, augmentation of the pollinator community of lowbush blueberry did not significantly increase reproduction. These include pollinator-mediated transfer of self-pollen followed by subsequent fruit abortion due to inbreeding depression and resource limitation for fruit maturation. Management practices that focus on increasing outcross pollen receipt, or plant resources for fruit set may have greater overall benefit than pollinator augmentation alone.