Photo 1. Blueberry blossom showing the characteristic flowers of this plant. Note that the flowers are closed, bell-shaped and pendant. Photo credit: Nicole Castle, contributed to Plant Village.
Blueberries are native to eastern North America, unlike many of the other crops cultivated in this region. In the northeastern United States, both cultivated and wild plants coexist. Wild and semi-wild blueberry species, which are in the “lowbush” category, are smaller in stature (ranging from 14 to 24 in) and produce smaller fruit with a sweeter taste. Cultivated species belong to the “highbush” category, with bushes that range in height between 6.6 to 9.8 ft produce larger fruit and account for most of the large-scale commercial production. Common cultivars include Rubel, Weymouth, Bluecrop, Elliot, and Jersey (see Blueberry Variety Selection in the Home Fruit Planting for an extended list of varieties).
Blueberry plants require pollination by bees. Flowers are closed, bell-shaped, pendant, and have anthers that are shorter than their stigma, which discourages self-pollination via wind or gravity (Photo 1). Therefore, bees need to move the pollen within or between flowers to achieve fruit development and plant reproduction. In addition, the pollen is hidden in the anthers and is only accessible through "buzz pollination" (also known as flower sonication). This means that for blueberry plants to reproduce, bee flower visitors must hold the stamens and rapidly vibrate their flight muscles to liberate the pollen grains through a small pore at the tip of the anthers.
Honey bee (Apis mellifera) colonies are regularly rented to achieve optimal pollination in commercial blueberry farms. However, honey bees cannot perform buzz pollination, making them ineffective pollinators for blueberry. On average, honey bees only deposit 11 pollen grains on the stigma of the flowers that they visit, while wild bees, such as mining bees and bumble bees (Photo 2), deposit on average 49 and 43 pollen grains per visit, respectively. Optimal pollination is therefore achieved by the joint visitation of wild bees and honey bees through an ecological process called complementarity. Many wild bees that are capable of buzz pollinating flowers free up pollen during their floral visits and these loose pollen grains can be later picked up by honey bees.
Even though honey bees are less effective at transferring pollen every time they visit a flower, there are many more individual honey bees than wild bees because of two reasons:
- First, honey bees live in colonies that are much larger than the other species; there are usually thousands to tens of thousands of individuals in every colony.
- Second, the high numbers of rented colonies brought to blueberry fields during bloom significantly increase the abundance of honey bees visiting blueberry flowers.
Although wild social bees such as bumble bees will build colonies that grow during the season, at the time of blueberry bloom colonies are just getting started. Therefore, both honey bees and wild bees work as a team to pollinate blueberries and are essential to maximize yields in this crop.
Because Pennsylvania is located within the native range of blueberry plants, the wild bee community that visits blueberry flowers comprises a large number of species, some of which specialize in blueberry pollination. Among the native wild blueberry specialists, there are two species of mining bees (Andrena carolina and Andrena bradleyi; Photo 2A) and the southeastern blueberry bee (Habropoda laboriosa; Photo 2B). Other native bees that commonly visit blueberry flowers include bumble bees (Bombus spp.; Photo 2C), mason bees (Osmia spp.), sweat bees (Lasioglossum spp.), carpenter bees (Xylocopa virginica) and a large number of other mining bee species (Andrena spp.).
Photo 2. Common wild bees visiting blueberry flowers include (A) mining bees in the genus Andrena (photo credit Anthony Vaudo), (B) the southeastern blueberry bee Habropoda laboriosa (photo credit Elsa Youngsteadt), and (C) bumble bees in the genus Bombus (photo credit Kristal Watrous).
In a recent survey of bees visiting wild and cultivated blueberries in Central Pennsylvania, we observed at least 14 and 8 wild bee species visiting cultivated and wild blueberry species, respectively (Table 1). The most diverse groups of bees visiting blueberries in Pennsylvania were mining bees (6 species) and bumble bees (5 species). Notably, the specialist southeastern blueberry bee was not observed during these surveys. Pennsylvania is part of the native range of this specialist bee, and it has been registered in southeastern and central counties of the state (Adams, Bucks, York, Huntingdon, and Lycoming).
Table 1. List of bee visitors to cultivated and wild blueberry plants in central Pennsylvania. An asterisk (*) indicates bee species that are not native to eastern North America.
|Bee Species||Cultivated Blueberry||Wild Blueberry or Relatives|
Each blueberry plant produces thousands of flowers every season. Each flower must be visited by multiple bee species to achieve optimal pollination that will translate into larger and more marketable fruit. For Pennsylvania, it is recommended to supplement high stocking densities of honey bees with surrounding natural habitat that supports other spring native bees species. The combination of large numbers of honey bees and native bees will guarantee optimal pollination for cultivated blueberries. Future research could help improve our understanding of wild bees providing the ecosystem service of pollination to our valuable blueberry crop in Pennsylvania.
An enjoyable 1-hour webinar summary of this type of research from current conditions in Michigan and British Columbia is available to watch. (scroll down to the webinar on blueberries).
Gibbs, J., Elle, E., Bobiwash, K., Haapalainen, T., & Isaacs, R. (2016). Contrasting pollinators and pollination in native and non-native regions of highbush blueberry production. PloS one, 11(7), e0158937.
Javorek, S. K., Mackenzie, K. E., & Vander Kloet, S. P. (2002). Comparative pollination effectiveness among bees (Hymenoptera: Apoidea) on lowbush blueberry (Ericaceae: Vaccinium angustifolium). Annals of the Entomological Society of America, 95(3), 345-351.
Kilpatrick, S.K., J. Gibbs, M.M. Mikulas, S. Spichiger, N. Ostiguy, D. Biddinger, and M.M. López-Uribe. 2018. Checklist of the Bees of Pennsylvania.
Rogers, S. R., Tarpy, D. R., & Burrack, H. J. (2014). Bee species diversity enhances productivity and stability in a perennial crop. PLoS One, 9(5), e97307.