Identity and diversity of pollens collected by two managed bee species while in blueberry fields for pollination

Authors: Graham, Kelsey; MILBRATH, MEGHAN - Michigan State University; KILLEWALD, MICHAEL - Michigan State University; SOEHNLEN, ANNUET - Michigan State University; ZHANG, YAJUN - Michigan State University; ISAACS, RUFUS - Michigan State University

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2023
Publication Date: 7/27/2023
Citation: Graham, K.K., Milbrath, M.O., Killewald, M., Soehnlen, A., Zhang, Y., Isaacs, R. 2023. Identity and diversity of pollens collected by two managed bee species while in blueberry fields for pollination. Environmental Entomology. https://doi.org/10.1093/ee/nvad072.
DOI: https://doi.org/10.1093/ee/nvad072

Interpretive Summary: Managed honey bees and bumble bees are critical for optimizing the pollination of highbush blueberry. Growers bring both species of bees to their farms during bloom to increase fruit yields. However, there have been concerns among commercial beekeepers that their colonies suffer from poor health following pollination stints in blueberry fields. Blueberry pollen has low protein content and is considered insufficient on it's own to sustain healthy colonies. Therefore, honey bees and bumble bees likely need to find supplemental pollen in order to maintain or grow the colony during this time. To better understand the pollen preferences and needs of commercial colonies during blueberry pollination, we sampled bee collected pollen from honey bee and bumble bee colonies at 15 commercial blueberry farms across two years in southwest Michigan. We then identified the collected pollen to characterize the plant communities utilized for pollen by both bee species and to compare across bee species and farms. We also characterized the surrounding landscape around farms to correlate the collection of plant pollens with landscapes. We found that bumble bees collect from a wider diversity of plants compared to honey bees and that the community of plants visited was different between bee species and between years. Plants that were important pollen resources included buckthorn (Rhamnus/Frangula), willow (Salix), cherry (Prunus), brambles (Rubus), white clover (Trifolium repens), and oak (Quercus). Barren landscapes, mixed forests, evergreen forests, water, and herbaceous wetlands were landscapes that were strongly correlated with primary pollen sources and may therefore represent important landscapes for supporting healthy colonies during blueberry pollination.

Technical Abstract: Honey bees and bumble bees are commonly used for crop pollination, yet the nutritional needs and foraging behavior of the colonies often require them to collect pollen resources other than the focal crop. Robust nutrition is key for maintaining healthy colonies, which in turn are important for successful crop pollination and for beekeeping businesses. To better understand the types of pollen collected by bees managed for blueberry pollination, we identified pollen collected by Apis mellifera and Bombus impatiens colonies during blueberry bloom across two years. Both species were on the same farms, allowing us to directly compare which species they brought back to the colony. We then characterized the surrounding landscape to determine which landscape features were correlated with pollen foraging. Bumble bees collected a wider diversity of pollens compared to honey bees, whereas honey bees were more focused on abundant resources. Despite blueberries being the most abundant resource in the landscape, it was not the most collected by either bee species in 2018. However, it was the most collected pollen for bumble bees in 2019 and bumble bees collected substantially more blueberry pollen than honey bees in both years. In 2018, buckthorn (Rhamnus/Frangula) and willow (Salix) pollens were a high proportion of pollen collected by both bees. In 2019, stone fruit (Prunus) and willow (Salix) pollens were collected at high rates and proportions by both species. Brambles (Rubus) and white clover (Trifolium repens) were also common pollen sources for honey bees, whereas oak (Quercus) was collected by bumble bees. Some land cover types were positively correlated with collection of many pollen types, including barren landscapes, mixed forests, evergreen forests, and water. Herbaceous wetlands were also associated with the collection of buckthorn (Rhamnus/Frangula), willow (Salix), and stone fruit (Prunus) pollen, which were abundantly collected by both bee species. There was no correlation between landscape diversity and pollen diversity suggesting that colonies will forage based on nutritional requirements rather than resource availability.