Cranberries are highly reliant on insect pollination to achieve optimal yield. Most growers ensure the delivery of pollination services through rented honey bee colonies, though recent population declines due to a combination of factors have called the sustainability of this practice into question. Wildflower plantings have been utilized in other cropping systems, including blueberry, almond, and strawberry to augment the wild pollinator community and ensure delivery of pollination services and increasing overall farm sustainability. Here we assessed how wildflower plantings affect the wild bee community in Wisconsin cranberry, to determine how the presence of a wildflower planting may affect bee visitation to cranberry flowers, and to evaluate how wild bee biodiversity and visitation affect cranberry yield.
We found a higher Simpson diversity index outside of cranberry beds than within them, and higher within wildflower plantings than on conventionally-managed field edges (Figure 1). Simpson’s diversity was not different between cranberry beds adjacent to wildflower plantings and those adjacent to conventionally-managed field edges. These results suggest that pollinator gardens increased bee diversity on cranberry marshes but not yet in cranberry beds near pollinator gardens.
Bee species richness and abundance
Looking at species richness (number of species collected) and abundance (number of bees per species) separately, we found no impact of the pollinator gardens on either of these metrics between cranberry beds adjacent to wildflower plantings and cranberry beds adjacent to conventionally-managed field margins, nor was it different between wildflower plantings and conventionally-managed field edges. Species richness increased every year in our study and wild bee abundance was higher outside of cranberry than within cranberry beds (Figure 2). These results suggest that there is a larger abundance of wild bees on field margins that could be tapped into for cranberry pollination by fostering their populations with pollinator gardens and other pollination practices.
Bee visitation to cranberry (Figure 3) was not significantly different between cranberry beds adjacent to wildflower plantings or conventionally-managed field edges. As growers stock honey bee hives to insure optimal pollination, it was not surprising to see that honey bees visited cranberry flowers more often than wild bees. These results suggest that while pollinator gardens are not yet leading to wild bees spilling over into cranberry, they are also not drawing honey bees away from cranberry.
Mean berry weight (Figure 4) was not different in cranberry beds adjacent to wildflower plantings or conventionally-managed field edges, was not affected by visitation rates from any bees, and was not different between years. Wild bee species richness was positively correlated with increased mean berry weight, with every additional wild bee species adding approximately 0.01 grams of weight to fruit, thus emphasizing the importance to foster bee richness and diversity on cranberry marshes.
Cranberry yield (Figure 5) was not different in cranberry beds adjacent to wildflower plantings or conventionally-managed field edges and was not affected by visitation rates from any bees. Yield was lower in 2019 than 2018 or 2020, and variations in yield were best explained by the marsh berries were collected from.
Wildflower plantings on cranberry marshes add an extra layer of support to the wild pollinator community and improve the overall resilience of pollination services to cranberry. The native floral resources afforded by pollinator gardens provide alternate means of forage for wild and honey bees. These resources may boost the overall health and resilience of honey bee colonies and may lead to greater sustainability of honey bee-mediated pollination.
Our result support the recommendation that pollinator gardens increase the diversity of wild bees on cranberry marshes. We recommend planting at least ¼ acre pollinator garden but suggest that this is likely on the lower end of an optimal size to promote pollinators in cranberry. The mix of plants we used was well received by growers and we provided recommendations for site preparation, planting, and maintaining gardens as well as managing weeds in pollinator gardens at different events. Other publications on protecting and promoting wild pollinators were provided to growers and are available as short and longer versions.
We would like to thank all the participating cranberry growers who established and maintained wildflower plantings, and allowed us to conduct research on their marshes, we could not do our research without you, your help, your knowledge, and insights! We would also like to thank the WCB and DATCP for funding this research.
Happy growing season!This article was posted in Cranberry, Insects and tagged bees, Christelle Guédot, Cranberries, cranberry, Nolan Amon, Pollination, pollinators, wild bees, wildflower.