Annual cover crops for managed and wild bees: Optimal plant mixtures depend on pollinator enhancement goals

Authors: MALLINGER, RACHEL - University Of Florida; Franco, Jose; PRISCHMANN-VOLDSETH, DEIRDRE - North Dakota State University; Prasifka, Jarrad

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2018
Publication Date: 12/25/2018
Citation: Mallinger, R., Franco Jr, J.G., Prischmann-Voldseth, D.A., Prasifka, J.R. 2018. Annual cover crops for managed and wild bees: Optimal plant mixtures depend on pollinator enhancement goals. Agriculture, Ecosystems and Environment. 273(1):107-116. https://doi.org/10.1016/j.agee.2018.12.006.
DOI: https://doi.org/10.1016/j.agee.2018.12.006

Interpretive Summary: Changes in land-use that have reduced availability of flowers in the Northern Great Plains may have negative effects on pollinators. Cover crops may help to provide flowers for both managed and wild bees. In this study, we found that low-diversity mixes including buckwheat and phacelia provide a high amount of flowers throughout the season, which resulted in a high number of bee visits. However, these two plants were attractive primarily to managed honey bees and generalist bumble bees. Including sunflower, a crop developed from a native plant, can increase the attractiveness of the cover crop to other bees, including rare species, but may reduce attractiveness to honey bees and bumble bees. Therefore, the best plant mix depends on whether the goal is to provide flowers for managed or generalist bees versus rare wild bees. This study shows that cover crops are valuable for pollinators, including rare species. However, it also shows that there is a trade-off between mixes that attract a large number of common generalists or managed bees versus mixes that attract fewer bees but ones that are rare. These results are important to land managers and producers who are interested in planting cover crops to provide flowers for managed and wild bees.

Technical Abstract: Agricultural intensification and landscape simplification have reduced flowering plant abundance and diversity worldwide, with subsequent negative effects on pollinators. To combat these effects, research and conservation have focused on incorporating flowering plants into agricultural landscapes through on-farm natural areas or flowering crops. However, while cover crops provide numerous agroecosystem services, they have received little attention for their role in providing pollinator resources. In this study, we examined the value of cover crops for providing supplemental floral resources for both managed and wild bees in the U.S. Northern Great Plains, where a large proportion of honey bee colonies spend the summer. Specifically, we compared 1, 2, 3, and 6-species cover crop mixes to one another, and to an annual native wildflower mix, in order to evaluate how plant identity and diversity affect season-long floral cover and bee visitation rates. We found that low-diversity cover crop mixes including buckwheat provided the highest season-long floral cover, higher than the annual native wildflower mix, and that total bee visitation rates were in turn positively related to floral cover. Neither floral cover nor bee visitation rates increased with seed mix diversity. However, different bees were attracted to different cover crops; honey bees and bumble bees were most attracted to mixes containing phacelia, while large-bodied solitary bees were more attracted to mixes including cultivated sunflower and the wildflower mix, illustrating that the optimal plantings for attracting generalist or managed bees may differ from those attracting specialist wild bees. Additionally, while similar numbers of bee species visited buckwheat, phacelia, and cultivated sunflower, the majority of species visiting phacelia and buckwheat are relatively common, while cultivated sunflower attracted more rare species. Our results highlight the value of annual cover crops for providing pollinator resources in agricultural landscapes, particularly on rotated land where perennial flower plantings are infeasible, but illustrate trade-offs between attracting a large number of common generalist or managed bees versus attracting fewer bees but species that may be of greater conservation concern.