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

Mallinger, Rachel E., Franco, Jose G., Prischmann-Voldseth, Deirdre A., Prasifka, Jarrad R.
Agriculture, ecosystems & environment 2019 v.273 pp. 107-116
Bombus, Helianthus annuus, Phacelia, agricultural land, agroecosystems, annuals, buckwheat, cover crops, experimental design, flowering, honey bee colonies, honey bees, indigenous species, intensive farming, landscapes, pollinators, rare species, solitary bees, summer, wild flowers, Great Plains region, United States
Agricultural intensification and landscape simplification have reduced flowering plant abundance and diversity worldwide, with subsequent negative effects on pollinators. To mitigate 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 floral resources for pollinators. In this study, we examined the value of cover crops for providing supplemental summer floral resources for both managed and wild bees in the Northern Great Plains of the United States, where a large proportion of honey bee colonies spend the summer. Specifically, using an experimental randomized plot design, we compared floral resources and bee visitation rates across 1, 2, 3, and 6-species cover crop mixes as well a 6-species annual native wildflower mix. We found that cover crop treatments including buckwheat provided the highest summer-long floral cover, higher than the annual native wildflower mix in which only two of six species bloomed, and that total bee visitation rates increased with higher floral cover. However, different bees were attracted to different treatments; 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 to the wildflower mix, illustrating that the optimal plantings for attracting social or managed bees may differ from those attracting solitary wild bees. Additionally, while similar numbers of bee species visited the three main plants in our treatments (buckwheat, phacelia, and cultivated sunflower) more rare species visited cultivated sunflower while more common bees visited buckwheat and phacelia. 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 less feasible, but illustrate trade-offs between attracting a large number of common generalist or managed bees versus attracting fewer individuals but species of potential conservation concern.