On-farm wildflower plantings generate opposing reproductive outcomes for solitary and bumble bee species

Authors: Angelella, Gina; MC CULLOUGH, CHRISTOPHER - Washington Department Of Agriculture; O'ROURKE, MEGAN - US Department Of Agriculture (USDA)

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2025
Publication Date: 4/10/2025
Citation: Angelella, G.M., Mc Cullough, C.M., O'Rourke, M.E. 2025. On-farm wildflower plantings generate opposing reproductive outcomes for solitary and bumble bee species. Environmental Entomology. https://doi.org/10.1093/ee/nvaf033.
DOI: https://doi.org/10.1093/ee/nvaf033

Interpretive Summary: Pollinator habitat strips or meadows can be planted to enhance floral and nesting resources for pollinators and enhance pollinator populations on farms. There is ample evidence linking such plantings to greater pollinator abundance on farms, but it is less clear whether these population increases result primarily from attraction and aggregation or whether they can enhance pollinator reproduction. USDA researchers in Wapato WA in collaboration with University researchers measured reproduction in a solitary bee species, the alfalfa leafcutter bee (Megachile rotundata), and a bumble bee species, the common Eastern bumble bee (Bombus impatiens) as well as parasitism rates on farms with and without wildflower plantings. They found the alfalfa leafcutter bee increased reproduction on farms with wildflower plantings, but the common Eastern bumble bee had lower nest growth and greater rates and intensity of Nosema infection on farms with wildflower plantings. This work demonstrates wildflower plantings have the potential to benefit some species, but could increase micro-parasite infections in others. It will be important to determine how to compose wildflower plantings to provide beneficial resources while minimizing parasitism in wild and managed bees.

Technical Abstract: Pollinator habitat can be planted on farms to enhance floral and nesting resources, and subsequently, pollinator populations. There is ample evidence linking such plantings to greater pollinator abundance on farms, but less is known about their effects on pollinator reproduction. We placed Bombus impatiens Cresson (Hymenoptera: Apidae) and Megachile rotundata (F.) (Hymenoptera: Megachilidae) nests out on 19 Mid-Atlantic farms in 2018, where half (n = 10) the farms had established wildflower plantings and half (n = 9) did not. Bombus impatiens nests were placed at each farm in spring and mid-summer and repeatedly weighed to capture colony growth. We quantified the relative production of reproductive castes and assessed parasitism rates by screening for conopid fly parasitism and Nosema spores within female workers. We also released M. rotundata cocoons at each farm in spring and collected new nests and emergent adult offspring over the next year, recording female weight as an indicator of reproductive potential and quantifying Nosema parasitism and parasitoid infection rates. Bombus impatiens nests gained less weight and contained female workers with Nosema spore loads over 150 times greater on farms with wildflower plantings. In contrast, M. rotundata female offspring weighed more on farms with wildflower plantings and marginally less on farms with honey bee hives. We conclude that wildflower plantings likely enhance reproduction in some species, but that they could also enhance microsporidian parasitism rates in susceptible bee species. It will be important to determine how wildflower planting benefits can be harnessed while minimizing parasitism in wild and managed bee species.