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

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.