Diversity of bumble bees and butterflies in Minnesota roadsides depends on floral diversity and abundance but not floral native status

Habitat loss is an important driver in the rapid decline of many insects. The restoration of roadside habitat has been touted as an opportunity to support communities of bumble bees and butterflies, two insect groups of conservation concern. However, it is unclear how current roadside restoration methods translate to pollinator abundance and diversity. Here, we ask how the plant communities of roadsides seeded with native and non-native seed mixes affect bumble bee and butterfly communities 2–20 years after planting. We found that bumble bee and butterfly abundance and diversity were positively correlated with floral abundance and diversity. However, the pollinator community in plots planted with diverse native seed mixes did not differ from that found in plots seeded with species-poor non-native mixes, likely because many forbs in the native seed mixes had poor establishment, and all plots were readily colonised by species that were never planted. While plots with more native flowers did not necessarily support higher pollinator diversity, we did find several native plants that established well in roadsides and supported pollinators, including goldenrods (Solidago spp.), milkweeds (Asclepias spp.) and wild bergamot (Monarda fistulosa). Our data suggest that managing a more diverse and abundant floral community will benefit bumble bees and butterflies. Refining the composition of native seed mixes, adding more floral abundance to non-native mixes and investing in management for native plant communities may increase the habitat value of roadsides for pollinator communities.