A rusty-patched bumble bee foraging on bee balm. (Photo: Susan Carpenter)
In our Wisconsin climate, January is an unlikely month for actual gardening, but seeding and planning for the new season is underway at the Arboretum. In restorations, seeds harvested last season have been mixed and spread on cleared and prepared sites. In the Arboretum native plant garden, where most areas are well-established, we have done some overseeding. We also enhance the garden by ordering and planting additional plants every year. There is always space for one more native plant!
To help home, school, and community gardeners establish new gardens or enlarge existing ones, Friends of the Arboretum offers an online plant sale (order plants through March 31, with pick up in May) as well as the in-person sale under the tent on May 17.
To serve widespread interest in creating pollinator habitat and supporting populations of the federally endangered rusty-patched bumble bee (RPBB), FOA has designed a plant mix comprising native plants that are RPBB “favorites.” This set of thirty-two plants will bear flowers sequentially throughout the season, depending on weather conditions and temperature. Spring blooming plant species like wild geranium (Geranium maculatum), Virginia waterleaf (Hydrophyllum virginianum), and leadplant (Amorpha canescens) support bumble bee queens after they emerge from their hibernation chambers and forage for pollen and nectar as they each establish a colony. After about a month, if nest establishment is successful, late spring and summer blooming plants like coneflowers (Echinacea spp.), bee balm (Monarda fistulosa), Culver’s root (Veronicastrum virginianum), Joe-pye weeds (Eutrochium spp.) and giant hyssops (Agastache spp.) are important in providing resources as the colony first builds in size to several hundred worker bees, and then shifts to reproduction. Male bees visit flowers for nectar, but the newly hatched gynes (future queens) visit flowers for nectar and may also collect pollen. Because mated new gynes are the only colony members to overwinter, they must have adequate late summer and fall floral resources like native thistles (Cirsium spp.), goldenrods (Solidago spp.) and asters (Symphyotrichum spp. and other aster genera).
Growing these plants in your garden will benefit the rusty-patched bumble bee as well as other bee species (including native solitary bees) and additional pollinators. Many of these plants are also used by specialist bee species that only visit one species or genus of plant. Networks of pollinators and plants are complex and many questions remain about them.
A recent study identified wild white indigo (Baptisia alba), milkweeds (Asclepias spp.) and Silphium spp. as additional genera that were preferentially selected by bumble bees in restoration areas. While bumble bees often visited several non-native species, this was mostly seen when those plants were the most abundant in the area. An interesting finding was that the three most common bumble bee species differed significantly in their feeding niches, which may have been due to differences in phenology (timing of colony development) but also may reduce competition for resources among them when their life histories overlap. The rarer bee species’ diets were less fully understood but showed more overlap among those bee species. This study was carried out in an area where the rusty-patched bumble bee is no longer found, so doing a similar analysis within remaining rusty-patched range could be instructive for understanding bees’ floral preferences (accounting for overall plant abundance) and for managing high-quality habitat for this rare bee.
During deep winter, while the bumble bee gynes are dormant in their overwintering chambers, we can plan plantings that will support them and their colonies next season.
– Susan Carpenter, native plant garden curator
