Have you ever taken a few moments to stop and admire flowers at the UWM Saukville Field Station? If you pay attention to the pollinators visiting the same flowers that caught your eye, you may notice a special species occurring at the Station—the rusty patched bumble bee (Bombus affinis).
The rusty patched bumble bee is an endangered bumble bee species listed by the U.S. Fish and Wildlife Service in 2017, following a significant range reduction and population decline. To date, the Upper Midwest remains one of the few strongholds for the species. Workers (non-reproductive females) and males of this species are fairly easy to identify, as their abdomen bears a distinctive patch of reddish-brown hairs surrounded by a border of yellow hairs (hence the common name, rusty patched bumble bee). Queens are a bit more difficult to identify, as they generally lack that rusty patch, but can be identified by their entirely black face, a black spot on an otherwise yellow thorax, and two segments of yellow hairs on the upper portion of the abdomen.
Bombus affinis queen on Hypericum perforatum by Mary Powley
I have spent the last two years in eastern Wisconsin documenting rusty patched bumble bees and other bumble bee species, comparing detection and occupancy differences in urban and rural habitats. One of my sites was the Saukville Field Station, where the rusty patched bumble bee was personally observed. Based on my experience, here are a few notes for those who may want to look for this species on your next walk at the Field Station.
Bumble bees are active from spring to fall, utilizing different habitats each season. Follow the flowers—from the wildflowers carpeting the forest floor in spring to the dazzling blooms of the open grasslands mid-summer to the wetland plants debuting in late-summer—where there’s flowers, there’s bumble bees.
The best time to spot a rusty patched bumble bee is mid-July to mid-August, when colony size is greatest, and the workers are at their busiest. Queens are generally spotted in late spring and late summer, but are less easily found given their cryptic nature. Males are often found in mid-summer until the end of the season, when the current colony’s life cycle has ended.
Male or worker rusty patched bumble bee on a wild bergamot flower.
Photo by Danielle Bell
Bumble bees are best spotted when they stop to forage for pollen or nectar on flowers. Some floral favorites of rusty patched bumble bee that occur at the Field Station include wild bergamot (Monarda fistulosa), Culver’s root (Veronicastrum virginicum), common St. John’s wort (Hypericum perforatum), and spotted Joe-pye weed (Eutrochium maculatum).
