Rusty-patched Bumble Bee, Bombus affinis
Excerpted from Bumble Bees of Wisconsin
This once common bumble bee is the first federally protected species of bumble bee, and first bee to
be placed on the endangered species list.
Hair medium and even. Thorax yellow, with a straight or “T” shaped band of black hair between the
wings. First abdominal segment fully yellow, with the second segment yellow and usually containing a
patch of orange or rusty-brown hair. Unlike B. griseocollis, this patch is entirely surrounded by yellow
hairs. Queens often without the rusty-patch, making the second abdominal segment entirely yellow.
Males have similar color patterns to workers, but occasionally with more brown hair on the second
abdominal segment. Males also lack corbiculae, or the flattened midleg of the hind leg used for
Rusty patched bumble bee colonies have an annual cycle. In spring, solitary queens emerge and find
nest sites, collect nectar and pollen from flowers and begin laying eggs, which are fertilized by sperm
stored since mating the previous fall. Workers hatch from these first eggs and colonies grow as workers
collect food, defend the colony, and care for young. Queens remain within the nests and continue laying
eggs. In late summer, new queens and males also hatch from eggs. Males disperse to mate with new
queens from other colonies. In fall, founding queens, workers and males die. Only new queens go into
diapause (a form of hibernation) over winter - and the cycle begins again in spring.
Queens emerge in late spring (typically early May), with peak worker numbers in July and August. New
queens and drones present from July to September.
Historically, Bombus affinis was found throughout the upper Midwest and Northeastern United States.
Its range has been reduced by over 90%, and is now only found in small pockets, with areas around
Madison seeing some of the most consistent numbers in recent years. Other recordings include areas
in the driftless, near Milwaukee, and near Baraboo.
Bumble bees gather pollen and nectar from a variety of flowering plants. The rusty patched emerges early
in spring and is one of the last species to go into hibernation. It needs a constant supply and diversity of
flowers blooming throughout the colony’s long life, April through September.
Slo-Mo Footage of a Bumble Bee Dislodging Pollen
Look Inside a Bumblebee Nest
How to Build a Bumble Bee House
Development of Colony and Nest in the Bumblebee