Bombus affinis, commonly known as the rusty patched bumble bee, is a species of bumblebee endemic to North America.

[3] This species consumes nectar and pollen from a variety of plants, including Abelia grandiflora, Asclepias syriaca, and Linaria spp.

[3] The colony odor is very similar to that of Bombus terricola, which makes it difficult for predators and parasites to differentiate between the two species.

[7] B. affinis is a member of the subfamily Apinae,[3] and is most closely related phylogenetically to B. franklini, which is another species of bumble bee endemic to North America.

[3] Unlike many other members of the genus Bombus, B. affinis workers and queens are characterized by different color patterns, which allow observers to tell them apart.

[9] They also both have completely yellow hair on the majority of their abdomens, except for a small section near the area closest to the rear end of the bee.

[9] Thus, while workers and queens share similarities in certain aspects of coloration, also differences occur in body size and the presence or absence of rust-colored patches of hair.

[9] Because of its body size and furry appearance, however, this bee is often confused with other species of bumblebee, such as B. citrinus, B. griseocollis, B. perplexus, and B. vagans.

[9] B. affinis requires three different types of habitats (each for foraging, nesting, and hibernating)[8] which are geographically close to one another, making this species particularly vulnerable to extinction.

[3] While B. affinis' habitat used to be highly prevalent, a large decline has occurred in recent years, possibly due to increased land development and agricultural use.

[5] The Ontario Ministry of Natural Resources has begun a recovery project aimed at protecting the species and critical habitats centered in Pinery Provincial Park.

[5] These threats have been proposed as the cause of population decline: pathogen spillover from other species, pesticide use, and habitat fragmentation and loss.

[4] Bell Bowl Prairie in Illinois is a known habitat of B. affinis;[11] it was partially demolished in March 2023 to make way for an expansion of the Chicago Rockford International Airport.

[8] Solitary queens are the first to emerge and begin searching for a colony, while also collecting nectar and pollen to feed her future brood.

[9] Eggs hatch about four days after fertilization, but take up to 5 weeks to become completely developed adults depending on temperature and food availability.

[13] Similar to other social insects, egg laying by B. affinis workers is suppressed by the presence of a dominant queen that is capable of reproducing.

[10] B. bohemicus does not demonstrate this behavior, and actually has worse chances of survival in nests where a dominant queen is not present, due to increased aggression by B. affinis members.

[10] However, while ovarian development is not impacted by the presence of B. bohemicus, overall reproductive success is decreased due to consumption of the host eggs and larval ejection.

[13] Because members are haplodiploid making males haploid and females diploid, so genetic relatedness is asymmetrical, causing workers to be more closely related to their sisters than their brothers.

[13] Thus, all members which are workers should demonstrate favor for a predominantly female population (if the queen is the major egg layer in the colony).

[7] B. bohemicus ends hibernation shortly after B. affinis, and searches for their established nests, but the exact mechanism is unknown.

[7] By flying low to the ground and searching carefully (even by looking near leaves and debris), B. bohemicus may first locate the entrance of a B. affinis nest, and then verify its findings by odor identification.

[13] Should B. bohemicus make a mistake in invading the nests of other species, they will be attacked by the queen, whose violent actions often end in their death.

[7] B. affinis consumes the nectar and pollen of a variety of nototrobic plant species, including Lobelia siphilitica, Linaria vulgaris, and Antirrhinum majus.

[19] While its transmission is not well understood, A. bombi causes multiple negative effects, including increased death of worker bees, and the prevention of formation of new colonies.

[3] In 2008, three recent events were reported to have led to the decline of B. affinis' agricultural role: pathogen spillover, pesticide use, and habitat loss.