Bombus affinis, commonly known as the rusty patched bumble bee, is a species of bumblebee endemic to North America. Its historical range in North America has been throughout the east and upper Midwest of the United States, north to Ontario, Canada, where it is considered a "species at risk", east to Quebec, south to Georgia, and west to the Dakotas. Its numbers have declined in 87% of its historical habitat range. On January 10, 2017, the United States Fish and Wildlife Serviceplaced B. affinis on the list of endangered species, making the rusty patched bumblebee the first bee to be added to the list in the continental United States (seven species of yellow-faced bees native to the Hawaiian islands were added in 2016).
Members of B. affinis are relatively large in size, and like other species of bumblebees, are known to be eusocial organisms. Most nests constructed by B. affinis are built underground, and are commonly found in old rodent burrows. Nests created in captivity can house up to 2,100 members, but they are typically much smaller in the wild. This species consumes nectar and pollen from a variety of plants, including Abelia grandiflora, Asclepias syriaca, and Linaria spp. The colony odor is very similar to that of Bombus terricola, which makes it difficult for predators and parasites to differentiate between the two different species.
Large differences are seen in morphology between both queens and workers, and males and females. Queens are about 20–22 mm (0.79–0.87 in) in length and 9–11 mm (0.35–0.43 in) in width, which is larger than workers that are typically about 10–16 mm (0.39–0.63 in) in length and 6–9 mm (0.24–0.35 in) in width. Both queens and workers have black hair that covers their heads, much of their legs, and the bottom of their abdomens. 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. Workers have a slight mixing of yellow and black hairs near the base of the wings, which forms a discernible "V" shape, as well as a rust-colored patch of hair on the middle portion of the abdomen. 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. Regardless of the caste within the colony, all members of B. affinis have significantly shorter tongues than any other species of bumblebee. 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.
Furthermore, male and female workers of B. affinis also differ in terms of their appearance and body size. Male workers are typically slightly larger than females (13–17.5 mm (0.51–0.69 in) in length), and have a few off-white/pale hairs present on tops of their heads. They also have black hair which sometimes streaks across the tops of their abdomens (which are typically yellow). Finally, males can even have pale yellow hair on their abdomens, as opposed to the normal shade of yellow observed in females and queens.
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