Half-black Bumble Bees vs. Similar Species:
It is possible to confuse half-black bumble bees with the two-spotted bumble bee (shown in the entry directly above in this guide), because both bees exhibit a curved "W'-shape where their yellow hair meets the black hairs of their lower abdomen. Half-black bumble bees, however, look unique in profile. Their top two abdominal segments are distinctly yellow, forming a yellow tunic that extends to the bottom of the second abdominal segment. The margin between the yellow and black portions of their abdomens tends to be sharply demarcated. By contrast, on most two-spotted bumble bees, only the first abdominal segment is yellow. The second is black on the sides with a small patch of yellow in the middle (on the top of the bee). Some variation in two-spotted bumble bees does occur, however; in male bees, the first two abdominal segments are occasionally entirely yellow. In such cases, the hair on the third segment tends to be black interspersed with yellow, and not purely yellow as is true of the half-black bumblebee.
Half-black bumble bees are at times indistinguishable from another species called Sanderson bumble bee (Bombus Sandersoni), which is fairly rare. The best description of the distinctions between half-black and Sanderson bumble bees can be found, in our estimation, in The Connecticut Bumble Bee Guide of the Connecticut Entomological Society (http://ctentsoc.org/wp-content/uploads/2014/04/CT-Bombus-Guide.pdf). The distinctions are nevertheless small: Sanderson bumble bees are somewhat smaller than half-blacks; Sanderson workers and males are typically between 2/5 and 3/5 inches. Female Sanderson bumble bees often have a black band on the thorax between the wings and show black hairs scattered through the yellow hairs on their second abdominal segments. Male half-black and Sanderson bumble bees show subtler differences. At times, visual differentiation between them is not possible, and DNA tests are required for those driven to distinguish the two species.