Identification

Bombus affinis are most easily identified by their characteristic rust-colored patch located on the middle of their abdomen. This patch is prevalent on workers, but may vary in appearance in queens and male bees. The thorax is entirely yellow except for  a small black patch which narrows to a "V" shape towards the rear. The top half of their abdomen is yellow and rusty-patched and all other abdominal segments are black.

Fig 1. Lateral view of Bombus affinis queen (left) and male bee (right). Photos via Vermont Center for Ecostudies.

Evolutionary History

Fig 2.  Phylogenic tree depicting evolutionary history and relationships of bee taxa. Image via Paleontological Research Institution.

The Rusty Patched Bumble bee belongs to a rich taxa of pollinators whose evolutionary history can be traced back to the same period during which we observe the first flowering plants. Genetic analysis suggests that the first bees appeared 130 million years ago, likely evolving from predatory wasps (order Hymenoptera) as an evolutionary response to increased pollen availability and other adaptions of plants which established symbiotic plant-pollinator relationships. Although bees were not the first animal pollinators, over time they have established their spot as arguably the most successful pollinating group. Diversification of bees through evolutionary history has lead to the evolution highly specialized mechanisms to optimize pollen collection and transfer. For example, members of the family Apidae, such as B. affinis, have specialized adaptations to the hind legs, making them larger, flatter, and evolving "pollen baskets" which store pollen. Today, B. affinis is one of over 20,000 pollinating bee species, 4,000 of which are native to the northern portion of North America. 

Fig 3. Bee with specially evolved "pollen baskets" called corbicula, located on the hind leg. Image via Georgia Tech College of Engineering.

Population distribution

Rusty patched bumble bee populations were once widely distributed across the southeastern portion of Canada and the northeastern United States, but has declined from an estimated 87% of its historic range in recent years. As a result of decreased range, populations have also suffered significant decrease in relative abundance. 

Fig 4. Decreased habitat range has also led to decrease in populations. Bombus affinis range map and observations, with a black line representing the historical range of B. affinis, and points representing recorded observations over time. Map via Xerces Society for Invertebrate Conservation.

Ecological Role

Like other bee species, the rusty patched bumble bee acts as an essential generalist pollinator within its ecosystems. B. affinis has been reported to pollinate a variety of plants vital to both environmental health and human health, including berries, fruit flowers, clover, milkweed, and other wildflowers.