Bombus affinis

Cresson, 1863

Rusty-patched Bumble Bee, Rusty Patched Bumble Bee

Bombus affinis, the rusty-patched , is a large eusocial bumble bee to North America. Once among the most common bumble bees in the eastern and upper Midwest United States and southern Canada, it has declined by approximately 87-90% across its historical range since the late 1990s. It was listed as endangered in Canada in 2010 and became the first in the continental United States to receive federal protection under the Act in January 2017. The is distinguished by a distinctive rust-colored patch on the second abdominal segment of and has notably shorter tongues than other bumble bee species.

Bombus (Bombus) affinis by the Smithsonian. Used under a CC0 license.

Pronunciation

How to pronounce Bombus affinis: /ˈbɒmbəs əˈfɪnɪs/

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Identification

The rust-colored patch on the second abdominal segment of is diagnostic. The ' unusually short tongue length distinguishes it from all other North American bumble bees. Workers can be separated from queens by their smaller size and presence of the rust patch; queens have a more extensive black thoracic spot reaching the sides of the . Males are distinguished from workers by pale hairs and larger size. The colony odor is very similar to Bombus terricola, which may complicate detection by olfactory methods.

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Habitat

Requires three distinct types in close proximity: foraging areas with diverse flowering plants, nesting sites, and hibernation sites. Nests are constructed underground, typically in soft soil 16-18 inches below the surface, commonly in old rodent burrows. Nesting locations include ditches along roadsides, wetlands, fields, and occasionally above-ground situations such as abandoned furniture or gaps between building foundations and insulation. Foraging habitats include sand dunes, farmland, marshes, wooded areas, and montane forests. Hibernation sites are poorly documented but likely involve underground burrows or rotting logs. The occupies temperate climates and can withstand colder temperatures than most bumble bees, occurring at elevations up to 1600 m. In the Appalachian region, strongly associated with high-elevation, heavily forested landscapes with west to northwest-facing aspects.

Distribution

Historical range extended across eastern and upper Midwest United States, from Georgia north to Quebec and Ontario, Canada, and west to the Dakotas. Currently restricted to disjunct in the upper Midwest (primarily Minnesota, Wisconsin, Iowa, Illinois) and the Central Appalachian Mountains (Virginia, West Virginia). The has been extirpated from most of its northeastern and north-central range. Approximately 84% of recent observations occur on U.S. National Forest lands in the Appalachian region, suggesting these areas harbor important remaining .

Seasonality

Queens emerge in early spring, before most other Bombus . Active foraging occurs from April through October, requiring extended floral blooming periods. Colony initiation begins in spring; colonies decline in fall with new queens entering . Males and die with onset of winter. Detection probability shows a steep seasonal decline from July through September, with highest detectability in early July requiring fewer surveys to confirm presence.

Diet

Consumes nectar and pollen from diverse plant . Documented floral associations include Monarda fistulosa, Eutrochium maculatum, Veronicastrum virginicum, Pycnanthemum virginianum, Abelia grandiflora, Asclepias syriaca, Linaria spp., Lobelia siphilitica, Linaria vulgaris, Antirrhinum majus, Hydrangea, Actaea, Rhododendron, Tilia, Laportea, Rubus, Securigera, Trifolium, and various Asteraceae. Appalachian show regional differentiation in forage use compared to upper Midwest populations, with highest plant diversity in late summer. The species is a primary of Dicentra cucullaria (Dutchman's breeches), which has a flower structure specifically adapted to B. affinis foragers.

Host Associations

Bombus bohemicus - obligate Cuckoo that invades B. affinis nests. Females locate nests by odor from a distance without physical contact. lives alongside host and , rearing its own with host worker assistance. B. affinis exhibits oophagy, larval ejection, and direct ejection of parasites in response to invasion. B. bohemicus has higher survival in nests with queens present due to reduced worker aggression.
Melittobia acasta - Chalcid documented as nest in Minnesota and Wisconsin nests in 2020.
Apicystis bombi - Protozoan infecting approximately 3% of B. affinis, particularly prevalent in Ontario. Infects gut then spreads systemically, causing mortality, preventing new colony formation, limiting ovarian development, and reducing queen lifespan. Likely introduced to North America via commercial Bombus terrestris.

Life Cycle

colony cycle. Solitary queens emerge in spring, search for nest sites, and collect nectar and pollen to feed first . hatch in approximately four days; development to adulthood takes up to five weeks depending on temperature and food. initially feeds larvae alone; assume foraging duties once they eclipse. Colony reaches optimal worker numbers mid-summer, then queen switches to producing males and new queens. Colony sizes range from 50-400 individuals in wild nests; captive colonies may reach 2,100. Mating occurs near nest entrances with males chasing . New queens enter for winter; males and workers die with cold weather. Colonies persist 4-5 months. Queens live approximately 77 days on average. Orphaned colonies occur when queens die during colony development.

Behavior

Eusocial with - system and reproductive division of labor. Workers are suppressed from -laying by queen and behavioral dominance; in queen absence, worker aggression increases and ovarian development proceeds. Foraging activity is reduced during hot and humid conditions. Workers construct individual wax and . Colony odor closely resembles that of Bombus terricola, potentially confusing and . Males exhibit patrolling and chasing near nests to locate mates.

Ecological Role

Important of native plants and agricultural crops. Documented to pollinate 65 plant including cranberries, plums, apples, onions, and alfalfa. Obligate mutualist with Dicentra cucullaria for . Contributes to food security and function through pollination services. Serves as for specialized including Bombus bohemicus.

Human Relevance

First in the continental United States to be listed as endangered under the Act (January 10, 2017). Listed as endangered in Canada in 2010. Subject of intensive conservation efforts including restoration, survey protocols, and recovery planning. Agricultural importance as of food crops and medicinally used plants by First Nations. Decline attributed to spillover from commercial bees, neonicotinoid (particularly threatening due to underground nesting), habitat loss and fragmentation, and climate change. Subject of award-winning documentary 'A Ghost in the Making.' Minnesota designated it as state bee.

Similar Taxa

Bombus terricolaShares very similar colony odor, making differentiation difficult by smell; both in subgenus Bombus and share similar color patterns with black and yellow banding
Bombus citrinusGeneral appearance with yellow and black coloration; distinguished by lack of rusty patch and longer tongue
Bombus griseocollisSimilar size and furry appearance; distinguished by different abdominal color pattern and longer tongue
Bombus perplexusOverlapping range and similar general ; distinguished by lack of diagnostic rusty patch
Bombus vagansSimilar use and appearance; distinguished by longer tongue and different color pattern

More Details

Conservation status timeline

First petitioned for listing in 2010 by Robbin Thorp and Xerces Society; U.S. Fish and Wildlife Service proposed protection in 2016; final listing January 10, 2017. decline estimates range from 87% loss to 90% overall population reduction since late 1990s.

Genomic resources

sequenced as part of the Beenome100 project, providing tools for conservation genetics and monitoring.

Survey methodology

Detection probability varies dramatically seasonally: six 30-minute surveys in early July yield 95% detection confidence, while more than ten surveys required in early August. Hot, humid conditions and reduced quality decrease detection probability.

Geographic isolation

Appalachian appear genetically and geographically isolated from upper Midwest populations, with distinct associations and forage plant use, requiring separate conservation prioritization.