Bombus melanopygus

Nylander, 1848

black-tailed bumble bee, black tail bumble bee, orange-rumped bumblebee

Bombus melanopygus is a polymorphic native to western North America, recognized for its striking color variation between geographic regions. The species exhibits Müllerian mimicry with two distinct forms: a northern 'Rocky Mountain' form with ferruginous (reddish-orange) second and third abdominal segments, and a southern 'Pacific' form with black mid-abdominal segments. These morphs were historically described as separate species (B. edwardsii for the black form) but genetic analyses confirm they represent a single species with secondary contact between historically isolated lineages. The species is among the earliest bumble bees to emerge in spring in its range and has been documented utilizing diverse including urban and agricultural areas.

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

Pronunciation

How to pronounce Bombus melanopygus: /ˈbɔm.bʊs mɛ.ləˈnɒ.pɪ.ɡəs/

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Identification

Distinguished from similar western bumble bees primarily by the combination of black terminal abdominal segments (the 'black tail') and geographic location. The ferruginous abdominal segments in northern can resemble Bombus mixtus or B. rufocinctus, but B. melanopygus lacks the mixed or banded color patterns of those . The black form in California can be confused with B. vosnesenskii (yellow-faced ), but B. melanopygus has a black while B. vosnesenskii has a yellow face. The two color forms of B. melanopygus meet in a narrow transition zone in northern California and southern Oregon where intermediate or mixed populations may occur.

Images

Appearance

Medium-sized with variable abdominal coloration depending on geographic origin. Northern (B. m. melanopygus) display ferruginous (reddish-orange) coloration on the second and third abdominal segments, earning the 'orange-rumped bumblebee.' Southern populations (B. m. edwardsii) have black mid-abdominal segments. All forms have a black tail (terminal abdominal segments), consistent with the 'black-tailed' common name. The color is controlled by a single Mendelian locus with the ferruginous .

Habitat

Utilizes diverse types across its range including coastal scrub, chaparral, montane meadows, forest edges, and urban environments. Documented in agricultural areas and residential gardens. One of few regularly observed in developed urban areas such as San Francisco. Associated with early-blooming shrubs in the Ericaceae , particularly manzanita (Arctostaphylos) and Ceanothus, which provide critical early-season floral resources.

Distribution

Western North America from Alaska and British Columbia south to Baja California, and east to Idaho and the Rocky Mountains. The ferruginous form (melanopygus) predominates from Oregon northward and in the Mountain West; the black form (edwardsii) is most common in California and southern Oregon. A narrow mimicry transition zone approximately 300 km wide occurs in northern California and southern Oregon where the two forms meet and intergrade.

Seasonality

Among the earliest-emerging bumble bees in its range; queens active from January in southern portions of range (California) to later spring in northern areas. Documented records include January 1-10 in the Sacramento Valley and San Francisco Bay Area. Males observed unusually early in season in some years (late January), suggesting possible or early colony initiation.

Diet

forager documented on diverse flowering plants including manzanitas (Arctostaphylos), Ceanothus, goldenbushes (Ericameria), wild buckwheats (Eriogonum), lupines (Lupinus), penstemons (Penstemon), rhododendrons (Rhododendron), willows (Salix), sages (Salvia), and clovers (Trifolium). Observed foraging on both native plants and ornamental garden including lavender and roses.

Host Associations

Apocephalus borealis - Zombie fly; known of this phorid fly

Life Cycle

Eusocial colony cycle with queens. Nests established underground or aboveground in structures. Colony development follows typical pattern: , colony founding, production, male and new queen production, and colony . Specific timing varies with latitude and elevation.

Behavior

Exhibits Müllerian mimicry, conforming to regional mimicry complexes: the ferruginous form resembles other red-banded bumble bees in the Rocky Mountain region, while the black form resembles the yellow-faced (Bombus vosnesenskii) in the Pacific coastal region. Assortative mating between color forms has been inferred as a partial reproductive barrier in the contact zone. Genetic data support secondary contact between historically isolated lineages with ongoing .

Ecological Role

Important early-season in western North American ; provides pollination services to diverse native plant and agricultural crops. Contributes to maintenance of plant communities through buzz pollination and foraging . Serves as for insects, contributing to complexity.

Human Relevance

Subject of science monitoring efforts including the California Bumble Bee Atlas. Frequently observed in urban gardens and agricultural settings, providing accessible opportunities for public engagement with native conservation. Used as for early spring pollinator activity in citizen science contests. Contributing pollinator to crops and ornamental plants; to urbanization makes it a model for studying pollinator to anthropogenic landscapes.

Similar Taxa

Bombus vosnesenskiiYellow-faced ; black form of B. melanopygus resembles this but B. vosnesenskii has yellow while B. melanopygus has black face
Bombus edwardsiiFormerly recognized as separate for the black abdominal form; now synonymized under B. melanopygus based on genetic evidence of conspecificity
Bombus mixtusFuzzy-horned bumble bee; similar reddish coloration in some but has more variable banding and different geographic range

Misconceptions

The two color forms were historically treated as separate (B. melanopygus and B. edwardsii), leading to potential confusion in older literature. Genetic analyses confirm they represent a single species with polymorphic coloration maintained by Müllerian mimicry selection rather than species-level divergence.

More Details

Mimicry transition zone

A narrow (~300 km) contact zone in northern California and southern Oregon where ferruginous and black color forms meet and intergrade. The zone exhibits mito-nuclear discordance: mitochondrial haplotypes transition further south than color pattern, suggesting complex evolutionary dynamics involving selection, dominance effects, and assortative mating.

Conservation status

Not currently listed as threatened; described as 'one of the few bumblebees still found regularly in San Francisco,' suggesting relative compared to other western bumble bee that have experienced significant declines.