Bombus pensylvanicus

Bombus pensylvanicus
	Queen in Hanover County, VA
	Male in Illinois, US
Conservation status
	; Vulnerable (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Apidae
Genus:	Bombus
Subgenus:	Thoracobombus
Species:	B. pensylvanicus
Binomial name
	Bombus pensylvanicus; (De Geer, 1773)
	The general range of Bombus pensylvanicus. (Dashed line indicates former range)
Synonyms
	Apis pensylvanicaDe Geer, 1773; Apis americanorumFabricius, 1775; Apis antiguensisFabricius, 1775; Apis nidulansFabricius, 1793; Bombus pallidusCresson, 1863; Bombus pennsylvanicusCresson, 1863 (Emend.); Psithyrus cevalliaeCockerell, 1899; Bombus americanaHoward, 1901 (Missp.); Bombus titusiAshmead, 1902; Bombus pennsylvanicus v. umbratusFriese, 1931;

Last updated May 04, 2024

Bombus pensylvanicus, the American bumblebee, is a threatened species of bumblebee native to North America. It occurs in eastern Canada, throughout much of the Eastern United States, and much of Mexico.^[1]

Once the most prevalent bumblebee in the southern United States, populations of Bombus pensylvanicus have decreased significantly in recent years,^[3] including in its scientific namesake state of Pennsylvania, where its numbers are considered critically low.^[4] Overall the population has dropped nearly 90% in just the last 20 years.^[5]

Bombus pensylvanicus tends to live and nest in open farmland and fields. It feeds on several food plants, favoring sunflowers and clovers,^[6] and functions as a pollinator.

Taxonomy and phylogenetics

Bombus pensylvanicus belongs to the order Hymenoptera (consisting of ants, wasps, bees, and sawflies), the family Apidae (consisting of Cuckoo, Digger, Carpenter, Bumble, and Honeybees), the Subfamily Apinae (consisting of Honey, Orchid, Bumble, Long-horned, and Digger Bees), and the genus Bombus (consisting of bumblebees).^[3] Within Bombus, B. pensylvanicus belongs to the subgenus Thoracobombus, which includes species such as Bombus armeniacus, Bombus pauloensis, Bombus dahlbomii, Bombus fervidus, Bombus humilis, Bombus morio, Bombus muscorum , Bombus pascuorum, Bombus pomorum, Bombus ruderarius, Bombus sylvarum, and Bombus transversalis.Bombus pensylvanicus is sometimes mistaken for B. terricola or B. auricomus,^[7] but its closest relative is Bombus sonorus, found in Mexico and Arizona.^[8] Scientists sometimes treat Bombus sonorus as a subspecies of Bombus pensylvanicus, although each species maintain differences in male genitalia.^[9] Intermediate individuals of B. pensylvanicus and B. sonorus have been found in areas of geographic overlap,^[8] but further evidence is needed to distinguish whether B. sonorus is a subspecies of B. pensylvanicus.^[6]

Description and identification

Bombus pensylvanicus is a widespread species, characterized as long-tongued. In comparison to its similar species B. sonorus, B. pensylvanicus has a darker color pattern and is located in the eastern United States.^[6] Characteristics of B. pensylvanicus include: a yellow thoracic dorsum, a black posterior, 3 initial alternating black and white tergal segments, a long and lanky malar space,^[3] and short hair.^[6] These characteristics resemble those of B. fervidus and B. auricomus, leading to confusion among species.^[3]B. pensylvanicus is similar in color and range to Bombus fervidus .^[10] In terms of characteristics within the hive, a larger queen measures 22–26 mm compared with a worker at 13–19 mm. Males have an outward penis valve head with a broad banana shape^[6] and often have extensive yellow on the thoracic dorsum posteriorly.^[3]

Whilst bees normally forage close to their nests, it has been observed that individuals can return from up to 1.5 miles away, though there is likely individual variation in homing ability and the time it takes a bee to find its way back to the nest. The method utilized by the bees is most likely trial and error, not a sixth sense or homing instinct, because the time taken to return to the nest varies.^[11] Males become more common in late summer.

Intraspecific Variation

S.A. Cameron observed that bees of the genus Bombus tend to have a comparable morphology throughout their range, thus signifying that behavioral adaptations may play a large part in colonizing different habitats.^[8] Mimetic evolution is postulated to account for both interspecific and intraspecific variation in color pattern. Eastern North American B. pensylvanicus and western North American B. sonorus are taxa that have a similar morphology but distinct color patterns and different male genitalia. In areas where the two taxa overlap there is genetic introgression between B. sonorus and B. pensylvanicus, suggesting that the two taxa may actually be conspecific (as they produce fertile offspring) and best considered as subspecies.^[8]

Distribution and habitat

Bombus pensylvanicus ranges from the Eastern Great Plains to eastern and central US and southern Canada, and Mexico.^[6] The species has become rarer, declining in number mainly in northern parts of its range.^[3]B. pensylvanicus generally nests in fields of long grass, but may sometimes nest underground. The species utilizes bundles of hay or long grass to create sheltered nests above ground. Some nest in established crevices and burrows, such as old bird nests, rodent burrows and cinder blocks.^[3] This species has even been noted to nest in human-made objects such as buckets and barns.^[11]

Colony cycle

Bombus pensylvanicus maintains a reproductive cycle similar to other species of bumblebee.^[12] Environmental factors as well as accessibility of resources affects the cyclic advancement of the colony. The cycle begins in February and ends around November or December.^[13] Female bees can yield eggs without the need for mating, a process known as haplodiploidy. Unfertilized eggs develop into males, whereas fertilized eggs develop into female workers or queen bees.^[12] The reproductive cycle begins in July/August, when a male mates with a freshly hatched queen. The fertilized queen stays in hibernation until spring of the next year, waiting for the optimal conditions to search for a nest. In March, the queen bee gathers pollen and nectar, as a source of nutrition and to build a wax pot, and establishes her colony.^[12] These colonies most likely arrange and initiate in February.^[13] Female workers develop through the pollen collected as it stimulates the ovaries to create eggs, which are fertilized from the males of year before. The queen continues to warm the eggs initially, then the eggs continue their life cycle of development: first larval stage, then pupae, and lastly adult female workers. The female workers care for the nest and eggs, whereas the queen lays eggs. This process of egg to adult bee takes about 4–5 weeks.^[12] Initial workers forage and increase the colony size by bringing resources for growth. Thus, workers that hatch later in the year, around midsummer, tend to be larger than initial worker bees.^[12] The hives continue to grow,^[13] and in late summer there can be more than 200 worker bees. At the point when the size of the hive is sufficient, the queen stops production of a chemical that prevents ovarian production of certain females leading to specify the production of queen bee eggs and male eggs.^[12] Certain female workers may produce their own eggs, but the queen will usually get rid of them. The constant battle between the queen and workers continue until late summer when the workers sting the queen to death. The cycle begins again in the winter as the queen bee eggs hatch and the worker bees die.^[12]

Colony Hierarchy

B. pensylvanicus has a varying stability within its colony cycle. Queens are considered the dominant caste because they are usually the largest bees of the colony. As worker male bees grow larger in average wing length, they become the dominant caste as the number of queens decrease. Wing length of males vary depending upon the point of colony development. Initial males that found the colony tend to have a smaller wing length than the first or second generation of the colony. Food supply is scarce, at first, as the queen is the forager. Until July, the workers are the foraging caste and an enormous increase in body size is observed. The worker proportions decrease when reproductive males develop, representing a turning point in the colony, as male size increase until active bees develop to the size of a queen near November and December when the worker population dissipates. Queens maintain the least amount of standard deviation for average wing length and thus are the most stable caste in the colony. It is postulated that this is because queens are made in a short time span when colony resources have reached its threshold. The activity of B. pensylvanicus in a subtropical zone resemble that of species in temperate zones, maintaining periods where there are no active bees.^[13]

Interaction with other species

Predators

Although Bombus pensylvanicus maintains aposematic coloration and defensive stinger, it faces many predators. Predation is likely to be caused by attack to gain the resources of the hive, which contains carbohydrate and protein abundant nectar, larvae, and pollen. Predators consist of mostly mammals such as skunks, bears, and raccoons. Furthermore, bumble bees are predated on by birds for food. Foragers are frequently predated by invertebrates. Crab spiders and cryptically colored ambush bugs ambush bees at flowers to catch them. Robber flies resemble bumble bees and clasp the bumble bees, insert them with enzymes, then eat their internal organs. Mallophora bomboides is a robber fly species that preys specifically on B. pensylvanicus and uses it as a model for Batesian mimicry.^[14] Wasps, such as the beewolf species Philanthus bicinctus , intercept bees then paralyze them with venom, using them to nourish the wasp’s larvae. Assassin bugs and dragonflies are also common predators of the bee.^[6]

Parasites

Bumble bees are generally host to a diversity of parasitoids in which the larvae grow inside the living host.^[6] The majority of parasitoids for bumble bees are flies and about 30 percent or more bees within the area can be infected. The process of parasitism consists of the fly attaching to the bee in flight and inserting her oviposits between the terga of the bee. The larval fly hatches within the bee host and develops by feeding on the host’s tissues. The bee lives for about two weeks before dying. The fly then pupates and spends the winter inside the bee, fully developed, before it emerges the following year. Bombus pensylvanicus is host to one "cuckoo" bumble bee species, B. variabilis.^[7] Hibernating queen bumble bees are parasitized by a nematode worm, Sphaerularia bombi . This parasite does not reduce life span, but instead causes the sterilization of the queen. It has been observed that affected queens forage two to three weeks later than those that are unaffected.^[6] Parasitic microorganisms also use bees as their host. Parasitic microorganisms’ effects may be lethal or sublethal. Pathogens may be transmitted within a colony or the bee may be infected at flowers. Tracheal mites ( Locustacarus buchneri ) leads to reduced foraging efficiency by living in the bee’s alveoli. Certain protozoans and fungi consume the host tissue or gut substances of the bumble bee’s digestive tract, decreasing foraging efficiency, life span, and thus the colony fitness. Bees may contain symbiotic bacteria that offer some immunity to pathogens. Further exposure to habitat loss as well as pesticide exposure may lead to bee predisposition, thus promoting the species’ decay.^[6]

Mimicry

Since bumble bees are characterized by a striking color pattern as well as a defensive sting, they are involved in mimetic complexes (both Müllerian mimicry and Batesian mimicry) with other insects that also gain reduced predation.^[6]Bombus pensylvanicus is mimicked by various sawflies, day-flying moths (e.g. Hemaris diffinis ), beetles, flies, and other bees, such as carpenter and digger bees.

Behavior

Pollinator preference

Wesselingh and Arnold (2000) studied pollinator preferences on Iris fulva (red-flowered) and Iris brevicaulis (blue-flowered). B. pensylvanicus preferred purple-flowered hybrids. The bees continued to visit the nearest flowers the majority of the time, demonstrating that movements were usually between a diversity of flower types rather than prioritizing only one type of flower. Thus a lack of intermediate genotypes of iris hybrids is not due to pollinator preference by B. pensylvanicus but rather pollinating behavior is done through mixed mating of alternating flower types of different pollination syndromes.^[15]

Resource partitioning

Johnson tested for intraspecific size resource utilization differences in B. pensylvanicus. In Minnesota, flowers with short corollas and long corollas existed in single and mixed species stands. Foragers with short corollas and shorter proboscises (tongue) were discovered in mixed species stands. Johnson concluded that B. pensylvanicus foragers would preference the corolla length that corresponds with their proboscis length. Further comparison of conspecific foragers of mixed versus single species stands revealed a shorter proboscis length for mixed species in comparison to single species stand for the short corolla. This study postulated that a diversity of flowering species may influence the specific bee that pollinates the species for single species stand.^[16]

Gene flow and decline

Bumblebee species have been found to deteriorate substantially in 1940-1960, and continue to decrease presently. Ranges of Bombus pensylvanicus have specifically decreased in Illinois, coinciding with agricultural investment within the state.^[17] Lozier and Cameron evaluated genetic structure using microsatellite markers in Illinois to compare genetic variation of historical versus contemporary collections within B. pensylvanicus. It was found that B. pensylvanicus had greater population structure, indicating reduced gene flow and dispersal among populations. It was found that genetic diversity has overall not been significantly altered over time, but there were some reductions in B. pensylvanicus. Slight losses of genetic diversity in B. pensylvanicus may be an indication of the species’ decline. These results were expected because of the recent decrease in population, which would cause declines in genetic diversity for severe bottleneck situations. Thus, the alteration in gene flow may suggest potential future genetic differentiation of the B. pensylvanicus.^[17]

Status

Current research states that Bombus pensylvanicus is uncommon and declining quickly.^[5]^[3] As stated in previous sections, the northern range of B. pensylvanicus has significantly decreased. Once the most abundant species throughout the southern United States, B. pensylvanicus is now a rare species that has been extirpated in certain areas and has suffered declines in others.^[4] Conservation efforts are encouraged in order to maintain the species, including in agriculture with wildlife-friendly techniques including hedgerows and pest management.^[17]

Related Research Articles

A bumblebee is any of over 250 species in the genus Bombus, part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related genera are known from fossils. They are found primarily in higher altitudes or latitudes in the Northern Hemisphere, although they are also found in South America, where a few lowland tropical species have been identified. European bumblebees have also been introduced to New Zealand and Tasmania. Female bumblebees can sting repeatedly, but generally ignore humans and other animals.

Bombus terrestris, the buff-tailed bumblebee or large earth bumblebee, is one of the most numerous bumblebee species in Europe. It is one of the main species used in greenhouse pollination, and so can be found in many countries and areas where it is not native, such as Tasmania. Moreover, it is a eusocial insect with an overlap of generations, a division of labour, and cooperative brood care. The queen is monogamous which means she mates with only one male. B. terrestris workers learn flower colours and forage efficiently.

The early bumblebee or early-nesting bumblebee is a small bumblebee with a wide distribution in most of Europe and parts of Asia. It is very commonly found in the UK and emerges to begin its colony cycle as soon as February which is earlier than most other species, hence its common name. There is even some evidence that the early bumblebee may be able to go through two colony cycles in a year. Like other bumblebees, Bombus pratorum lives in colonies with queen and worker castes. Bombus pratorum queens use aggressive behavior rather than pheromones to maintain dominance over the workers.

The tree bumblebee or new garden bumblebee is a species of bumblebee common in the European continent and parts of Asia. Since the start of the twenty-first century, it has spread to Great Britain. These bumblebees prefer habitats that others do not, allowing them to pollinate flowers in areas that many other species do not get to.

Bombus lapidarius is a species of bumblebee in the subgenus Melanobombus. Commonly known as the red-tailed bumblebee, B. lapidarius can be found throughout much of Central Europe. Known for its distinctive black and red body, this social bee is important in pollination.

Bombus polaris is a common Arctic bumblebee species. B. polaris is one of two bumblebees that live above the Arctic Circle. The other is its social parasite Bombus hyperboreus. B. polaris is a social bee that can survive at near freezing temperatures. It has developed multiple adaptations to live in such cold temperatures. B. polaris has a thicker coat of hair than most bees, utilizes thermoregulation, and makes insulated nests.

Bombus hortorum, the garden bumblebee or small garden bumblebee, is a species of bumblebee found in most of Europe north to 70°N, as well as parts of Asia and New Zealand. It is distinguished from most other bumblebees by its long tongue used for feeding on pollen in deep-flowered plants. Accordingly, this bumblebee mainly visits flowers with deep corollae, such as deadnettles, ground ivy, vetches, clovers, comfrey, foxglove, and thistles. They have a good visual memory, which aids them in navigating the territory close to their habitat and seeking out food sources.

Bombus lucorum, the white-tailed bumblebee, is a species of bumblebee, widespread and common throughout Europe. This name has been widely used for a range of nearly identical-looking or cryptic species of bumblebees. In 1983, Scholl and Obrecht even coined the term Bombus lucorum complex to explain the three taxa that cannot be easily differentiated from one another by their appearances. A recent review of all of these species worldwide has helped to clarify its distribution in Europe and northern Asia, almost to the Pacific. B. lucorum reaches the Barents Sea in the North. However, in southern Europe, although found in Greece it is an upland species with its distribution never quite reaching the Mediterranean.

The two-spotted bumble bee is a species of social bumble bee found in the eastern half of the United States and the adjacent south-eastern part of Canada. In older literature this bee is often referred to as Bremus bimaculatus, Bremus being a synonym for Bombus. The bee's common name comes from the two yellow spots on its abdomen. Unlike many of the other species of bee in the genus Bombus,B. bimaculatus is not on the decline, but instead is very stable. They are abundant pollinators that forage at a variety of plants.

Bombus vosnesenskii, the yellow-faced bumblebee, is a species of bumblebee native to the west coast of North America, where it is distributed from British Columbia to Baja California. It is the most abundant species of bee in this range, and can be found in both urban and agricultural areas. Additionally, B. vosnesenskii is utilized as an important pollinator in commercial agriculture, especially for greenhouse tomatoes. Though the species is not currently experiencing population decline, urbanization has affected its nesting densities, and early emergence of the B. vosnesenskii has been implicated in the increasing lack of bee diversity on the West coast.

Bombus occidentalis, the western bumblebee, is one of around 30 bumblebee species present in the western United States and western Canada. A recent review of all of its close relatives worldwide appears to have confirmed its status as a separate species.

Bombus fervidus, the golden northern bumble bee or yellow bumblebee, is a species of bumblebee native to North America. It has a yellow-colored abdomen and thorax. Its range includes the North American continent, excluding much of the southern United States, Alaska, and the northern parts of Canada. It is common in cities and farmland, with populations concentrated in the Northeastern part of the United States. It is similar in color and range to its sibling species, Bombus californicus, though sometimes also confused with the American bumblebee or black and gold bumblebee. It has complex behavioral traits, which includes a coordinated nest defense to ward off predators. B. fervidus is an important pollinator, so recent population decline is a particular concern.

Bombus impatiens, the common eastern bumble bee, is the most commonly encountered bumblebee across much of eastern North America. They can be found in the Eastern temperate forest region of the eastern United States, southern Canada, and the eastern Great Plains. Because of their great adaptability, they can live in country, suburbs, and even urban cities. This adaptability makes them a great pollinator species, leading to an increase in their commercial use by the greenhouse industry. This increase consequently led to their farther spread outside their previous distribution range. They are considered one of the most important species of pollinator bees in North America.

Bombus ruderatus, the large garden bumblebee or ruderal bumblebee, is a species of long-tongued bumblebee found in Europe and in some parts of northern Africa. This species is the largest bumblebee in Britain and it uses its long face and tongue to pollinate hard-to-reach tubed flowers. Bumblebees are key pollinators in many agricultural ecosystems, which has led to B. ruderatus and other bumblebees being commercially bred and introduced into non-native countries, specifically New Zealand and Chile. Since its introduction in Chile, B. ruderatus has spread into Argentina as well. Population numbers have been declining and it has been placed on the Biodiversity Action Plan to help counteract these declines.

Bombus muscorum, commonly known as the large carder bee or moss carder bee, is a species of bumblebee in the family Apidae. The species is found throughout Eurasia in fragmented populations, but is most commonly found in the British Isles. B. muscorum is a eusocial insect. The queen is monandrous, mating with only one male after leaving a mature nest to found its own. Males mate territorially and the species is susceptible to inbreeding and bottlenecks. The species builds its nests on or just under the ground in open grassland and forages very close to the nest. In recent years, populations have significantly declined due to loss of natural habitat. B. muscorum is currently listed as vulnerable in Europe by the European Red List of Bees.

Bombus frigidus, the frigid bumblebee, is a rare species of bumblebee largely found in Canada and parts of the United States.

Bombus dahlbomii, also known as the moscardón, is a species of bumblebee endemic to southern South American temperate forests. B. dahlbomii is one of the largest bee species in the world, with matured queens growing up to 40 mm (1.6 in) long. Because of its size and furry appearance, the species has been described as "flying mice" colloquially, and "a monstrous fluffy ginger beast" by British ecologist David Goulson.

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 Service placed 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.

Bombus terricola, the yellow-banded bumblebee, is a species of bee in the genus Bombus. It is native to southern Canada and the east and midwest of the United States. It possesses complex behavioral traits, such as the ability to adapt to a queenless nest, choose which flower to visit, and regulate its temperature to fly during cold weather. It was at one time a common species, but has declined in numbers since the late 1990s, likely due to urban development and parasite infection. It is a good pollinator of wild flowers and crops such as alfalfa, potatoes, raspberries, and cranberries.

Bombus vancouverensis is a common species of eusocial bumblebee of the subgenus Pyrobombus. B. vancouverensis inhabits mountainous regions of western North America, where it has long been considered as a synonym of Bombus bifarius, and essentially all of the literature on bifarius refers instead to vancouverensis. B. vancouverensis has been identified as one of the two species of bumblebee observed to use pheromones in kin recognition. The other is the frigid bumblebee, Bombus frigidus.

References

1 2 Hatfield, R., et al. 2015. Bombus pensylvanicus. The IUCN Red List of Threatened Species. Downloaded on 09 March 2016.
↑ Bombus pensylvanicus. Integrated Taxonomic Information System (ITIS).
1 2 3 4 5 6 7 8 "Species Bombus pensylvanicus - American Bumble Bee". Bug Guide. 25 July 2015.
1 2 Adams, Abigail (October 16, 2021). "American Bumblebee Could Soon Be Considered an Endangered Species in the United States". Yahoo! Finance. The petition lists seven other states — Connecticut, Massachusetts, Michigan, West Virginia, New York, Pennsylvania, and Delaware — with critically low numbers of American bumblebees.
1 2 Turner, Ben (September 21, 2021). "American bumblebee could be officially declared endangered". livescience.com.
1 2 3 4 5 6 7 8 9 10 11 Williams, Paul H.; Thorp, Robbin W.; Richardson, Leif L.; Colla, Sheila R. (2014). Bumble Bees of North America: An Identification Guide. Princeton Field Guides. Princeton University Press. p. 147. ISBN 978-0-691-15222-6. LCCN 2013945435.
1 2 Colla, S., et al. Bumble Bees of the Eastern United States. US Forest Service and Pollinator Partnership. March, 2011.
1 2 3 4 Cameron, S. A.; Hines, H. M.; Williams, P. H. (2007). "A Comprehensive phylogeny of the bumble bees (Bombus)" (PDF). Biological Journal of the Linnean Society. 91: 161–188. doi: 10.1111/j.1095-8312.2007.00784.x .
↑ NatureServe. 2015. Bombus pensylvanicus. NatureServe Explorer Version 7.1. Accessed 9 March 2016.
↑ "Bombus fervidus". Discover Life.
1 2 Rau, Phil (December 1, 1924). "Notes on Captive Colonies and Homing of Bombus Pennsylvanicus De Geer". Annals of the Entomological Society of America. 17 (4): 368–381. doi:10.1093/aesa/17.4.368. ISSN 0013-8746.
1 2 3 4 5 6 7 "Bombus pensylvanicus: Reproduction". University of Wisconsin La Crosse. 2013.
1 2 3 4 de la Hoz, J. D. (2006). "Phenology of Bombus pennsylvanicus sonorus Say (Hymenoptera: Apidae) in Central Mexico". Neotropical Entomology. 35 (5). doi:10.1590/S1519-566X2006000500004.
↑ Brower, Lincoln P.; Westcott, Peter W. (1960-09-01). "Experimental Studies of Mimicry. 5. The Reactions of Toads (Bufo terrestris) to Bumblebees (Bombus americanorum) and Their Robberfly Mimics (Mallophora bomboides), with a Discussion of Aggressive Mimicry". The American Naturalist. 94 (878): 343–355. doi:10.1086/282137. ISSN 0003-0147. S2CID 83531239.
↑ Wesselingh, R. A.; Arnold, M. L. (2000). "Pollinator behaviour and the evolution of Louisiana iris hybrid zones". Journal of Evolutionary Biology. 13 (2): 171–180. doi: 10.1046/j.1420-9101.2000.00153.x .
↑ Johnson, Robert A. (1986). "Intraspecific resource partitioning in the bumble bees Bombus ternarius and B. pennsylvanicus" (PDF). Ecology. 67 (1). Ecological Society of America: 133–138. doi:10.2307/1938511. JSTOR 1938511.
1 2 3 Lozier, J. D.; Cameron, S. A. (2009). "Comparative genetic analyses of historical and contemporary collections highlight contrasting demographic histories for the bumble bees Bombus pensylvanicus and B. impatiens in Illinois". Molecular Ecology. 18 (9): 1875–1886. doi:10.1111/j.1365-294X.2009.04160.x. PMID 19344350. S2CID 34671129.

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[iucn-1] 1 2 Hatfield, R., et al. 2015. Bombus pensylvanicus. The IUCN Red List of Threatened Species. Downloaded on 09 March 2016.

[itis-2] Bombus pensylvanicus. Integrated Taxonomic Information System (ITIS).

[bugguide-3] 1 2 3 4 5 6 7 8 "Species Bombus pensylvanicus - American Bumble Bee". Bug Guide. 25 July 2015.

[Adams2021-4] 1 2 Adams, Abigail (October 16, 2021). "American Bumblebee Could Soon Be Considered an Endangered Species in the United States". Yahoo! Finance. The petition lists seven other states — Connecticut, Massachusetts, Michigan, West Virginia, New York, Pennsylvania, and Delaware — with critically low numbers of American bumblebees.

[Turner2021-5] 1 2 Turner, Ben (September 21, 2021). "American bumblebee could be officially declared endangered". livescience.com.

[Williams2014-6] 1 2 3 4 5 6 7 8 9 10 11 Williams, Paul H.; Thorp, Robbin W.; Richardson, Leif L.; Colla, Sheila R. (2014). Bumble Bees of North America: An Identification Guide. Princeton Field Guides. Princeton University Press. p. 147. ISBN 978-0-691-15222-6. LCCN 2013945435.

[eastern-7] 1 2 Colla, S., et al. Bumble Bees of the Eastern United States. US Forest Service and Pollinator Partnership. March, 2011.

[Cameron-8] 1 2 3 4 Cameron, S. A.; Hines, H. M.; Williams, P. H. (2007). "A Comprehensive phylogeny of the bumble bees (Bombus)" (PDF). Biological Journal of the Linnean Society. 91: 161–188. doi: 10.1111/j.1095-8312.2007.00784.x .

[9] NatureServe. 2015. Bombus pensylvanicus. NatureServe Explorer Version 7.1. Accessed 9 March 2016.

[10] "Bombus fervidus". Discover Life.

[rau-11] 1 2 Rau, Phil (December 1, 1924). "Notes on Captive Colonies and Homing of Bombus Pennsylvanicus De Geer". Annals of the Entomological Society of America. 17 (4): 368–381. doi:10.1093/aesa/17.4.368. ISSN 0013-8746.

[bioweb-12] 1 2 3 4 5 6 7 "Bombus pensylvanicus: Reproduction". University of Wisconsin La Crosse. 2013.

[juan-13] 1 2 3 4 de la Hoz, J. D. (2006). "Phenology of Bombus pennsylvanicus sonorus Say (Hymenoptera: Apidae) in Central Mexico". Neotropical Entomology. 35 (5). doi:10.1590/S1519-566X2006000500004.

[14] Brower, Lincoln P.; Westcott, Peter W. (1960-09-01). "Experimental Studies of Mimicry. 5. The Reactions of Toads (Bufo terrestris) to Bumblebees (Bombus americanorum) and Their Robberfly Mimics (Mallophora bomboides), with a Discussion of Aggressive Mimicry". The American Naturalist. 94 (878): 343–355. doi:10.1086/282137. ISSN 0003-0147. S2CID 83531239.

[wesselingh-15] Wesselingh, R. A.; Arnold, M. L. (2000). "Pollinator behaviour and the evolution of Louisiana iris hybrid zones". Journal of Evolutionary Biology. 13 (2): 171–180. doi: 10.1046/j.1420-9101.2000.00153.x .

[johnson-16] Johnson, Robert A. (1986). "Intraspecific resource partitioning in the bumble bees Bombus ternarius and B. pennsylvanicus" (PDF). Ecology. 67 (1). Ecological Society of America: 133–138. doi:10.2307/1938511. JSTOR 1938511.

[lozier-17] 1 2 3 Lozier, J. D.; Cameron, S. A. (2009). "Comparative genetic analyses of historical and contemporary collections highlight contrasting demographic histories for the bumble bees Bombus pensylvanicus and B. impatiens in Illinois". Molecular Ecology. 18 (9): 1875–1886. doi:10.1111/j.1365-294X.2009.04160.x. PMID 19344350. S2CID 34671129.

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