Bombus pensylvanicus

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).[4] Within Bombus, B. pensylvanicus belongs to the subgenus Thoracobombus, which includes species such as Bombus armeniacus, Bombus atratus, 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 and B. auricomus,[5] but its closest relative is Bombus sonorus, found in Mexico and Arizona.[6] Scientists sometimes treat Bombus sonorus as a subspecies of Bombus pensylvanicus, although each species maintain differences in male genitalia.[7] Intermediate individuals of B. pensylvanicus and B. sonorus have been found in areas of geographic overlap,[6] but further evidence is needed to distinguish whether B. sonorus is a subspecies of B. pensylvanicus.[3]

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.[3] 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,[4] and short hair.[3] These characteristics resemble those of B. fervidus and B. auricomus, leading to confusion between species.[4] B. pensylvanicus is similar in colour and range to Bombus fervidus.[8] In terms of characteristics within the hive, a larger queen measures 22–26 mm compared to worker at 13–19 mm. Males have an outward penis valve head with a broad banana shape,[3] and often have extensive yellow on the thoracic dorsum posteriorly.[4]

While 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 method, not a sixth sense or homing instinct because the bees maintained varying variation in time taken to return to the nest.[9] Males become more common in late summer.

Bombus pensylvanicus ranges from the Eastern Great Plains to eastern and central US and southern Canada, and Mexico.[3] The species has become rarer, declining in number mainly in northern parts of its range.[4] 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, or in cinder blocks.[4] This species has even been noted to nest in human made objects, like buckets or barns.[9]

Bombus pensylvanicus maintains a reproductive cycle that it similar to other species of bumblebee.[10] 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.[11] 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.[10] 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.[10] These colonies most likely arrange and initiate in February.[11] 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.[10] 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.[10] The hives continue to grow,[11] 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.[10] 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.[10]

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.[15] 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.[15]

Current research states that Bombus pensylvanicus is uncommon and most likely declining. 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. Conservation efforts are encouraged in order to maintain the species including agriculture with wildlife-friendly techniques including hedgerows and pest management.[15]