Bombus variabilis

Bombus variabilis, the Variable Cuckoo Bumble Bee, is a parasitic species of bumble bee primarily located in North America. Its preferred host species, Bombus pensylvanicus^[1], has dealt with population decline due to habitat loss and climate change. This makes it more difficult for Bombus variabilis to grow in numbers, with less colonies available to raise their young.^[2] The bee possesses a dark face and yellow  vertex with dark brown wings. Its thorax has variations between black spots or patches. Females only have black abdomens but males may vary in their hair color patterns.^[3]

Variable Cuckoo Bee
Conservation status
Critically Endangered  (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Apidae
Genus:	Bombus
Subgenus:	Psithyrus
Species:	B. variabilis
Binomial name
Bombus variabilis Cresson, 1872. (n.d.). Bombus variabilis. ITIS. https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=714847#null

Taxonomy

Bombus variabilis is a species of Bumble Bee most commonly found in North America. It is of the order Hymenoptera, with a family of Apidae and genus Bombus.^[4]

Bombus variabilis was first described by Cresson (1872) and has been classified under the subgenus Psithyrus, along with all other cuckoo bumble bees.^[5]

The cuckoo bumble bees (of subgenus Psithyrus) are a group that do not collect pollen or start their own colonies. Instead, the females take over the nests of other Bombus species and rely on the host workers to raise their young. In North America, they are most often linked with the American bumble bee (Bombus pensylvanicus), which is believed to be their primary host.^[6]

It is believed that B. intrudens, B. variabilis, B. sololensis, B. guatemalensis, and B. mysticus might all belong to the same species. The main differences between them only lie in their color pattern, and there is no strong evidence that they are truly separate species.^[7][8]

Species Descriptions and Adaptations

The Variable Cuckoo Bumble Bee has adapted to require a successful host species for survival. They no longer have the ability to store pollen or rear their own young. This species uses a mated female to infiltrate a different hive of bees and take over the queen bee. The new queen Variable Cuckoo Bumble Bee will then use pheromones or physical attacks to force the worker bees of the existing colony to feed both the offspring and the new queen herself.^[9]

This parasitic relationship has therefore made the survival of Bombus variabilis dependent on the survival of the host species. The preferred host species is the American Bumble Bee (Bombus pensylvanicus).^[10] There have also been observations that the host species include B. sonorus.^[11]

Populations of the American Bumble Bee has been dwindling as well which puts the Variable Cuckoo Bumble Bee at an even greater risk of extinction. The conservation of the host species is greatly connected to conservation of the Variable Cuckoo Bumble Bee.^[10] Conservation through captivity is likely considered unrealistic, so the best way to make an effort for the broad conservation of bumble bees includes protecting known habitats from deforestation, pesticide exposure, and spillover of pathogens.^[9]

Phylogenetics

The Bombus variabilis belongs to the subgenus Psithyrus, but is unique in its own way compared to other species in its subgenus as it developed a different way of life through the effects of natural selection.^[12] Females are reasonably sized, ranging from 18-22mm. Their bodies are covered with short hairs that are colored black on their face. The hairs are also mostly black on the sides of the thorax, contrasting to the B. citrinus. Additionally, on the upper side of the thorax they have largely black hairs, unlike many other cuckoo bees.^[12]

Even though the Bombus variabilis exhibits distinct differences compared to the other members in the subgenus due to natural selection, it also maintains some characteristics that link it to related species. It is considered most similar to the lemon cuckoo bumble bee, named Bombus citrinus. Apart from its developmental differences to adapt to its lifestyle, it still exhibits similar traits to others which shows evolutionary relationships. The Bombus variabilis exhibits traits of host specialization, the act of adapting to a specific host. ^[12]

An analysis done to improve the phylogeny and classification of Bombus indicated that the 209 species examined can be grouped into fifteen subgenera and most of those groups came from the same ancestry and stick together on the phylogenetic tree. The DNA findings also agree with what scientists already believed based on the bees' physical traits. The phylogenetic trees created also prove that the subgenera fall into two clear groups: short-faced and long-faced bumble bees. ^[13]

Habitat and Distribution

Bombus variabilis follow the same distribution pattern as their host species, the American Bumblebee (Bombus pensylvanicus). Both species have historically inhabited: the southern extremes of Canada, the majority of Mexico, and the spread from the east coast of the United States all the way to the non-mountainous/desert regions of western states adjacent to California and Oregon.^[14]

B. variabilis map

Because Bombus variabilis has a parasitic relationship to its host, the steady decline in the population of the American Bumblebee has resulted in the same trend to be observed in the Bombus variabilis.^[15] Before 1949 the host species for Bombus variabilis accounted for 28.1% of the Bumblebee population of Illinois; As of 2007 the American Bumblebee only represents 4.4%.^[16] Should the decline of the population of the American Bumblebee continue, both the species and the species dependent on it might go extinct.

In general, both Bombus variabilis and Bombus pensylvanicus choose to live on the surface of the ground in grassy areas and occasionally choose to live subterraneanly.^[7]