Bombus affinis Cresson, 1863
The Rusty-patched Bumblebee (Bombus affinis) was at one time among the more common and widespread bumblebees in eastern North America. However, this is no longer the case. The first decade of the 21st century saw growing concern about declining pollinator populations in general in several regions of the world, with particular attention focused on bees, and much of the available data on declining bee populations has focused on bumblebees. Although populations of some bumblebee species appear to be robust, many others have apparently gone extinct in recent years or suffered dramatic declines. Bombus affinis is one of several North American bumblebee species that have experienced clear declines. Colla and Packer (2008) documented an impoverishment of the bumblebee community in general in southern Ontario (Canada) between the early 1970s and the first decade of the 21st century and found that B. affinis, in particular, declined dramatically in abundance not only in southern Ontario but throughout its native range. There is evidence of declines in three other North American bumblebees as well (all four belong to the subgenus Bombus): B. franklini and B. occidentalis in the west and B. terricola in the east. Bombus franklini, which had a historically small geographic distribution, is thought to be at the brink of extinction (or possibly extinct). Bombus affinis, B. terricola, and B. occidentalis have much larger historical ranges, but have disappeared from numerous sites where they were previously common. (Colla and Packer 2008 and references therein)
Conservation and Management
The reasons for the sudden decline of Bombus affinis, a previously common species throughout its large range, are unknown. It has been suggested that, along with other vulnerable North American species in the subgenus Bombus (B. occidentalis, B. terricola, and the possibly extinct B. franklini), the species has suffered from introduced diseases transmitted from managed bumblebee colonies used for greenhouse pollination. In addition, habitat loss and the widespread use of certain pesticides likely represent significant threats. (Evans et al. 2008; Colla 2010)
A variety of circumstantial evidence supports the hypothesis that at least some recent bumblebee declines in North America have been driven or exacerbated by the spread of Nosema bombi, an obligate intracellular microsporidian fungal parasite found commonly in bumblebees throughout Europe, via commercial bumblebee rearing facilities that introduced this pathogen from Europe. Pathogenic effects of N. bombi may vary depending on the host species and reproductive caste, but they may include reductions in both colony growth and individual life span and fitness. Further research will be necessary to clarify the role of pathogens in bumblebee declines. (Cameron et al. 2011 and references therein) Based on a combination of modeling, laboratory experiments, and literature review, Otterstatter and Thomson (2008) found strong support for the hypothesis that spillover of Crithidia bombi (a destructive internal trypansomatid protozoan parasite) from bumblebees reared commercially to pollinate greenhouse crops has contributed to the ongoing decline of wild Bombus in North America. Wild bumblebees may also be negatively impacted by the spread of the bumble bee tracheal mite Locustacarus buchneri to wild populations from commercially reared colonies (Evans et al. 2005 and references therein).
In the 1970s, Bombus affinis was among the more common and widespread bumblebee species in eastern North America. Dramatic declines were noted by the mid-1990s in both Canada and the United States. In Canada, extensive targeted searches from 2005 to 2009 detected just three individuals (one in 2005 and two in 2009). Similar population crashes have been observed in the U.S. (Colla 2010 and references therein)
According to an analysis by Cameron et al. (2011), the relative abundances of four North American bumblebee species have declined by up to 96% and their geographic ranges have contracted by an estimated 23% to 87%, some within a span of two decades. In the recent large-scale bumblebee survey carried out by Cameron et al., which included the capture and identification of nearly 17,000 individuals (most of which were released), only 22 B. affinis individuals were found, and although this species was once found throughout the eastern United States and northern Midwest, individuals were detected at just three locations in Illinois and one in Indiana (yielding an estimated range reduction of 87%).
Grixti et al. (2009) found that bumblebee species richness in Illinois (U.S.A.) declined substantially during the middle of the century (1940 to 1960). Four species were locally extirpated: B. borealis, B., B. terricola and B. variabilis. The ranges of B. affinis, B. fraternus, B.pensylvanicus and B. vagans have also decreased dramatically in Illinois. The major decline in the Illinois bumblebee fauna coincided with large-scale agricultural intensification in Illinois, suggesting one likely factor driving bumblebee declines.
Because of the particular haplodiploid mode of sex determination characteristic of bees, which normally yields diploid females and haploid males but results in the production of non-viable diploid males when allelic diversity is low (and hence homozygosity is high), effective population size relative to census size is much reduced and bee populations in general may therefore be especially vulnerable to extinction as population size shrinks (Zayed and Packer 2005).
Bombus affinis queens and workers differ slightly in coloration (an uncommon feature in bumble bees). Other than size, the primary difference between queens and workers is the presence of a medial rusty patch on the second abdominal segment of the worker. The hairs of workers are entirely black on the head, the bottom of the thorax, and in large part on the legs. The rest of the thorax has mostly yellow hair, with a black area in the middle of the thorax. Hairs are entirely yellow on the first two abdominal segments and black on the rest of the abdomen. On workers, there is more black intermixed with yellow near the base of the wings, forming something of a band between the wings, and black hairs extend posteriorly in a narrow "V" that partially bisects the yellow on the scutellum. The second abdominal segment has a rusty reddish patch centrally, with yellow hairs around the edges of the segment. Bombus affinis males have hairs largely black on the head, but with a few pale hairs intermixed near the top of the head. Black hairs sometimes form an obscure band across the middle of the thorax, but the hair on the thorax is otherwise largely pale yellowish. The first two abdominal segments have pale yellow hair. The hair on the rest of the abdominal segments is black. (Evans et al. 2008)
Bombus affinis has been collected in a wide variety of habitats including mixed farmland, sand dunes, marshes, and both urban and wooded areas (Colla 2010).
The flight season of Bombus affinis is longer than that of most other North American bumblebees and it visits numerous plant genera in diverse habitats (Colla 2010).
Bombus affinis have relatively short tongues. On average, the tongues of workers are around 5 to 7 mm in length (some other bumblebee species have tongues as long as 10 mm). Their short tongues make them unable to access the nectar in flowers with deep tubes, although they sometimes use their mandibles to chew holes in the bottom of these flowers to access the nectar from the outside of the flower, thus cheating the flower of pollination. (Evans et al. 2008 and references therein)
The historical range of Bombus affinis extends from southern Ontario and southwestern Quebec (but apparently not New Brunswick, contrary to apparently erroneous literature references) in Canada south to Georgia and west to the Dakotas in the United States, with occurrences in the southern portion of the range limited mainly to higher elevations (Thorp and Shepherd 2005; Colla 2010).
Bombus affinis typically nests underground in abandoned rodent burrows located from six to eighteen inches below the surface. Occasionally nests are constructed on the surface in areas such as clumps of grass on the ground. Thus, nesting sites may be limited by the abundance of rodents and the presence of undisturbed grassland. (Evans et al. 2008 and references therein).
Brood cells and honey pots are made of wax produced by the queen and workers. Like other bumblebees, B. affinis have an annual life cycle (i.e., 1 year = 1 generation). Mated queens emerge from diapause in the spring to begin feeding and searching for potential nest sites to initiate new colonies. The queen collects nectar and pollen from flowers to support the production of her eggs (which are fertilized by sperm she has stored since mating the previous fall) and produces a brood of workers. In the early stages of colony development, the queen is responsible for all food collection and care of the young. As the colony grows, workers take over the duties of food collection, colony defense, and care of the young. The queen then remains within the nest and spends most of her time laying eggs. As the summer progresses, the colony reaches maximum worker production and begins producing males and potential queens (queen production is dependent on access to sufficient quantities of pollen). These reproductive individuals leave the colony and mate. After mating, young queens enter diapause and overwinter. The males and workers decline as fall approaches until they die in the winter. (Evans et al. 2008; Colla 2010).
The largest B. affinis colony on record produced 2,100 individuals in captivity (MacFarlane 1974, cited in Colla 2010), but in the wild colonies are much smaller (Colla 2010). More typically, B. affinis colonies consist of a queen and between 50 and 400 workers at their peak (Evans et al. 2008).
Bombus affinis is a "pollen-storer", meaning the larvae live in cells and are fed individually by adults opening the brood clump as the larvae develop. Pollen-storing adults emerge relatively equal in size compared to "pocket-making" bumble bee species, which tend to produce workers that vary greatly in size due to unequal food distribution within the brood clumps during development. (Colla 2010)
Macfarlane (1974, cited in Colla and Packer 2008) observed B. affinis visiting at least 65 plant genera. These bees have been observed biting holes (i.e. nectar-robbing) in flowers with long corolla tubes such as Jewelweed (Impatiens capensis), Yellow Toadflax (Linaria vulgaris) (R. Gegear pers. comm., cited in Colla and Packer 2008), and Cow Vetch (Vicia cracca) (Harder 1983, cited in Colla and Packer 2008).
Bombus affinis is one of two host species for the socially parasitic Bombus (Psithyrus) ashtoni (the other host being the also declining B. terricola) (Fisher 1983). In 2010, Colla reported that B. ashtoni had not been observed in a decade. Bombus species in the subgenus Psithyrus lack pollen-collecting corbiculae and rely on the host workers for the rearing of reproductives. Sladen (1912, cited in Fisher 1983a) reported that the host queen was always killed or displaced by the invading Psithyrus, while Plath (1934, cited in Fisher 1983a) found that in the case of the two North American Psithyrus species he studied (B. ashtoni and B. citrinus) the queen was seldom killed. Subsequent investigations showed that B. ashtoni never kills queens of either of its host species, B. affinis and B. terricola (Fisher 1983a). Fisher found that B. ashtoni is incapable itself of suppressing ovarian development in queenless workers of B. affinis. An alternative strategy to physiological suppression of oogenesis is behavioral domination. Fisher reported that B. ashtoni females were often seen mauling workers, i.e., grasping and pulling them underneath the body as if to sting, but not actually doing so. This behavior only occurred in colonies which had no queen or had a queen that had lost her dominance (ovarian development of workers is normally supressed by the queen). Physical dominance by B. ashtoni females would not prevent oogenesis, but could eliminate or minimize egg laying by fecund workers. Fisher presented data suggesting that females of B. ashtoni benefit by the supression of host worker reproduction by the host queen. At the same time, B. ashtoni females prevent Bombus affinis males and queens from developing by selectively eating eggs and ejecting larvae. The non-aggressive invasion strategy used by B. ashtoni is quite distinct from that of other Psithyrus species, such as B. citrinus, which according to Fisher always kills or displaces its B. impatiens host queen. These other species presumably possess themselves the physiological or behavioral means of eliminating or reducing the frequency of egg laying by host workers in the nests they invade and thus do not need to rely on the host queen to control the workers. (Fisher 1983a) Fisher (1983b) investigated host nest finding by B. ashtoni. These bees search for nests within one to two weeks of host queen emergence at a time prior to emergence of the first worker brood and therefore cannot use odor trails of workers to recognize nests. In laboratory experiments, Fisher showed that B. ashtoni females can recognize host nest odor without actual contact with the nest or with worker-laid trails, successfully distinguishing nests of B. affinis and B. terricola from those of B. bimaculatus and from controls consisting only nest material.
Martin et al. (2010) investigated the cuticular hydrocarbon cues of 14 European Bombus species, including 5 socially parasitic species ("cuckoo bees" in the subgenus Psithyrus) (B. affinis, B. ashtoni, or any other North American species, were not among the species studied, but the general conclusions of the analysis by Martin et al. likely apply more broadly). They found that found that bumblebees possess species-specific alkene positional isomer profiles that are stable over large geographic regions and are mimicked by three host-specific Psithyrus parasites. In three host-cuckoo associations where mimicry is poor, possibly as a result of recent host shifts, these cuckoos produce dodecyl acetate a known chemical repellent that allows the cuckoos to invade their host colonies. Thus, various Psithyrus species may use both mimicry and repellents to invade host colonies.
Microscopic endoparasites recorded infecting B. affinis include Sphaeruluria bombi (a nematode infecting 10% of overwintered queens) and the apicomplexan protozoan Apicystis bombi (Neogregarinida: Ophrocystidae) (Macfarlane et al. 1995, cited in Colla 2010). Other parasites that are known to infect sympatric Bombus species are Nosema bombi (Microsporidia: Nosematidae) and the trypanosome protozoan Crithidia bombi (Kinetoplastea: Trypanosomatidae), both of which may be acquired at flowers via fecal transmission (Colla et al. 2006). Nosema bombi has recently been found infecting B. affinis (Cameron et al. 2011), but infection of B. affinis by C. bombi has apparently not yet been documented (possibly because of the relative recency of the presumed introduction of this parasite from Europe in combinatuon with the rarity of B. affinis in recent years) (Colla 2010). Several species of parasitoid conopid flies (Diptera: Conopidae) attack foraging bumblebees on the wing and lay their eggs inside the bee’s abdomen. (Gillespie 2010 and references therein)
Conopids and C. bombi can affect colony reproduction and worker foraging behavior. Nosema bombi may reduce colony fitness and worker survival. These parasites could affect local abundance of bumblebee populations and C. bombi and N. bombi have been tentatively implicated in the overall decline of bumblebees. In a study in Massachusetts, Gillespie (2010) found a high level of parasitism of bumblebees by C. bombi, N. bombi, and conopid flies (although no B. affinis were encountered in this study). (Gillespie 2010 and references therein)
Bombus affinis has been shown to be an excellent pollinator of cranberry and also to pollinate other important crops such as plum, apple, alfalfa, and onion for seed production. Evans et al. provide a long list of wild plants known to be visited by B. affinis. (Evans et al. 2008 and references therein).
Evolution and Systematics
Systematics and Taxonomy
Bombus affinis was first described by Cresson in 1863. Although the taxonomy of some bumble bee species is controversial, the status of B. affinis as a distinct, valid species is not (Cameron et al. 2007).