Bombus variabilis 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Hymenoptera Apidae

Scientific Name: Bombus variabilis Cresson, 1872
Common Name(s):
English Variable Cuckoo Bumblebee
Apathus intrudens Smith, 1861
Bombus guatemalensis Cockerell, 1912
Bombus intrudens (Smith, 1861)
Bombus mysticus Frison, 1925
Bombus sololensis Franklin, 1913
Taxonomic Source(s): Williams, P.H. 1998. An annotated checklist of bumble bees with an analysis of patterns of description (Hymenoptera: Apidae, Bombini). Bulletin of the British Museum (Natural History) 67: 79-152.
Taxonomic Notes: Williams (1998) and Williams et al. (2014) consider that Bombus intrudensB. variabilis, B. sololensis, B. guatemalensis, and B. mysticus might all be parts of one species based on morphological evidence, noting the lack of any documented reason (other than differences in colour pattern) why these five taxa should not be considered conspecific. Further study is hampered by a lack of fresh material of North American B. variabilis for analysis of genetic relationships. Although B. intrudens is the oldest available name for this species, the name B. variabilis has been in common use for the species since 1950, and it could be argued that the latter name be used in the interest of stability (Williams 1998).

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A2bc ver 3.1
Year Published: 2016
Date Assessed: 2014-08-19
Assessor(s): Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S.
Reviewer(s): Ascher, J., Cannings, S., Inouye, D., Jha, S., Lozier, J., Vandame, R.V., Williams, P., Woodard, H. & Yanega, D.
Contributor(s): Antweiler, G., Arduser, M., Ascher, J., Bartomeus, N., Beauchemin, A., Beckham, J., Cromartie, J., Day, L., Droege, S., Evans, E., Fiscus, D., Fraser, D., Gadallah, Z., Gall, L., Gardner, J., Gill, D., Golick, D., Heinrich, B., Hinds, P., Hines, H., Irwin, R., Jean, R., Klymko, J., Koch, J., MacPhail, V., Martineau, R., Martins, K., Matteson, K., McFarland, K., Milam, J., Moisan-DeSerres, J., Morrison, F., Ogden, J., Packer, L., Pineda Diez de Bonilla, E.P., Richardson, L., Savard, M., Scott, V., Scully, C., Sheffield, C., Sikes, D., Strange, J., Surrette, S., Thomas, C, Thompson, J., Vandame, R.V., Veit, M., Wetherill, K., Williams, N., Williams, P., Winfree, R., Yanega, D. & Zahendra, S.
Facilitator/Compiler(s): Foltz Jordan, S., Hatfield, R., Colla, S. & MacPhail, V.
This species is considered one of the rarest of all North American bumble bee species, having been collected only a few times in the past twenty years, and not at all in the last decade (Williams et al. 2014 and supporting database). The decade by decade relative abundance trend has been consistently downward since the 1920s, reaching zero in the recent decade (see Figure 2 in the Supporting Information, attached) (Hatfield et al. 2014). North of Mexico, this species has exhibited 100% decline in relative abundance, extent of occurrence (EOO), and persistence between the recent (2002-2012) and historic (1805-2001) time periods, pointing toward a Critically Endangered category for the species in this region. In Mexico, there are very few known records of this species in the centre and north of the country, but it looks more abundant in the south (Chiapas) and in Guatemala (Rémy Vandame, pers. comm. 2014). Further sampling is needed to clarify the status of this species. Based on the above calculations and trends, along with published reports of bumble bee decline and the assessors' best professional judgement, we recommend this species for the Critically Endangered category at this time.
For further information about this species, see 21215168__Bombus_variabilis.pdf.
A PDF viewer such as Adobe Reader is required.

Geographic Range [top]

Range Description:In North America, this species is found in the Eastern Temperate Forest and Great Plains region of the midwestern U.S., with scattered occurrences on the southeastern coastal plain, southern Texas, and southern Arizona (Williams et al. 2014).

Bombus intrudens also occurs in Central America, including Mexico and Guatemala (Williams et al. 2014). The ECOSUR database (2015) includes 133 recent (2009-2013) records of this species, mainly from Guatemala and southeast Mexico (Chiapas state), and few records from other Mexican states (Guerrero, Jalisco, Oaxaca and Veracruz). In addition, there are 21 Mexican records of this species from a number of collections with unavailable record information (Vandame, R. 2014, pers. comm).
For further information about this species, see 21215168__Bombus_variabilis.pdf.
A PDF viewer such as Adobe Reader is required.
Countries occurrence:
Guatemala; Mexico (Chiapas, Guerrero, Jalisco, Oaxaca, Veracruz)
Possibly extinct:
Canada (Québec); United States (Arizona, Arkansas, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Maryland, Mississippi, Missouri, Nebraska, New Hampshire, New York, North Carolina, North Dakota, Ohio, Oklahoma, South Carolina, Tennessee, Texas, Wisconsin)
Additional data:
Continuing decline in extent of occurrence (EOO):Yes
Range Map:Click here to open the map viewer and explore range.

Population [top]


Several North American studies have found decline in the relative abundance and distribution of this species over time. For example, in a study of 56 sites across Illinois, comparisons between recent (2007) surveys and historic records (beginning in 1900) found that although B. variabilis (=intrudens) historically occurred in all geographic regions of the state, it was completely absent in recent surveys and museum collections (2000-2007) (Grixti et al. 2009). This species was last recorded in Illinois in 1980, and is now considered extirpated from the state (Grixti et al. 2009). A larger study considering over 69,000 bumblebee specimens collected from 1865-2010 from various institutions assessed changes in relative abundance and occupancy of 21 North American species and ranked their conservation status throughout their entire U.S. and Canadian ranges (Colla et al. 2012). Although this study considered a larger scale than previous studies, the species found to be declining range-wide were generally consistent with those found to be declining in regional studies. Of all North American bumblebees examined, B. variabilis (=intrudens) was found to exhibit the most decline, showing severe decline in both persistence and relative abundance. This species completely absent from all samples in the most recent time period (1991–2009), despite 94 historical records (Colla et al. 2012).

We evaluated this species’ spatial distribution over time across its North American range using a measure of change in the extent of occurrence (EOO) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014). We also assessed changes in the species’ relative abundance, which we consider to be an index of abundance relevant to the taxon, as specified by the IUCN Red List Categories and Criteria (IUCN 2012). For all three calculations we divided the database into historical (1805-2001, N=128,572) and current (2002-2012, N=74,682) records. This time frame was chosen to meet the IUCN criteria stipulation that species decline must have been observed over the longer of three generations or 10 years. Average decline for this species was calculated by averaging the change in abundance, persistence, and EOO. We used these measures of change between the recent and historic time periods to estimate the population trend that has occurred in the past 10 years, and to inform the application of an IUCN category. This analysis yielded the following results (see also the Supporting Information for Figure 1 - map of change in EOO over time; and Figure 2 - relative abundance trend):
  • Current range size relative to historic range: 42.91%   
  • Persistence in current range relative to historic occupancy: 0.00%*    
  • Current relative abundance relative to historic values: 0.00%*
  • Average decline: 79.71%*
There are no records of this species in North America in the last decade. Our analysis suggests that this species has declined in relative abundance consistently since the 1920s, making it one of the rarest in its range, and possibly extinct (Hatfield et al. 2014) throughout its range north of Mexico. The range size calculation reflects the difference in sampling effort between the historical, and the current sampling period, and recognizes that the lack of recent records may be a artefact of sampling.

*The persistence and relative abundance measures do not reflect the population in Mexico and Central America. However, since the majority of this species' range occurs north of Mexico, and the range loss estimate is very conservative (before correcting for sampling bias, the range loss for this species is greater than 99%), we are still confident that this species meets the quantitative threshold for a Critically Endangered status.
For further information about this species, see 21215168__Bombus_variabilis.pdf.
A PDF viewer such as Adobe Reader is required.
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes

Habitat and Ecology [top]

Habitat and Ecology:Bombus intrudens is a cuckoo bee, a term used for a specialized lineage of bumble bees (subgenus Psithyrus) that has lost the ability to collect pollen and to rear their brood. As such, these bees do not found their own nests, but instead, usurp the colonies and worker forces of other bumble bee species. To do this, a mated female enters the nest of another bumble bee species, kills or subdues the queen of that colony, and forcibly (using pheromones and/or physical attacks) "enslaves" the workers of that colony to feed her and her developing young. Since all of the resulting cuckoo bee offspring are reproductive (not workers), they leave the colony to mate, and the mated females seek out other nests to attack. Males of this species patrol circuits in search of mates. Before finding and invading a host colony, females feed directly from flowers. The adult food plants of this species include Aster, Cirsium, Echinacea, Helianthus and Solidago (Williams et al. 2014).

Cuckoo bees often attack a broad range of host species, but some specialize in attacking the members of just one species or subgenus. Bombus variabilis (=intrudens) is recorded as breeding as a parasite of colonies of Bombus pensylvanicus (Williams et al. 2014): females usurp established colonies and lay eggs which the workers of the host colony then rear to adulthood. Its host species is associated with open fields and grasslands (Colla and Dumesh 2010).

Generation Length (years):1
Movement patterns:Not a Migrant

Use and Trade [top]

Use and Trade: This species is not used.

Threats [top]

Major Threat(s): Generally, bumble bees (Bombus spp.) are threatened by a number of factors including habitat loss, pesticide use, pathogens from managed pollinators, competition with non-native bees, and climate change (reviewed in Goulson 2010, Williams et al. 2009, Williams and Osborne 2009, Fürst et al. 2014, Cameron et al. 2011, Hatfield et al. 2012). Reduced genetic diversity resulting from any of these threats can be particularly concerning for bumble bees, since their method of sex-determination can be disrupted by inbreeding, and since genetic diversity already tends to be low in this group due to their colonial life cycle (i.e., large numbers of bumble bees found locally may represent only one or a few queens) (Packer and Owen 2001, Zayed and Packer 2005, Goulson 2010, Hatfield et al. 2012, but see Cameron et al. 2011 and Lozier et al. 2011).

As this bee is a parasitic species, it is dependent on its host species throughout its large range in the United States. The main threat to this species is the significant widespread decline of the only known host species, Bombus pensylvanicus (Cameron et al. 2011), including the colour pattern known as B. sonorus. This Psithyrus species uses B. pensylvanicus as a host; it historically occurred in the northern part of B. pensylvanicus' range, which is where B. pensylvanicus has declined most severely. This is likely linked to the decline of suitable grassland habitat for the host species (Grixti et al. 2009). However, the lack of recent B. variabilis (= intrudens) records despite the presence of its host species indicates other threats may be involved. Alternatively, perhaps host abundances have dropped below a threshold required to sustain healthy populations of this species in the United States (Natureserve 2013). Regardless, B. varians (=intrudens) is likely vulnerable to environmental stressors such as climate change, habitat loss and pesticide exposure, however the extent of impact of these stressors is unknown, given the sparsity of recent information for this species.

Natural wildfires and prescribed burning may benefit bees by creating open forage in otherwise unsuitable habitat. As such, the suppression of natural fires can result in habitat loss for bees and other grassland species, particularly in forested regions. In light of this, prescribed burning is frequently used as a conservation management tool to restore natural ecosystems (e.g., grasslands), increase biodiversity (particularly plant species), and control invasive species (e.g., Brockway et al. 2002, Hatch et al. 2002). However, depending on fire intensity, duration, season, frequency, and patchiness, prescribed fire may result in population loss for pollinators, particularly at sites where few individuals of a species exist (e.g., Swengel 1996). As such, both fire suppression and fire itself may threaten this species in some areas.

Conservation Actions [top]

Conservation Actions: Actions should focus on conservation management of host species in North America as captive rearing of this species is likely not an option. Due to the lack of recent specimens across this species' large range, targeted surveys to locate extant populations are priority for this species. If located, any extant populations should be protected from habitat loss, pesticide exposure and pathogen spillover.

Specific conservation and research needs for this species have not been identified. Research needs for North American bumble bees (as a whole) are summarized in Cameron et al. (2011), the final report for the 2010 North American Bumble Bee Species Conservation Planning Workshop.

Citation: Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S. 2016. Bombus variabilis. The IUCN Red List of Threatened Species 2016: e.T21215168A21215249. . Downloaded on 16 August 2018.
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