Bombus crotchii 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Hymenoptera Apidae

Scientific Name: Bombus crotchii Cresson, 1878
Common Name(s):
English Crotch bumble bee
Taxonomic Source(s): Cresson, E.T. 1878. Descriptions of new species of North American bees. Proceedings of the Academy of natural Sciences of Philadelphia 1878: 181-221.

Assessment Information [top]

Red List Category & Criteria: Endangered A2bc ver 3.1
Year Published: 2015
Date Assessed: 2014-08-19
Assessor(s): Hatfield, R., Jepsen, S., Thorp, R., Richardson, L. & Colla, S.
Reviewer(s): Ascher, J., Jha, S., Williams, P., Lozier, J., Cannings, S., Inouye, D., Yanega, D., Woodard, H., Strange, J., Griswold, T., Sagot, P., Vandame, R. & Pineda, E.
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., Richardson, L., Savard, M., Scott, V., Scully, C., Sheffield, C., Sikes, D., Strange, J., Surrette, S., Thomas, C, Thompson, J., 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 limited in distribution to southwestern North America. It was historically common in the Central Valley of California, but now appears to be absent from most of it, especially in the centre of its historic range. The past decline of 67.51% (based on EOO, persistence, and relative abundance) suggests that this species should be listed as Endangered (EN) using criteria A2bc. The A2c criterion includes a decline in habitat quality; in the northern Central Valley there has been extensive agricultural intensification and the southern part of its range is experiencing rapid urbanization. Note that recent collection effort has not been sufficient throughout this species' historic range, although there has been a moderate level of sampling in the recent decade. 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 Endangered Red List category at this time.

Geographic Range [top]

Range Description:This species occurs primarily in California, including the Mediterranean region, Pacific Coast, Western Desert, Great Valley, and adjacent foothills through most of southwestern California (Williams et al. 2014, Zungri 2005). It has also been documented in southwest Nevada, near the California border. In addition, this species occurs uncommonly in Baja California, Mexico, where it has been documented in the city of El Progreso in the Sierra de Juarez Mountain Range (Labougle 1990, Williams et al. 2014). In his thesis on Bombus of Mexico and Central America, Labougle (1990) notes that he has seen only one specimen of this species in Mexico, and mostly relied on specimens from California for his study. There are only 12 known records of this species in the ECOSUR (2014) database of Mexican bumble bee records (including ECOSUR and other collections) (R. Vandame pers. comm. 2014).
Questionable Records: This species is represented by a single known record in Arizona and Utah in the database supporting Williams et al. (2014), although the Utah record (1936) is considered suspect (out of range), and the Arizona record (1956) has limited locality information ("Flying H Ranch"), and is also suspect (J. Strange pers. comm. 2014). According to Robbin Thorp (pers. comm. 2014) the Arizona record more likely should be mapped to Shasta County, CA where there is a quarter horse ranch of that name near Cottonwood which would fit with the northern range of B. crotchii. Additional erroneous records shown on the Discover Life (2014) distribution map for this species include Pennsylvania (obviously out of range) and two records in coastal Humboldt County. Robbin Thorp (pers. comm. 2014) has examined numerous bumble bees from the Humbolt County sites and never found B. crotchii, but notes that there is a dark color form of B. nevadensis at the sites that could easily be confused with B. crotchii. As such, it is suspected by Thorp that these records represent identification errors. In addition, a record of this species on Discover Life map from Baja California Sur is suspected to be in error (listed as Ensenada de los Muertos, but likely should have been simply Ensenada instead, which is in Baja California del Norte (R. Thorp pers. comm. 2014). None of the records described above were included in our map or analysis (Hatfield et al. 2014).

For a graph and map of relative abundance and range changes of this species over time see the Supplementary Material.


For further information about this species, see 44937582_Bombus_crotchii.pdf.
A PDF viewer such as Adobe Reader is required.
Countries occurrence:
Mexico (Baja California); United States (California)
Additional data:
Continuing decline in extent of occurrence (EOO):Unknown
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:We evaluated this species’ spatial distribution over time using a measure of change in the extent of occurrence (EOO; see Figure 2 in the Supplementary Material) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014). We also assessed changes in the species’ relative abundance
(see Figure 1 in the Supplementary Material), 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 timeframe 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. This analysis yielded the following results (see also the graph of relative abundance and map of change in EOO over time Supplementary Material): 
  • Current range size relative to historic range: 74.67%
  • Persistence in current range relative to historic occupancy: 20.48%
  • Current relative abundance relative to historic values: 2.32%
  • Average decline: 67.51%
This analysis suggests sharp declines in both relative abundance and persistence over the last ten years. This species was historically common in the Central Valley of California, but now appears to be absent from most of it, especially in the center of its historic range (Hatfield et al. 2014). 

Note that this analysis did not consider the Mexican distribution of this species, however, since this species is known from very few records in just one area of Mexico (Labougle 1990, R. Vandame pers. comm. 2014), the Mexican data would not have significantly impacted the above results. Remy Vandame (pers. comm. 2014) notes that the species is of high conservation concern in Mexico, although more sampling effort is needed to confirm its status in the region. There are very few historical records in Mexico (12 specimens from Baja California), and recent surveys in the region have not encountered this species (R. Vandame pers. comm. 2014).
For further information about this species, see 44937582_Bombus_crotchii.pdf.
A PDF viewer such as Adobe Reader is required.
Current Population Trend:Decreasing
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:

Bombus crotchii inhabits open grassland and scrub habitats. Nesting occurs underground. Males perch and chase moving objects in search of mates. This species is classified as a short-tongued species, whose food plants include Asclepias, Chaenactis, Lupinus, Medicago, Phacelia, and Salvia (Williams et al. 2014).

Bumble bees are social insects that live in colonies composed of a queen, workers, and reproductives (males and new queens). Colonies are annual and only the new, mated queens overwinter. These queens emerge from hibernation in the early spring and immediately start foraging for pollen and nectar and begin to search for a nest site. Nests are often located underground in abandoned rodent nests, or above ground in tufts of grass, old bird nests, rock piles, or cavities in dead trees. Initially, the queen does all of the foraging and care for the colony until the first workers emerge and assist with these duties. Bumble bees collect both nectar and pollen of the plants that they pollinate. In general, bumble bees forage from a diversity of plants, although individual species can vary greatly in their plant preferences, largely due to differences in tongue length. Bumble bees are well-known to engage in “buzz pollination,” a very effective foraging technique in which they sonicate the flowers to vibrate the pollen loose from the anthers. Tomatoes (Solanaceae), blueberries (Ericaceae), and many other important food plants are pollinated by bumble bees in this way.

Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):1

Use and Trade [top]

Use and Trade: There is no trade or commercialization of this taxon.

Threats [top]

Major Threat(s): Agricultural intensification in California's northern Central Valley and rapid urbanization in the southern Central Valley are threats that may have impacted B. crotchii, since this species was historically common in the Central Valley but now appears to be absent from much of its historic range, especially in the central part of its range (Hatfield et al. 2014, R. Thorp pers. comm. 2014). Note, however, that in the more northern parts of this species range, this species has been detected in agricultural landscapes in Yolo County and Contra Costa County in recent years (R. Thorp pers. comm. 2014). Climate change, specifically increasing aridity, is an additional threat; this species has a very narrow climatic specialization compared to most bumble bees (NatureServe 2014).  Bumble bees, as a whole, are threatened by a number of additional factors including pesticide use, pathogens from managed pollinators, and competition with non-native bees (reviewed in Goulson 2010, Williams et al. 2009, Williams and Osborne 2009, Cameron et al. 2011, Fürst et al. 2014, 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 the colonial life cycle (i.e., even large numbers of bumble bees may represent only one or a few queens) (Goulson 2010, Hatfield et al. 2012 but see Cameron et al. 2011, Lozier et al. 2011).

Conservation Actions [top]

Conservation Actions:

Conservation needs: Specific conservation needs for this species have not been identified. Due to the inherent vulnerability of many bumble bee species and importance of supporting wild bee populations for pollination services, the following general conservation practices are recommended: 

  1. Restore, create and preserve natural high-quality habitats to include suitable forage, nesting and overwintering sites. 
  2. Restrict pesticide use on or near suitable habitat, particularly while treated plants are in flower.
  3. Promote farming practices that increase of nitrogen-fixing fallow (legumes) and other pollinator-friendly plants along field margins.  
  4. Minimize exposure of wild bees to diseases transferred from managed bees. 
  5. Avoid honey bee introduction to high-quality native bee habitat.

Research needs: Hatfield et al. (2014) suggest that recent collection effort has not been sufficient throughout this species' historic range. Thorp (pers. comm. 2014) notes concerns about how much collection effort has been conducted in the central area of the species range in recent years. The area has been under intense conversion to agriculture and there has been reduced collecting and monitoring in the area.

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. 2015. Bombus crotchii. The IUCN Red List of Threatened Species 2015: e.T44937582A46440211. . Downloaded on 19 November 2017.
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