Suckley's Cuckoo Bumble Bee (Bombus suckleyi): COSEWIC assessment and status report 2019

Official title: COSEWIC Assessment and Status Report on the Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) in Canada 2019

Committee on the status of Endangered Wildlife in Canada (COSEWIC)
Threatened 2019

COSEWIC assessment summary

Assessment Summary – November 2019

Common name: Suckley’s Cuckoo Bumble Bee

Scientific name: Bombus suckleyi

Status: Threatened

Reason for designation: This bumble bee is a nest parasite of other bumble bees and depends on its hosts to rear its young. It is found in all provinces and territories except Nunavut. It is more frequent in the west than in the east and always much less frequent than its hosts. Despite significantly increased search effort for bumble bees in Canada over the last two decades, fewer individuals of this species have been encountered than in the past. There has been a decline of more than 30% in relative abundance compared to all bumble bees (indicating a population decline) and a decline in area of occupancy. The decline has been particularly severe in areas where the species was historically more frequent, in British Columbia and Alberta. The primary threat is the steep decline of the host bumble bee species, again in British Columbia and Alberta. The major threats to the hosts are the escape of pathogen-infected bumble bees from managed colonies in commercial greenhouses, pesticide use (particularly neonicotinoids), and climate change.

Occurrence: Yukon, Northwest Territories, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Québec, New Brunswick, Prince Edward island, Nova Scotia, Newfoundland (Island only, no species confirmed in Labrador)

Status history: Designated Threatened in November 2019.

COSEWIC executive summary

Suckley’s Cuckoo Bumble Bee
Bombus suckleyi

Wildlife Species Description and Significance

Suckley’s Cuckoo Bumble Bee is one of six true cuckoo bumble bee species occurring in North America. Both sexes are medium-sized (15–25 mm length). Females are slightly larger than males and have an abdomen with shiny black terga (dorsal abdominal segments) and yellow hairs near the tip; males have a similar colour pattern, but with more yellow hair on the abdomen. Unlike nest-building bumble bees, female cuckoo bumble bees do not possess a corbicula (pollen basket) on the hind leg as they do not collect pollen for their offspring.

Suckley’s Cuckoo Bumble Bee can be distinguished from the similar Gypsy Cuckoo Bumble Bee by the prominent triangular ridges on the underside of the last segment of the abdomen. Males also typically have more yellow hairs on the body than Gypsy Cuckoo Bumble Bee.

Suckley’s Cuckoo Bumble Bee is an obligate social parasite of nest-building bumble bees of the subgenus Bombus. Of the four species in this subgenus in Canada, Western Bumble Bee is the only confirmed host in western Canada, while Yellow-banded Bumble Bee is the suspected host in eastern Canada due to co-occurrence of the two species in much of its eastern range of Suckley’s Cuckoo Bumble Bee. Additional suspected hosts include Rusty-patched Bumble Bee (Ontario and Québec) and Cryptic Bumble Bee (northwestern Canada) because they are also in subgenus Bombus (like the confirmed host) and co-occur in the range of Suckley’s Cuckoo Bumble Bee. However, there is no direct evidence that either of these are hosts.

Three of the host and probable host species have been assessed at risk in Canada by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Western Bumble Bee occidentalis subspecies (Threatened) and mckayi subspecies (Special Concern), Yellow-banded Bumble Bee (Special Concern) and Rusty-patched Bumble Bee (Endangered). Cryptic Bumble Bee, a Holarctic species and potential host, has not been assessed by COSEWIC and has a conservation status of Secure.

Distribution

Suckley’s Cuckoo Bumble Bee has an extensive distribution from the southern United States to the subarctic regions of Canada (Yukon) and east to the island of Newfoundland (not confirmed from Labrador). In Canada, the species has been recorded in all provinces and territories except Nunavut. The species is more abundant in western Canada, and most collection sites are from west of Manitoba.

Canadian records of Suckley’s Cuckoo Bumble Bee date from 1897 (British Columbia) and 1901 (Ontario) to 2019 (Saskatchewan and Yukon). Additional records within the last ten years are from Alberta (2018), British Columbia (2013) and the island of Newfoundland (2010). The distribution of Suckley’s Cuckoo Bumble Bee is limited by the distribution, and presumably abundance, of its host bumble bee species, although other factors appear to be important because it has not been collected evenly throughout its hosts’ range.

Habitat

Suckley’s Cuckoo Bumble Bee occurs in diverse habitats including open meadows and prairies, farms and croplands, urban areas, boreal forest, and montane meadows. Records are from sea level to 1200 m although the species could potentially occur at higher elevations where its host(s) occur. In the early spring, hosts typically establish nests in abandoned underground rodent burrows or other dry natural hollows; because Suckley’s Cuckoo Bumble Bee is a nest parasite these same host residence sites also serve as its habitat. Adults have been recorded feeding on pollen and nectar from many flowers.

Biology

Suckley’s Cuckoo Bumble Bee is an obligatory social parasite of nest-building bumble bees, and therefore does not produce a eusocial colony with distinct castes (i.e. no workers). The species has an annual life cycle. Mated females emerge in the spring, slightly later than host nest-building species (e.g., hosts emerge March – April and cuckoos emerge April – June, sometimes later in higher latitudes and/or elevations) and begin to search for potential host nests. Successful nest parasitism by female cuckoos occurs after hosts have established colonies with some workers, but only if the host nest is not so large that the host workers can defend the colony and drive out the cuckoo. Once a host nest is found, the female cuckoo subdues (or kills) the host queen, and ultimately takes over egg laying in the nest; the workers of the original host queen care for the cuckoo’s offspring. Cuckoos emerge throughout the summer and with higher numbers produced in late summer and early autumn. New female and male cuckoos produced in the host nest emerge to feed on nectar, and then mate. Mated females ultimately select an overwintering site, presumably near nest-building host species. Males and the original egg-laying female die at the onset of cold weather.

Population Sizes and Trends

Limited information on the Canadian on global population size and trends for Suckley’s Cuckoo Bumble Bee is available. Most bumble bee surveys target all Bombus and do not specifically target cuckoo bumble bees. Historically, surveys have included all bumble bees and have mostly been conducted haphazardly or by wandering transects through suitable habitat, and have focused on recording new subpopulations, natural history and habitat information of bumble bees in general. Within the past 20 years, there have been extensive bumble bee surveys and academic research focused on pollinators, including bumble bees, and Suckley’s Cuckoo Bumble Bee has been recorded during this work. The species is inherently less abundant than other bumble bees because it does not produce a worker caste and is less common than its hosts.

Historical data show Suckley’s Cuckoo Bumble Bee appears to have always been more common in western Canada than in eastern Canada. The species has not been recorded from southern Ontario since the 1970s despite extensive search effort in the past twenty years. However, throughout other parts of its range the species remains present, albeit uncommon, where hosts occur.

Threats and limiting factors

The major threat to Suckley’s Cuckoo Bumble Bee is the decline of its host species: Western Bumble Bee, in western Canada, and likely Yellow-banded Bumble Bee in eastern Canada. Both Western Bumble Bee and Yellow-banded Bumble Bee were once more common and widespread, and subpopulations have been declining through much of their range, likely due to pesticide use (including neonicotinoids), pathogen spillover (specifically within high intensity agricultural areas), and floral resource and habitat loss from agricultural intensification and natural systems modifications (e.g., fire suppression, natural shrub encroachment into open areas).

Protection, Status and Ranks

Suckley’s Cuckoo Bumble Bee has no legal status and is not protected in Canada by any federal or provincial legislation. The species is globally ranked as Critically Imperilled (G1) and nationally in Canada as Vulnerable (N3) (NatureServe 2018). The species is assessed as Critically Endangered (CR) by the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Western Bumble Bee occidentalis (Threatened) and mckayi subspecies (Special Concern) have been assessed by COSEWIC but not listed under SARA. Yellow-banded Bumble Bee (Special Concern) has been assessed by COSEWIC and listed under SARA. Rusty-patched Bumble Bee has been assessed as Endangered by COSEWIC and listed under SARA.

Technical summary

Bombus suckleyi

Suckley’s Cuckoo Bumble Bee

Bourdon de Suckley

Range of occurrence in Canada: Yukon, Northwest Territories, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Québec, New Brunswick, Prince Edward island, Nova Scotia, Newfoundland (Island only, no species confirmed in Labrador)

Quantitative Analysis
Is the probability of extinction in the wild at least [20% within 20 years or 5 generations, or 10% within 100 years]? Insufficient data.

Threats (direct, from highest impact to least, as per IUCN Threats Calculator)

Was a threats calculator completed for this species? Yes, High impact; threats assessment completed December 16, 2018.

7.3 Other ecosystem modifications (mainly decline in abundance and distribution of host bumble bees) – Medium impact

2.1 Annual & perennial non-timber crops – Low impact

9.3 Agricultural & forestry effluents – Low impact

11.1 Habitat shifting and alteration – Low impact

11.2 Droughts – Low impact

What additional limiting factors are relevant?

1) Parasitism of bumble bees

2) Predators of adult bumble bees

3) Diploid male extinction vortex

4) Low genetic diversity in cuckoo bumble bees

5) Sustained nectar and pollen availability to both cuckoo and hosts

6) Cuckoo bumble bees are more vulnerable to extinction than their hosts

Data Sensitive Species

Is this a data sensitive species? No

Status History

COSEWIC: Designated Threatened in November 2019.

Status and Reasons for Designation:

Status: Threatened

Alpha-numeric codes: A2bce

Reasons for designation: This bumble bee is a nest parasite of other bumble bees and depends on its hosts to rear its young. It is found in all provinces and territories except Nunavut. It is more frequent in the west than in the east and always much less frequent than its hosts. Despite significantly increased search effort for bumble bees in Canada over the last two decades, fewer individuals of this species have been encountered than in the past. There has been a decline of more than 30% in relative abundance compared to all bumble bees (indicating a population decline) and a decline in area of occupancy. The decline has been particularly severe in areas where the species was historically more frequent, in British Columbia and Alberta. The primary threat is the steep decline of the host bumble bee species, again in British Columbia and Alberta. The major threats to the hosts are the escape of pathogen-infected bumble bees from managed colonies in commercial greenhouses, pesticide use (particularly neonicotinoids), and climate change.

Applicability of Criteria

Criterion A (Decline in Total Number of Mature Individuals): Meets criteria for Threatened, A2bce. Inferred reduction of greater than 30% in total number of mature individuals comparing most recent two ten year periods. The decline is based on: (b) an index appropriate to the taxon—when the decades 1999 – 2008 and 2009 – 2018 are compared, the number of mature individuals has declined by greater than 30% (67% using relative abundance of this species over all Bombus collected in Canada); (c) a decline in IAO comparing the two most recent ten year periods (approximately 56%) and in the quality of habitat (host species abundance) and (e) the effects of introduced taxa, hybridization, pathogens, pollutants, competitors or parasites.

Criterion B (Small Distribution Range and Decline or Fluctuation): Not applicable. EOO and IAO are larger than thresholds.

Criterion C (Small and Declining Number of Mature Individuals): Not applicable. Insufficient data to complete population estimates.

Criterion D (Very Small or Restricted Population): Not applicable.

Criterion E (Quantitative Analysis): Not applicable. Insufficient data to complete analysis.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2019)

Wildlife species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special concern (SC)
(Note: Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.)

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at risk (NAR)
(Note: Formerly described as “Not In Any Category”, or “No Designation Required.”)

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data deficient (DD)
(Note: Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” [insufficient scientific information on which to base a designation] prior to 1994. Definition of the [DD] category revised in 2006.)

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

The Canadian Wildlife Service, Environment and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife species description and significance

Name and classification
Taxonomic background

Phylum Arthropoda - arthropods

Class Insecta – insects

Order Hymenoptera – ants, bees, wasps

Infraorder Aculeata – stinging wasps

Family Apidae – bumble bees, honey bees, stingless bees, and many others

Subfamily Apinae

Genus Bombus – bumble bees

Subgenus Psithyrus – cuckoo bumble bees

Species Bombus suckleyi Greene, 1860

French common name: Bourdon de Suckley

English common name: Suckley’s Cuckoo Bumble Bee

Scientific name synonyms: Psithyrus latitarsus Morrill, 1903

Suckley’s Cuckoo Bumble Bee was described as Bombus suckleyi in 1860 from a male specimen collected by Dr. George Suckley at Puget Sound, Washington (Greene 1860). The female was described as Psithyrus latitarsus in 1903 from Montana (Morrill 1903) and later synonymized by Frison (1926).

Cuckoo bumble bees make up about 11% of all bumble bee species (Lhomme and Hines 2018), and historically were placed in a separate genus from other bumble bees because of their parasitic lifestyle. The earliest generic name used was Psithyrus Lepeletier, the name still used for the subgenus. While Psithyrus was considered a genus, multiple subgenera were also recognized in North America (e.g., Frison 1927; Thorp et al. 1983). More recent phylogenetic analysis of bumble bees (Cameron et al. 2007; Williams et al. 2008) recognized Psithyrus as a monophyletic unit (subgenus) within the genus Bombus, and sister group to the subgenus Thoracobombus (Hines 2008; Williams et al. 2008). Thoracobombus is a taxon particularly prone to nest usurpation (Sakagami and Nishijima 1973; Lhomme and Hines 2018).

Suckley’s Cuckoo Bumble Bee belongs to a clade (i.e. formerly subgenus Ashtonipsithyrus of Frison, though Lhomme and Hines (2018) called this the “bohemicus group” of Psithyrus) containing three to five additional species (Hines 2008; Lhomme and Hines 2018): Suckley’s Cuckoo Bumble Bee and Gypsy Cuckoo Bumble Bee (formerly B. ashtoni (Cresson) now B. bohemicus) from North America, Vestal Cuckoo Bumble Bee (B. vestalis Geoffroy) in the Palearctic, and Gypsy Cuckoo Bumble Bee (B. bohemicus s. str.) from the Palearctic, Arctic, and Orient; Lhomme and Hines (2018) also include B. coreanus (Yasumatsu) from the Orient, though little is known about this species, including its host(s) or its relatedness to other members.

Hines (2008) indicated that Suckley’s Cuckoo Bumble Bee was likely a sister taxon to the other species, splitting from them about 4 million years ago (mya) in North America, and with Vestal Cuckoo Bumble Bee splitting from Gypsy Cuckoo Bumble Bee in the Old World ca 2.5 mya. As such, Suckley’s Cuckoo Bumble Bee is closely related to Gypsy Cuckoo Bumble Bee. The ranges of both species overlap throughout much of Canada. Both species have had their mitochondrial DNA barcoded and have unique Barcode Index Numbers (BINs) with mean interspecies percent sequence divergence of 8.18% (Sheffield pers. data).

Morphological description

Most bumble bees are primitively eusocial^{Footnote 1} and all have four developmental stages (e.g., egg, larva, pupa, and adult). There are typically three adult forms or castes in bumble bee colonies: the queen (reproductive female), workers (non-reproductive females) and males. However, cuckoo bumble bees are social parasites in other bumble bee colonies and do not have a queen or worker caste (see Biology ). The morphological description of Suckley’s Cuckoo Bumble Bee is summarized below based on information in Morrill (1903), Thorp et al. (1983), and Williams et al. (2014).

Female (front cover and Figure 1): Body length 15–25 mm; breadth of abdomen 8–9 mm. The outer surface of the hind tibia (i.e. flattened segment of hind leg) is convex, with dense hair covering the surface, and without a corbicula (i.e. the shiny and hairless pollen basket of nest-building species). The hair on the face and top of the head is typically all black, occasionally with some yellow hairs at the posterior top of the head. The sides of the thorax are predominantly with yellow hair (with some exceptions). Hair on the anterior surface of thorax (i.e. in front of wings) is yellow and varies from yellow to black on the remaining dorsal surface. The first two abdominal segments have black hair, the 3^rd to 5^th abdominal segments are laterally variable yellowish-white, but usually white at least posteriorly in the middle of the 4^th segment. Like all cuckoo bumble bees, the tip of abdomen is recurved ventrally, with the ventral surface with two strong triangular carinae (ridges) visible in dorsal view.

Male (Figure 2): Body length 15–22 mm; breadth of abdomen 5–7 mm. Antenna medium length, with the flagellum (i.e. the long whip-like part consisting of the 2^nd to 11^th antennal segments) 3 times longer than scape (i.e. first antennal segment, that is attached to the face). Hair of hind basitarsus (i.e., the basal segment of the “foot” on hind leg) posterior fringe predominantly black, the first abdominal segments largely yellow, with some specimens with much black hair intermixed laterally, especially on 2^nd segment, the 3^rd, 5^th, and 6^th segments primarily yellow with black hairs present medially, the 4^th segment primarily yellow, the 7^th segment is entirely black (Figure 2). Proper identification of males may require examination of genitalia structures (Williams et al. 2014).

Suckley’s Cuckoo Bumble Bee from eastern Canada were identified correctly in major collections (e.g., Canadian National Collection of Insects, Arachnids and Nematodes [CNC]) but not mentioned in compilations of eastern bumble bees (i.e. Mitchell 1962; Laverty and Harder 1988; Colla and Dumesh 2010; Colla et al. 2011). The result was some misidentifications as Gypsy Cuckoo Bumble Bee, now corrected through review of the identifications in museum collections (see Taxonomic background ).

Females of both species have pronounced carinae on the 6^th sternum that is visible even in dorsal view, that of Suckley’s Cuckoo Bumble Bee being even more distinct than that of Gypsy Cuckoo Bumble Bee (Figure 1). Gypsy Cuckoo Bumble Bee females typically have black hair on the pleura (side of thorax; compare to which also is a useful diagnostic feature), though this may also occur on some specimens of Suckley’s Cuckoo Bumble Bee.

The eggs, larvae and pupae of Suckley’s Cuckoo Bumble Bee have not been described.

Population spatial structure and variability

Genetic variability and population structure have not been studied for Suckley’s Cuckoo Bumble Bee. Cytochrome oxidase 1 (COI) gene (i.e. DNA Barcode) sequences are available in the Barcode of Life Data system (BOLD) (www.barcodeoflife.org) from eight specimens from multiple sites in Canada; two sites from Yukon, one site in British Columbia, one site in Saskatchewan, and three specimens from two sites on the island of Newfoundland. All sequences are almost identical, with maximum interspecific distance across samples at 0.3% (average of 0.1%) (Sheffield pers. data) supporting one widespread species, and all sequences have been assigned to Barcode Index Number (BIN) BOLD: ABY1164 (Sheffield et al. 2017).

Designatable units

Sequence analysis of the COI (DNA barcode) gene (Sheffield et al. 2017), BIN assignment and other molecular based phylogenies (Hines 2008; Lhomme and Hines 2018) and the absence of other evidence of subspecific genetic structure support Suckley’s Cuckoo Bumble Bee being assessed as one designatable unit.

Special significance

Suckley’s Cuckoo Bumble Bee is a social parasite in other bumble bee colonies. The species likely plays a significant ecological role through its effect on host dynamics and distribution (Antonovics and Edwards 2011), as is likely true of most cuckoos (Sheffield et al. 2013). In general, bees and their less common cuckoo bees appear to be particularly sensitive to detrimental environmental impacts because of their sex determining mechanism (Zayed and Packer 2005). The smaller population size of cuckoo bumble bees enhances this effect (Williams et al. 2010) (see Limiting factors ). The rarity and extinction risk of hosts puts cuckoo bumble bees at much higher risk of decline (Suhonen et al. 2015). The species is also a floral visitor and pollinator.

Distribution

Global range

Globally, Suckley’s Cuckoo Bumble Bee occurs in North America and is primarily a western Nearctic species (Lhomme and Hines 2018) (Figure 3 and Figure 4). It is found from Alaska south to northern California and east to Colorado, Manitoba and South Dakota. East of the 100^th meridian the species becomes rarer but has been recorded east to the island of Newfoundland and south to Virginia. In the west it becomes rare north of the 60^th parallel. There are scattered records in central and northeastern North America (Williams et al. 2014).

Figure 4. Canadian extent of occurrence (EOO) of Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) based on databased museum collections and additional data compiled during status report preparation (1901–2018). The EOO (9,710,188 km²) is based on a minimum convex polygon created around all databased records and 9,160,823 km² within Canada’s extent of jurisdiction. Map created by the COSEWIC secretariat.

Canadian range

In Canada, Suckley’s Cuckoo Bumble Bee has been recorded in every province and territory except Nunavut and it is not recorded in Labrador. Suckley’s Cuckoo Bumble Bee occurs in most Canadian ecozones (as described in COSEWIC 2011), including: Boreal Cordillera, Mountain Cordillera, Taiga Plains, Boreal Plains, Prairies, Taiga Shield, Boreal Shield, Hudson Plains, Mixed Wood Plains and Atlantic Maritimes. The species has not been recorded from the Arctic ecozone. Most records are from western North America, with fewer records east of Manitoba (Table 1, Figure 3 and Figure 4 and Figure 5) (Williams et al. 2014; Sheffield pers. data and see Collections Examined). Its sister species, Gypsy Cuckoo Bumble Bee, is most frequent in eastern and boreal northwestern Canada (Figure 3 and Figure 5).

Figure 5. The relative abundance of Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) [white bars] and Gypsy Cuckoo Bumble Bee (Bombus bohemicus) [black bars] in Canada. These two species have a close phylogenetic relationship.

The earliest records of Suckley’s Cuckoo Bumble Bee in Canada are 1897 and 1899 from “British Columbia” but the locality information is not precise and hence they are not on the distribution maps. The earliest Suckley’s Cuckoo Bumble Bee record in Canada with precise locality data is 1901 from Ontario.

Suckley’s Cuckoo Bumble Bee is an obligate social parasite of nest-building host bumble bees and therefore does not produce a eusocial colony with distinct castes (i.e. workers are not produced) (see Biology ). The ranges of the main host bumble bee species are listed below and shown in Figure 7.

Figure 7. Canadian range of Suckley’s Cuckoo Bumble Bee (black dots) and the two host bumble bee species: Western Bumble Bee (southern subspecies B. occidentalis occidentalis and northern subspecies B. o. mckayi) (red dots) is a confirmed host (Hobbs 1965ab, 1966ab) and Yellow-banded Bumble Bee (B. terricola) (blue dots) is a presumed host (see discussion in Canadian range). The approximate range of Western Bumble Bee southern subspecies is 720,170 km², northern subspecies is 623,837 km² and Yellow-banded Bumble Bee is 7,913,612 km². Map created by the COSEWIC secretariat.

Western Bumble Bee (B. occidentalis Greene, northern mckayi and southern occidentalis subspecies) is a confirmed host for this species (Hobbs 1965ab, 1966; Lhomme and Hines 2018). Hobbs (1965ab, 1966ab) studied the nesting biology of Western Bumble Bee in Alberta and confirmed nest usurpation by Suckley’s Cuckoo Bumble Bee. Western Bumble Bee ranges in British Columbia, Alberta, southern Saskatchewan, Yukon and western Northwest Territories (Sheffield et al. 2016).

Yellow-banded Bumble Bee (B. terricola Kirby) is a presumed host of Suckley’s Cuckoo Bumble Bee in Manitoba, Ontario, Québec, the Maritime provinces, and Newfoundland (Lhomme and Hines 2018). Hobbs (1968) observed Suckley’s Cuckoo Bumble Bee in the nest of Yellow-banded Bumble Bee in Alberta but did not confirm it as a host. Yellow-banded Bumble Bee is in subgenus Bombus and closely phylogenetically related (i.e. sister species) to Western Bumble Bee. This is the main reason we consider Yellow-banded Bumble Bee a likely host, particularly in areas where Western Bumble Bee does not occur. Yellow-banded Bumble Bee ranges from the Rockies in eastern British Columbia through the boreal zone, southern Northwest Territories and southern half of Canada to the island of Newfoundland (Sheffield et al. 2016).

Rusty-patched Bumble Bee (B. affinis Cresson) is a possible host because it too is in subgenus Bombus and phylogenetically closely related to both Western Bumble Bee and Yellow-banded Bumble Bee. Yet there are no confirmed observations of Suckley’s Cuckoo Bumble Bee entering the nest or parasitizing this species, nor is it indicated by Lhomme and Hines (2018). However, Rusty-patched Bumble Bee has been observed as a host to the sister species, Gypsy Cuckoo Bumble Bee (Plath 1934). Rusty-patched Bumble Bee has a range in southern Ontario and Québec (Laverty and Harder 1988; Williams et al. 2014), and New Brunswick (Klymko and Sabine 2015). This species has not been observed in Canada since 2009 and is designated Endangered by COSEWIC (COSEWIC 2010).

Cryptic Bumble Bee (Bombus cryptarum) is a possible host because it is also in subgenus Bombus; however, there are no confirmed observations of Suckley’s Cuckoo Bumble Bee entering the nest or parasitizing this species (e.g., Williams et al. 2014; Lhomme and Hines 2018). The range of Cryptic Bumble Bee overlaps with Suckley’s Cuckoo Bumble Bee in most areas in western Canada.

Hobbs (1965ab; 1966ab) observed Suckley’s Cuckoo Bumble Bee entering the nests of Red-belted Bumble Bee (B. rufocinctus Cresson), Nevada Bumble Bee (B. nevadensis Cresson), White-shouldered Bumble Bee (B. appositus Cresson), and Yellow Bumble Bee (B. fervidus (Fabricius)) (Hobbs 1968). However, there is no further evidence that Suckley’s Cuckoo Bumble Bee queens usurped these potential hosts, nor that progeny were reared. In addition, these four bumble bees are not in the phylogenetically related subgenus Bombus.

Extent of occurrence and area of occupancy

Extent of occurrence (EOO) for Suckley’s Cuckoo Bumble Bee in Canada (Figure 4) is approximate and based on the databased museum collections, online, sight and collection records (see Collections Examined ). The approximate EOO based on a minimum convex polygon created around all databased records with known localities and dates (1901–2018) is 9,710,188 km² (9,160,823 km² within Canada’s extent of jurisdiction). The index of area of occupancy (IAO) (2 X 2 grid cells) for all known records is 5,136 km² and 112 km² for the most recent ten year period. Both calculations are undoubtedly underestimates because surveys in northern Canada are limited.

Search effort

Much time and effort have been invested (recently and historically) in surveys that focus on bumble bees, particularly in southern Canada. There are more data available for wild bumble bees than for most other North American insects. A recently compiled dataset (i.e. used for Williams et al. 2014) of approximately 236,260 bumble bee specimens from museums in Canada and the United States shows an increase in bumble bee specimens collected, particularly in the past 15 years (for some studies in Canada see Table 2).

Suckley’s Cuckoo Bumble Bee records in Canada used for the analysis in this report date from 1901–2018 (by decade 1898-2018). The species has been recorded from all provinces and territories except Nunavut (Table 1). The most recent records are from Saskatchewan (N=4) in 2018 (Sheffield pers. data), Alberta (N=1) in 2018 (Galpern pers. data), British Columbia (N=1) in 2013, Newfoundland and Labrador (N=3) in 2010 and Yukon (N=5) in 2018 (Cannings pers. comm. 2019). Suckley’s Cuckoo Bumble Bee was recorded from Woodrow (Saskatchewan) in 2019; however, we did not include these specimens in our analysis because we did not have bumble bee data from across the country assembled for 2019.

Suckley’s Cuckoo Bumble Bee records in Canada were plotted over all databased Bombus in Canada (based on museum collections and additional data compiled during status report preparation; 1899–2018) (Figure 8). In the absence of historical systematic survey efforts, this dataset serves as a proxy of bumble bee search effort throughout the country; we assume that Bombus collection events were not biased with respect to species being collected, and that if Suckley’s Cuckoo Bumble Bee was present at the time and place of collection it likely also would have been collected.

Figure 8. Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) records in Canada (black dots) plotted over all databased Bombus in Canada (open dot) based on databased museum collections and additional data compiled during status report preparation (1901–2018). We assume that Bombus collections were not biased and if there was Suckley’s Cuckoo Bumble Bee present, it would have been collected. Map created by the COSEWIC secretariat.

Surveys for bumble bees are typically by sampling methods that include hand netting, blue vane trapping, pan trapping, visual searches, and occasionally Malaise traps. Bumble bee surveys typically do not specifically target one species; the identification of a bumble bee on the wing can be difficult, results and surveyor error are difficult to repeat and quantify and is not always practical when surveying large areas. In the past few years, online citizen science forums such as iNaturalist^©, BugGuide^© and Bumble Bee Watch^© have become valuable sources of additional occurrence information.

There are some shortcomings to the bumble bee search effort and these factors make it difficult to interpret spatial and temporal patterns specifically for Suckley’s Cuckoo Bumble Bee. Surveys have not been systematic or comprehensive over time, most surveys have occurred in the southern parts of Canada and were done haphazardly and not quantified by time, distance searched, and number of surveyors. Cuckoo bumble bees are less likely to be collected during the middle of the active season because spring cuckoos are ensconced in host nests and it takes a few weeks before new females/males start to emerge from the host nest. In addition, as cuckoos produce no workers, they are less numerous than social bumble bees which can produce more than a hundred workers. In North America cuckoo bumble bees represent less than 4% of all bumble bees databased (Lhomme and Hines 2018). Regardless, cuckoo bumble bees have been recorded from March through September, suggesting emergence times can vary.

Recent bumble bee search effort (within the past 15 years), including the two main host species, for each province and territory is summarized in Table 2. Search effort is tallied by jurisdiction because there have been different jurisdictional initiatives over time, jurisdictional natural heritage or conservation data centres manage data independently; specimens are housed within jurisdictional museums and academic or local studies are not typically across jurisdictions. In summary, there have been at least 70 separate sites and more than 65,000 bumble bees collected since 2005 (Table 2). More than 50% of the records are from the most recent two decades. Most search effort has been in southern Canada and there remain large areas of subarctic Canada with little recent or historical data.

Habitat

Habitat requirements

Nesting habitat:

Suckley’s Cuckoo Bumble Bee is a social parasite, or cuckoo, in nests of hosts in the subgenus Bombus (Western Bumble Bee in the west, and probably Yellow-banded Bumble Bee in the east), but may occasionally parasitize other Bombus (Williams et al. 2014; Lhomme and Hines 2018). Nest-building bumble bees typically select abandoned underground rodent burrows as nests (Plath 1934) in various habitats such as montane meadows, old and fallow fields, farmlands, croplands, urban areas and woodlands.

Foraging habitat:

Suckley’s Cuckoo Bumble Bee does not collect pollen for its own nest provisions. They are generalist nectar foragers and have been recorded on several members of the Asteraceae: Aster (sensu lato) including Symphyotrichum, as well as Chrysothamnus, Cirsium, and Solidago. Recent observations and collection events in Saskatchewan also indicate that cotoneaster (Cotoneaster: Rosaceae) hedges are also attractive to female Suckley’s Cuckoo Bumble Bee as a nectar source (Sheffield pers. data).

Overwintering habitat:

Specific overwintering habitat requirements for Suckley’s Cuckoo Bumble Bee are unknown, but like other bumble bees, only mated females overwinter in the ground, in mulch or other decomposing vegetation, and in rotting logs near nesting sites (Macfarlane 1974). The species likely does not disperse far, mainly because it needs to remain near host nests to reproduce the following year.

Habitat trends

Suckley’s Cuckoo Bumble Bee has one of the largest ranges of all bumble bee species in Canada (Figure 3) and it is unlikely that specific habitat trends have caused its decline at such large scales.

The decline of Suckley’s Cuckoo Bumble Bee is linked to the declines of its two main host species. Habitat loss due to urbanization or intensive agriculture may threaten this species directly and indirectly (via its hosts) in the southern parts of its range in Canada. It is possible the increase in density of vegetable greenhouses within agricultural areas may be causing the decline of host species (Szabo et al. 2012) as well as field crops. The movement of both bumble bees and Western Honey bees is not tracked, and may be spreading diseases (see Threats). In south-central Canada, increased agriculture in ecozones inhabited by the Yellow-banded Bumble Bee (i.e. Prairies, Atlantic Maritime and Mixedwood Plains) show diminished wildlife habitat capacity (Javorek and Grant 2011). Climate change-induced habitat alteration may also negatively impact this species via the effect on its hosts, but more research is required.

Biology

Information is compiled from general bumble bee references (Alford 1975; Goulson 2003a; Benton 2006) and where applicable references are provided specifically for Suckley’s Cuckoo Bumble Bee or other cuckoos (Hobbs 1968; Lhomme and Hines 2018).

Life cycle and reproduction

Suckley’s Cuckoo Bumble Bee is a social parasite of nest-building bumble bees, but follows the same basic life cycle pattern of other bumble bee species and has a generation time of one year. In the spring, female cuckoos invade the nests of the host nest-building species and displace the resident host queen, either by killing or subduing her. The workers already in the nest (i.e. daughters of the host queen) are controlled by the queen cuckoo using chemical cues to rear both cuckoo and host workers (Michener 2000, 2007; Zimma et al. 2003). Eggs hatch approximately four days later, and the small larvae begin to feed on the pollen and nectar provisions collected by host workers. The larval stage of bumble bees has four instars. After almost two weeks of development, cuckoo larvae spin cocoons and pupate. Pupae develop for another two weeks before hatching as adult cuckoos. In total, development takes approximately five weeks but varies with temperature and food supply (Alford 1975). Male and female cuckoos typically emerge in the late summer, and after mating and the onset of frost, the males die, and mated females overwinter.

Information on Suckley’s Cuckoo Bumble Bee fecundity and development is limited (Hobbs 1965ab, 1966ab); however, information from the sister species Gypsy Cuckoo Bumble Bee is available and summarized here. Three Gypsy Cuckoo Bumble Bee females and six males were recorded when a Rusty-patched Bumble Bee colony was dug up on August 9^th (Plath 1934). This nest also contained the old Rusty-patched Bumble Bee queen and one hundred Rusty-patched Bumble Bee workers (Plath 1934). The colony was observed until the end of September and produced twenty-nine cuckoo males and sixty-one females. Although the injured Rusty-patched Bumble Bee queen was seen with a distended abdomen and laying eggs, no further Rusty-patched Bumble Bee males, workers or queens were produced. Fisher (1983) hypothesized that the presence of a live Rusty-patched Bumble Bee queen is required by Gypsy Cuckoo Bumble Bee females to suppress ovarian development of the worker caste, but that the cuckoo eats the eggs produced by the Rusty-patched Bumble Bee queen to reduce competition with her offspring.

Mating behaviour of Suckley’s Cuckoo Bumble Bee is likely similar to Gypsy Cuckoo Bumble Bee. Adult cuckoos visit flowers, both after emergence (sometime in the autumn) and females only, prior to nest invasion in the spring (Antonovics and Edwards 2011). Phenology differs with latitude and altitude but generally females emerge approximately one month after the host species (Plath 1934) and are detected until late summer. Males emerge in early summer and are detected until late autumn.

Though only Western Bumble Bee has been confirmed as a host, the distribution of Suckley’s Cuckoo Bumble Bee in Canada (outside the range of Western Bumble Bee) suggests that other nest-building bumble bees must also serve as hosts (Williams 2008; Hines and Cameron 2010; Lhomme and Hines 2018). It is rare that cuckoo bumble bees use a single host, and often they tend to use related species (i.e. members of the same subgenus or even species group). As such, it is likely that other members of the subgenus Bombus also serve as hosts (Hobbs 1965ab, 1966ab; Williams et al. 2014; Lhomme and Hines 2018).

Physiology and adaptability

When compared to nest-building bumble bees, female cuckoo bumble bees typically have a thicker, more protective exoskeleton, larger mandibles, a greater number of ovarioles and a longer venom gland (Fisher and Sampson 1992). As they do not collect resources for their own offspring, they do not possess a corbicula (i.e. pollen basket) for pollen carrying on their hind leg, and their abdomens generally have less hair, giving them a shinier appearance in dorsal view than nest-building species.

Dispersal and migration

The ability and rate of dispersal for Suckley’s Cuckoo Bumble Bee depends on its hosts’ population dynamics and distribution. In general, there is little information on natural dispersal rates for bumble bees. Regardless, given the patchiness of bumble bee habitat (e.g., Hatfield and LeBuhn 2007) and increased problems associated with small effective population sizes in haplodiploid insects (e.g., Zayed and Packer 2005) (see Limiting factors ), dispersal is likely important to survival. An important opportunity for dispersal occurs with the movement of reproductive individuals, primarily females in spring that disperse while searching for suitable nest sites (Goulson 2003a).

There is some evidence that bumble bees can disperse long distances. Males of the well-studied Buff-tailed Bumble Bee (B. terrestris, and host to Gypsy Cuckoo Bumble Bee in the Palearctic) are estimated to fly between 2.6 and 9.9 km from the colony of origin (Kraus et al. 2009). Additionally, Buff-tailed Bumble Bee was introduced to Tasmania in the early 1990s and has since spread at a rate of approximately 12.5 km per year (Stout and Goulson 2000). It is presumed that these values may also be true of other bumble bee species, including cuckoos, due to their large size.

Interspecific interactions

Suckley’s Cuckoo Bumble Bee, like its sister species the Gypsy Cuckoo Bumble Bee, is a social parasite of bumble bees in the subgenus Bombus, the most important being Western Bumble Bee, the only confirmed host for this species (Hobbs 1968; Lhomme and Hines 2018). Cuckoo bumble bees detect their host species using chemical cues (Fisher et al. 1993). In the west, the host species is Western Bumble Bee, but likely also includes Yellow-banded Bumble Bee and possibly Cryptic Bumble Bee. Hobbs (1965ab, 1966ab) recorded Suckley’s Cuckoo Bumble Bee in the nest of other species in Alberta, though these species have not been confirmed as hosts (Williams et al. 2014; Lhomme and Hines 2018).

Population sizes and trends

Sampling effort and methods

A large dataset of bumble bee records from Canada was assembled for this status report. This Canadian dataset includes records from a larger North American dataset of bumble bees originally used for the Williams et al. (2014) publication (see http://www.leifrichardson.org/bbna.html), supplemented with additional data from museums, personal collections, online observation sources (e.g., iNaturalist^©, BugGuide^©, Bumble Bee Watch^©) and other unpublished research datasets. This Canadian dataset is not inclusive of all Canadian bumble bee data; however, we have tried to obtain datasets that represent recent (within the last 15 – 20 years) collection effort from across the Canadian geographic range of Suckley’s Cuckoo Bumble Bee (Table 2).

This Canadian dataset was analyzed to assess changes in relative abundance (RA), EOO, and IAO of Suckley’s Cuckoo Bumble Bee by 10-year increments dating backwards from 2018. The type of data available to analyze historical bumble bee population trends are limited, and RA can be used as a proxy of abundance when data are not amenable to other analysis. For example, much of the data available is from opportunistic inventory and not geographically repeated; historical collection sites are not precisely georeferenced; or the data is a subset of a specific study.

There are assumptions to using RA when analyzing population trends. Most specimens in this dataset were collected by passive collection methods (e.g., pan traps, Malaise traps, blue vane traps) or hand netting. Passive collection methods are considered non-biased and we assume that a collector, photographer or observer would not bias themselves when capturing a bumble bee.

For analysis we worked backwards in 10-year increments from 2018 (1899 – 2018) for all of Canada (Figure 10). In addition jurisdictions with more than 75 records of Suckley’s Cuckoo Bumble Bee had figures prepared (Figures 11-15). Calculations were based on subsets of the data for:

1. RA was calculated for Suckley’s Cuckoo Bumble Bee / all databased Bombus records available for this analysis (red lines);
2. RA was calculated for Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee northern and southern subspecies and Yellow-banded Bumble Bee) (black lines)
3. RA was calculated for host bumble bees (Western Bumble Bee and Yellow-banded Bumble Bee) / all databased Bombus records available for this analysis (blue lines)
4. changes in IAO and EOO for Suckley’s Cuckoo Bumble Bee across the Canadian range

For RA of Suckley’s Cuckoo Bumble Bee to hosts we used the known host Western Bumble Bee and sister taxa Yellow-banded Bumble Bee (presumed host and only member of subgenus Bombus in much of the range). We excluded Rusty-patched Bumble Bee and Cryptic Bumble Bee from our analysis because these two species have not been confirmed as hosts.

Suckley’s Cuckoo Bumble Bee individuals are predicted to have a bimodal flight period. In the spring mated females emerge and actively look for an established host nest to parasitize; if successful it may take several weeks before the cuckoo progeny become adults. These adults will leave the nest, mate with conspecifics, and the mated queens will overwinter; it is unlikely these new queens would be successful if they attempted to parasitize large host colonies late in the season (e.g., the abundance of workers in the host nest would be great enough to prevent the cuckoo bumble bee from taking over the nest). Suckley’s Cuckoo Bumble Bee has been recorded throughout the months of May – September in Canada. Although the cuckoo bumble bee is less abundant in late June and July, emergence times vary according to geography, temperature, phenology and elevation, among other factors. Therefore, we included all bumble bees collected/observed throughout this time in the analysis.

Abundance

Cuckoo bumble bees are, by their nature, less common than non-cuckoo bumble bees (e.g., workers make up most bumble bees collected). Thus, cuckoo bumble bees will naturally make up a low proportion of bumble bee records compared to workers of non-cuckoo species. Lhomme and Hines (2018) showed that less than 6% of all bumble bees databased globally in the Global Biodiversity Information Facility (GBIF) were cuckoo bumble bees. In North America, 3.8% were cuckoo bumble bees, supporting this low proportion compared to non-cuckoos.

Fluctuations and trends

Little is known about the natural fluctuations and trends of bumble bee subpopulations. Even though surveys have been completed over large geographic areas of Canada (e.g., Cameron et al. 2011; Colla and Packer 2008; Colla et al. 2012; Sheffield pers. data; and many more), there are few studies that have repeatedly surveyed sites over an entire season, over several years, or gone back to those same places a decade later.

For bumble bees, a site may contain three or four common species and a handful of relatively rare ones (e.g., see Colla and Packer 2008). Common species often have stable subpopulations over time (e.g., large effective population sizes), whereas rare species will fluctuate and suffer from local stochastic extinction (e.g., small effective population sizes), may be uncommon members of the local bee fauna or may have more specific habitat requirements. Cuckoo bumble bees have the added complexity of being dependent on the host bee species’ presence, abundance, and subpopulation dynamics. Cuckoo bees in general are susceptible to changes in abundance of their host (Sheffield et al. 2013) and have a greater extinction risk than non-cuckoo bumble bees (Suhonen et al. 2015).

The results from RA analysis are in Table 3 and Figures 10-15. Only major trends in these data are discussed below.

Figure 10. Species relative abundance within Canada by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). Despite the commonness of the two hosts in the 1950s to 1990s, Suckley’s Cuckoo Bumble Bee remained uncommon. Also see Table 3.

Figure 11. Species relative abundance within British Columbia by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). The relative abundance of Suckley’s Cuckoo Bumble Bee was highest pre-1960s, with another large peak in the late 1980s and 1990s, and despite commonness of its main host (Western Bumble Bee) until the late 1990s, the cuckoo has remained uncommon in British Columbia; hosts and cuckoo are now very uncommon in the province. Also see Table 3.

Figure 12. Species relative abundance within Alberta by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). The relative abundance of Suckley’s Cuckoo Bumble Bee has fluctuated in Alberta, seemingly following the abundance of its hosts Western Bumble Bee southern subspecies (B. occidentalis occidentalis) and Yellow-banded Bumble Bee (B. terricola). Although uncommon, it has been detected recently in surveys across the southern half of the province; from the early 1990s it was detected in higher numbers than its hosts (hence the peak), though likely an artifact of sampling and/or data capture. Also see Table 3 which documents the increased search effort since the early 1990s.

Figure 13. Species relative abundance within Saskatchewan by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). The peaks in relative abundance of Suckley’s Cuckoo Bumble Bee have largely been due to infrequent sampling in the province, with bias towards the cuckoo over hosts, and hosts over other bumble bees (1950s), and thus does not reflect the true abundance. Also see Table 3.

Figure 14. Species relative abundance within Manitoba by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). The peak in relative abundance of Suckley’s Cuckoo Bumble Bee in the late 1960s is due to infrequent sampling, with bias towards the cuckoos over hosts, and thus does not reflect the true abundance. Also see Table 3.

Figure 15. Species relative abundance within Ontario by 10-year periods from 1899 until 2018 of 1) Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) / all databased Bombus records available for this analysis (red); 2) Suckley’s Cuckoo Bumble Bee / hosts only (Western Bumble Bee [both subspecies] [B. occidentalis occidentalis and B. o. mckayi], Yellow-banded Bumble Bee [B. terricola]) (black); and 3) host bumble bees (two species) / all databased Bombus records available for this analysis (blue). Despite an ample number of hosts (Yellow-banded Bumble Bee) until the 2000s, Suckley’s Cuckoo Bumble Bee appears to have always been rare in Ontario; trends are therefore difficult to document. Also see Table 3.

1) Suckley’s Cuckoo Bumble Bee / all databased Bombus records (SCBB/total) available for this analysis (red lines, Table 3 and Figures 10-15).

• Suckley’s Cuckoo Bumble Bee records comprise ca 2% of all Bombus in the Canadian dataset. Cuckoo bumble bees typically make up very low proportions of total bumble bee captures as there are no workers produced, and throughout the period from 1901 until 2018, this species has fluctuated with both increases and decreases in RA in Canada (Figure 10)
• Historically (i.e. pre.1969-1978), Suckley’s Cuckoo Bumble Bee made up a higher RA of bumble bees in Canada with a peak in RA of 0.18 occurring in the 1950s (Figure 10, red line); Suckley’s Cuckoo Bumble Bee has declined in RA in most subsequent decades
• The RA for Suckley’s Cuckoo Bumble Bee for the last three decades for all of Canada (Figure 10) are 0.019 from 1989-1998, 0.003 from 1999-2008 and less than 0.001 for 2009-2018; these represent declines of 84% between decades 1989-1998 and 1999-2008; and 67% between 1999-2008 and 2009-2018. Similar large declines in the last few decades are apparent for Yukon, British Columbia (Figure 11), and Alberta (Figure 12), though too few data are available for other jurisdictions

2) Suckley’s Cuckoo Bumble Bee / hosts (Western Bumble Bee occidentalis and mckayi subspecies and Yellow-banded Bumble Bee) (black lines, Figures 10-15).

The RA of Suckley’s Cuckoo Bumble Bee when calculated in relation to host species across its Canadian range (Figure 10, black line) did not follow the same general trend as its hosts to all bumble bees (Figure 10, blue line). Suckley’s Cuckoo Bumble Bee appears to have had a major population crash in the 1960s and 1970s (Figure 10, red and black lines) even though its hosts continued to be relatively common until the 1990s (Figure 10, blue line).

• In British Columbia where most occurrences have been recorded, the RA of Suckley’s Cuckoo Bumble Bee to host was highest pre-1960s (with a smaller peak again in the 1990s)
• For other jurisdictions, the data do not reveal declines in the same manner. In Yukon, Suckley’s Cuckoo Bumble Bee has historically been very uncommon, though a peak was observed (6 specimens) in 1999–2008, with populations declining with host abundance in the last decade. Similarly, Alberta (Figure 12) and Saskatchewan (Figure 13) have seen peaks in the RA of the cuckoo to its hosts, though these are an artifact of more cuckoos being caught than hosts, which skews the RA calculations. In general, the results are not as clear for Alberta and Saskatchewan, though declines in Suckley’s Cuckoo Bumble Bee are apparent between the late 1990s and 2000s for both jurisdictions

3) Host bumble bees (Western Bumble Bee and Yellow-banded Bumble Bee) / all databased Bombus records available for this analysis (blue dots, (Figures 10-15).

Between 1989-1998 and 1999-2008 the RA of hosts to all bumble bees declined by 86%, and again in the next decade by 24% (Table 3, Figure 10).

• On average, RA for Suckley’s Cuckoo Bumble Bee is 1–2% of all bumble bees databased in Canada (Table 3). RA for hosts/total bumble bees (see Table 3 and Figures 10-15) supports recent (i.e. last two decades) downward trends for Western Bumble Bee (occidentalis and mckayi subspecies, COSEWIC 2014) and Yellow-banded Bumble Bee (Colla and Packer 2008; Cameron et al. 2011; COSEWIC 2015)
• When the host bumble bee data are pooled (both species) and RA is calculated for Canada as well as for each jurisdiction, there are declines between decades
• In British Columbia, most Western Bumble Bee records are from southern regions of the province (occidentalis subspecies). There has only been one recent Suckley’s Cuckoo Bumble Bee record (2013, south of McBride) from within this range
• Throughout their ranges, the RA of hosts declined dramatically between 1989-1999 and 1999-2008 (Yukon, British Columbia, Manitoba, Ontario, Québec) and more recently, between 1999-2008 and 2009-2018 in Alberta and Saskatchewan

4) Changes in EOO and IAO for Suckley’s Cuckoo Bumble Bee (Table 4).

EOO and IAO were calculated in decade intervals from 1899 to 2018 for all of Canada.

• The total EOO was 9,160,823 km²; among the decade periods EOO fluctuated between 388,938 km² and 8,491,978 km². Changes in EOO do not appear to show evidence for strong declines and are likely more indicative of sampling intensity in each period given the broad geographic range
• The total IAO was 5,138 km². IAO fluctuated across the decades and showed a decline between both 1999 - 2008 (45%) and 2009 - 2018 (57%) (Table 4). These two declines occurred as sampling effort increased, but with limited sampling in the northern areas

Table 4. Percent changes in extent of occurrence (EOO) and index of area of occupancy (IAO) by decade for Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) in Canada. EOO and IAO calculations done using GeoCAT software (geocat.kew.org). Both EOO and IAO fluctuate between decades, though the IAO declined by 45% and 56% between decades 1989 – 1999 and 1999 – 2008, and 1999 – 2008 and 2009 – 2018, respectively.

Rescue effect

The low abundance of Suckley’s Cuckoo Bumble Bee and the possible continued declines of its main host species outside Canada make recolonization by rescue effect throughout its range in southern Canada unlikely.

Threats and limiting factors

Threats

The International Union for the Conservation of Nature – Conservation Measures Partnership (IUCN-CMP) threats calculator (IUCN-CMP 2006; Salafsky et al. 2008; Master et al. 2009) was used to classify and list threats to Suckley’s Cuckoo Bumble Bee. The calculated overall threat impact is high (Table 5).

Table 5. Threat classification table for Suckley’s Cuckoo Bumble Bee (Bombus suckleyi) across its geographic range in Canada based on the IUCN-CMP (World Conservation Union–Conservation Measures Partnership) unified threats classification system. For information on how the values are assigned see Master et al. (2009). Threats considered not applicable or negligible are included in this table; scored and unknown threats are discussed under subheadings in the report.

Species
Suckley’s Cuckoo Bumble Bee (Bombus suckleyi)

Date:
2019-04-04

Assessor(s):
Cory Sheffield (report writer), Jennifer Heron (report writer), David McCorquodale (Co-Chair), Kristiina Ovaska (Facilitator), Al Harris (Arthropods SSC), Sarah Semmler (Arthropods SSC), Elisabeth Shapiro (Canadian Wildlife Service), Rob Longair (Arthropods SSC), Purnima Govindarajulu (BC COSEWIC representative), Joanna Wilson (NWT representative), Colin Jones (Ontario representative), John Klymko (Arthropods SSC), Marie-France Chenier (COSEWIC Secretariat)

^{Classification of Threats adopted from IUCN-CMP, Salafsky et al. (2008).}

¹Impact – The degree to which a species is observed, inferred, or suspected to be directly or indirectly threatened in interest. The impact of each stress is based on Severity and Scope rating and considers only present and future threats. Threat impact reflects a reduction of a species population or decline/degradation of the area of an ecosystem. The median rate of population reduction or area decline for each combination of scope and severity corresponds to the following classes of threat impact: very high (75% declines), high (40%), medium (15%), and low (3%). Unknown: used when impact cannot be determined (e.g., if values for either scope or severity is unknown).

²Scope – Proportion of the species that can reasonably be expected to be affected by the threat within 10 years. Usually measured as a proportion of the species’ population in the area of interest. (Pervasive = 71–100%; Large = 31–70%; Restricted = 11–30%; Small = 1–10%)

³Severity – Within the scope, the level of damage to the species from the threat that can reasonably be expected to be affected by the threat within a 10-year or three-generation timeframe. Usually measured as the degree of reduction of the species’ population (Extreme = 71–100%; Serious = 31–70%; Moderate = 11–30%; Slight = 1–10%).

⁴Timing – High = continuing; Moderate = only in the future (could happen in the short term [< 10 years or 3 generations]) or now suspended (could come back in the short term); Low = only in the future (could happen in the long term) or now suspended (could come back in the long term); Insignificant/Negligible = only in the past and unlikely to return, or no direct effect but limiting

The predominant threat to Suckley’s Cuckoo Bumble Bee is the continued decline of host bumble bee subpopulations to abundances low enough to cause local extirpations of this cuckoo bee species (scored under 7.1 Other ecosystem modifications).

Where appropriate, consideration of threats to the host bumble bees are discussed concurrently. Threats are listed from highest to least impact under the associated headings below. The scope of most threats is difficult to quantify, mainly because much of the species’ range has not been surveyed for both Suckley’s Cuckoo Bumble Bee and host bumble bees.

Threat 7. Natural system modifications (medium impact)

7.3. Other ecosystem modifications (medium impact).

The most direct threat to Suckley’s Cuckoo Bumble Bee is the continuing decline of its host, Western Bumble Bee (COSEWIC 2015), and its assumed hosts Yellow-banded Bumble Bee (COSEWIC 2014) and Rusty-patched Bumble Bee (COSEWIC 2010). Approximately one-third of the Canadian range of Suckley’s Cuckoo Bumble Bee has experienced host bumble bee declines (COSEWIC 2014, 2015). Western Bumble Bee southern subspecies is designated Threatened with an inferred decline of about 50%. The approximate range of Western Bumble Bee southern subspecies is 720,170 km² and northern subspecies is 623,837 km² and Yellow-banded Bumble Bee is 7,913,612 km².

Threat 2. Agriculture and aquaculture (low impact)

2.1 Annual and perennial non-timber crops (low impact).

Habitat loss because of agricultural intensification is ongoing throughout southern portions of Canada, and primarily concentrated in the Prairies, Western Interior Basin and Mixedwood Plains ecozones, which contain some of the most highly urbanized and farmed regions in Canada (Javorek and Grant 2011; ESTR 2016). Much of Canada’s landscapes managed for agriculture have low capacities to support wildlife (Javorek and Grant 2011) and it is likely that Suckley’s Cuckoo Bumble Bee has been affected by agriculture-related habitat loss. The increased reliance on intensive agriculture over the past few decades has resulted in decreased quality foraging habitat for bumble bees globally (e.g. Williams 1989; Kosior et al. 2007), and intensive agriculture expansion has been correlated with declines in species richness and local extirpation of bumble bees in some areas (Grixti et al. 2009).

Farmland dedicated to hay production, particularly within areas of high agricultural yield, has declined in recent decades. For example, hay production in Ontario declined from approximately 1 million ha in 2001 to 696,000 ha in 2016 (decline of 31%); hay fields often also have a diversity of wildflowers, as well as abundant rodent populations, and serve as nest sites for hosts and cuckoo bumble bees. Field crops such as soybeans, grain and silage corn, winter and spring wheat, dry field beans, oats and rye increased in the same time span (Statistics Canada 2017). Soybeans are self-pollinated; and grain and silage corn are wind-pollinated, and some of these same crops also use neonicotinoids and other pesticides which are shown to adversely impact pollinators (see Threat 9.3). There is an amplified effect in the hierarchy of parasitism: factors negligible for the host bumble bee may be more serious for the cuckoo bumble bee (Sheffield et al. 2013).

Agricultural development and intensification reduce numbers of host species. Western Bumble Bee southern subspecies (COSEWIC 2014), Yellow-banded Bumble Bee (COSEWIC 2015), Rusty-patch Bumble Bee (COSEWIC 2010), as well as other bumble bees have declined within areas with intensive agriculture and the loss of natural areas from within agricultural landscapes (e.g., hedgerows, flowering weeds and natural patches of habitat). However, there are no range-wide studies.

Threat 9. Pollution (low impact)

9.3 Agricultural and forestry effluents (low impact).

At local scales pesticides could threaten host nesting subpopulations by decreasing the wildlife habitat suitability (Javorek and Grant 2011). In agricultural and urban areas, subpopulations of Suckley’s Cuckoo Bumble Bee and their hosts may be threatened by a variety of pesticides, including neonicotinoids. Neonicotinoids are a class of systemic pesticides that travel and accumulate throughout the plant, including in pollen and nectar, and specifically pose a threat to bees because they are harmful even at concentrations in the parts per billion (ppb) range (Environmental Protection Agency [EPA] 1994; Marletto et al. 2003). Neonicotinoids are commonly used on golf courses, ornamental plants and agricultural lands (Sur and Stork 2003). Large treated areas, such as golf courses, may expose bumble bees to large quantities of pesticides in otherwise suitable habitat (Tanner and Gange 2004). In dry conditions, contaminated soil can become airborne with tilling and contaminate adjacent areas where bees might be foraging or nesting (Krupke et al. 2012).

Imidacloprid (a neonicotinoid) was registered for use in the United States (in 1994) and Canada (1995) (Cox 2001, Pest Management Regulatory Agency [PMRA] 2001), coinciding with the first declines of Western Bumble Bee in western Canada. The effects of imidacloprid are not lethal to bumble bees when used as directed (e.g., Tasei et al. 2001); however, studies of its effects on bumble bees were only tested on Common Eastern Bumble Bee (B. impatiens), a commercially available species for which colonies are available and serve as an experimental model for North American bumble bee species (Gels et al. 2002; Morandin and Winston 2003).

Colonial insects which produce reproductive individuals at the end of the colony cycle can be negatively impacted by cumulative sub-lethal effects. Further study showed neonicotinoids had negative lethal and sub-lethal impacts on a European bumble bee in the same subgenus, including at levels found in crops treated as directed (Tasei et al. 2001; Whitehorn et al. 2012; Gill and Raine 2014).

Many species began exhibiting declines prior to the widespread use of neonicotinoids in North America (Colla et al. 2012). The data available on neonicotinoid use may not explain landscape levels of decline in some bumble bee species (Colla et al. 2013) but may contribute to declines at local scales.

Pesticides can have negative impacts on beneficial insects through direct exposure while foraging or in nesting habitat or indirect exposure while feeding on contaminated pollen and nectar. Effects can be lethal or sub-lethal depending on the chemical and/or concentration (Crall et al. 2017). Effects can also be synergistic with exposure to multiple pesticides (Gill et al. 2012), more specifically fungicides.

Threat 8. Invasive and other problematic species and genes (unknown impact)

8.1 Invasive non-native/alien species (unknown impact)

Multiple non-native/alien species potentially threaten subpopulations of Suckley’s Cuckoo Bumble Bee and associated host species. These are categorized and discussed below:

Pathogen spillover from managed bees

Pathogen spillover has been implicated in the significant declines of many wide-ranging animals (Morton et al. 2004; Power and Mitchell 2004) and is considered a major threat to bumble bees in North America. Pathogen spillover occurs when pathogens spread from a heavily infected ‘reservoir’ host population to a sympatric ‘non-reservoir’ host population (Power and Mitchell 2004). Managed bumble bees have been documented to have a higher than natural level of pathogens (Colla et al. 2006; Graystock et al. 2013a). The use of infected commercial bumble bees, including Common Eastern Bumble Bee, for greenhouse pollination is known to cause pathogen spillover into populations of wild bumble bees foraging nearby (Colla et al. 2006; Otterstatter and Thomson 2008).

Two unicellular parasitic species involved in pathogen spillover to wild bumble bees, Crithidia bombi (flagellate parasite) and Nosema bombi (fungal parasite), have detrimental effects on colony-founding queens, foraging workers and entire nests (Brown et al. 2000, 2003; Otterstatter et al. 2005). Commercial bumble bees have been found to have high prevalence of these parasites (approx. 34-80%; see Colla et al. 2006; Murray et al. 2013). These parasites are also found naturally in a variety of bumble bee species at lower levels (Macfarlane 1974; Macfarlane et al. 1995; Colla et al. 2006), but virulence in Suckley’s Cuckoo Bumble Bee and host bumble bees and remains unknown. Szabo et al. (2012) found that declines in the Yellow-banded Bumble Bee throughout its range in the United States and in the southern parts of its Canadian range were weakly correlated with the density of vegetable greenhouses, indicating pathogen spillover from managed greenhouse bees may be a factor threatening this species. Additional studies have found declining species to have higher pathogen loads in the wild compared to co-occurring species that are not declining (Cameron et al. 2011; Cordes et al. 2012); however, pathogen loads have been found to be highly variable in common bumble bees as well (5-44%) (Koch and Strange 2012; Malfi and Roulston 2014). Cordes et al. (2012) reported high prevalence of the microsporidium Nosema bombi (25%) in Suckley’s Cuckoo Bumble Bee, although the sample size consisted of four individuals.

Managed bumble bees

The only known landscape level change weakly correlated with declines in this species is the increasing density of vegetable greenhouses (Szabo et al. 2012). The use of managed bumble bees for field and crop pollination is likely increasing across this species’ range. Crops which use managed bumble bees include blueberry, cranberry, tomato, eggplant, cucumber, sweet pepper and strawberries. Bumble bees are primarily used for greenhouse crops but are also increasingly used for field crops. The use of bumble bees is increasing throughout Canada as they are more efficient in cooler temperatures, demand for these crops is growing and they are used as an alternative to honey bees, which have suffered major declines in recent years. Currently the movement of managed bumble bees within Canada is not tracked but the potential for these and honey bees to transmit or amplify diseases and other pests (e.g., small hive beetle) to wild bees is high throughout most provinces and territories.

In Canada, greenhouses using managed bees exist across southern BC, ON and QC and to a lesser extent in southern AB, NT and YT. In Canada, greenhouse area (including insect-pollinated vegetables such as tomatoes, cucumbers and peppers) increased 7% from 2015 to 2016 with over 15 million m² in 2016. Ontario leads the greenhouse vegetable sector accounting for more than two-thirds of all greenhouse vegetable area in Canada (70%), followed by British Columbia (20%) and Québec (6%) (Statistics Canada 2017). The increase in greenhouses translates into a decline in outdoor habitat for the bee, and a likely increase in the use of Common Eastern Bumble Bee as the greenhouse vegetable pollinator (see Threat 8.2). Pathogen spillover due to the increased use of managed bumble bees in greenhouse operations in recent decades has been implicated in the declines of the Yellow-banded Bumble Bee, the Rusty-patched Bumble Bee and the Western Bumble Bee (Thorp and Shepherd 2005; NRC 2007; Evans et al. 2008) and could provide an avenue for rapid and catastrophic disease outbreaks in the future.

Competition from managed honey bees

In agricultural and urban landscapes Suckley’s Cuckoo Bumble Bee and associated hosts likely compete for nectar and pollen with the introduced and managed Western Honey Bee (Apis mellifera). However, competition is difficult to quantify under natural conditions (Thomson 2006), so the impact in agricultural landscapes is largely unknown. The Western Honey Bee has been in North America for hundreds of years making it difficult to correlate the suspected decline of Suckley’s Cuckoo Bumble Bee and associated hosts with direct competition with managed honey bees. However, the number of managed Western Honeybee hives has increased, which thereby has likely increased competition, and there is increasing evidence that the honey bee poses threats to natural mutualisms (reviewed in Aizen et al. 2014), and that they do have direct impacts on wild bees. For instance, Cane and Tepedino (2016) calculate that during a single month an individual healthy honey bee colony can collect enough pollen that would otherwise produce 33,000 native bees, thus reducing overall fecundity of nesting native wild bees in the area.

Disease transfer

Recent studies have shown that honey bee diseases may be transmittable to bumble bees (e.g., Li et al. 2011; Peng et al. 2011). In Canada it is estimated that there are 600,000 honey bee colonies in use for pollination and honey production (Canadian Honey Council 2014) and this number is expected to grow (AAFC 2012). Given that disease is a rampant problem in managed honey bees, honey bees may pose a threat to native bumble bees. In the UK, honey bees have been documented transmitting Nosema ceranae to bumble bees (Graystock et al. 2013b). Other disease agents, such as viruses, are understudied but may pose a threat.

Other invasive species

The general threat of invasive species in the many parts of Canada is not well-studied; however, it has been identified as an important research priority (Langor et al. 2014).

Inappropriately marketed nectar/pollen plants

Other potential threats to bumble bees in urban areas are plants, including those labelled as “bee-friendly”, sold in nurseries contaminated with pesticides, and/or the use of pesticides for residential use. Bumble bee diversity and abundance was higher in gardens in France that abstained from pesticides than those that used pesticides (Muratet and Fontaine 2015), especially in gardens in urban areas. Thus, the use of insecticides and herbicides for garden, ornamental, and other residential purposes may pose a risk to all bumble bees, including this species.

8.2 Problematic native species (unknown impact)

The use of the highly competitive Common Eastern Bumble Bee, native to Canada in Ontario and Québec (Laverty and Harder 1988) but now used for pollination of greenhouse crops (e.g., tomato) and field crops (e.g., blueberry) across most of southern Canada may further impact Suckley’s Cuckoo Bumble Bee and its hosts. Common Eastern Bumble Bee may out-compete Suckley’s Cuckoo Bumble Bee for forage resources and host nesting habitats (Williams et al. 2014). The adverse impacts of bumble bees introduced for commercial pollination on native species is unknown in Canada but has been documented elsewhere (Williams and Osborne 2009; Goulson 2003b). Currently the use and movement of the Common Eastern Bumble Bee within and outside its native range within Canada is not being monitored in BC (Heron pers. comm. 2019), SK (Sheffield pers. comm. 2019), Yukon (Cannings pers. comm. 2019) or Northwest Territories (Carrière pers. comm. 2019). It is unlikely monitored in Alberta. Newfoundland and Labrador prohibit and monitor importation of bumble bees not naturally occurring in the province, including Bombus impatiens, a species widely used for greenhouse pollination (Humber pers. comm. 2019).

Threat 11. Climate change & severe weather (low impact)

11.1 – 11.2 (low impact)

Climate change is another possible threat (Williams and Osborne 2009). Bumble bee species shown to have narrow climatic tolerances are more vulnerable to extrinsic threats (Williams et al. 2009). Climatic tolerances for Suckley’s Cuckoo Bumble Bee are not currently known, but there is evidence one of the species’ hosts (Rusty-patched Bumble Bee) may be negatively impacted by climate change due to the increase in precipitation variability over time (Kerr et al. 2015).

Within the genus Bombus, some species have narrow climatic tolerances and are more vulnerable to extrinsic threats (Williams et al. 2009). A recent study of two bumble bee species that co-occur with Suckley’s Cuckoo Bumble Bee in eastern Canada and northeastern United States (Common Eastern Bumble Bee and Two-spotted Bumble Bee [B. bimaculatus]) determined that bee species are emerging 10 days earlier than a century ago due to climate change (Bartomeus et al. 2011). This could lead to mismatch of early spring forage (e.g., Miller-Rushing and Primack 2008; Bartomeus et al. 2011) or increase the likelihood of queens emerging earlier than normal (i.e. before the end of winter storms). Neither of these species are thought to be hosts for Suckley’s Cuckoo Bumble Bee; however, there may be similar patterns for the other host species.

Limiting factors

Numerous factors limit the abundance of cuckoo bumble bees:

1) Parasitism of bumble bees

A wide range of invertebrates parasitize bumble bees at all stages of the colony cycle (Schmid-Hempel 1998), and this includes cuckoo bumble bees. Spring queens which have wintered in the soil (including cuckoos) can be infected by nematodes (Sphaerularia bombi) or protozoa (Apicystis bombi) rendering them incapable of founding colonies. The internal mite Locustacarus buchneri is a common parasite that lives within the respiratory tubes and air sacs of many bumble bee species. Otterstatter and Whidden (2004) found unusually high prevalence of this parasite in Yellow-banded Bumble Bee in Alberta. Although Cryptic Bumble Bee, Western Bumble Bee and Yellow-banded Bumble bee made up only 18% of their total bumble bee sample (n= 4096), these three species accounted for 83% of infected individuals, with 9% of Yellow-banded Bumble Bee individuals infected (Otterstatter and Whidden 2004). Infection rates for nine other species studied ranged from 0-3.9% (Otterstatter and Whidden 2004). This parasite is known to adversely impact the health of bumble bees.

During the summer, bees may acquire parasites (e.g., Crithidia bombi, Nosema bombi), while foraging on flowers contaminated by infected bees. Nosema bombi is a microsporidian gut and tissue parasite of bumble bees which can reduce survival and foraging efficiency (Fisher and Pomeroy 1989). Nosema bombi infection is considered infrequent among wild bumble bees (average infection rates = 5–10%; Colla et al. 2006). Recent field surveys across the United States (Cameron et al. 2011) found the highest levels of N. bombi infection (i.e. over 35%) among declining bumble bee species, which supports the hypothesis that this parasite is a serious limiting factor. Thus, these parasites may have direct, and indirect impacts to Suckley’s Cuckoo Bumble Bee via their hosts; and a naturally occurring parasite may become a threat to the species.

2) Predators of adult bumble bees

Robber flies (Family Asilidae) and larger spiders (Arachnida) are predators of bumble bees (e.g., crab spiders [Thomisidae], jumping spiders [Salticidae] and orb weavers [Araneidae] (Copley pers. comm. 2019)). Thickheaded (Family Conopidae) and Humpbacked (Family Phoridae) flies are parasitoids of adult bumble bees. Raccoons, skunks, bears and other mammals are known to destroy and consume bumble bee colonies (Breed et al. 2004).

3) Diploid male extinction vortex

Bumble bees are haplodiploid organisms with complementary sex determination, which makes them extremely susceptible to extinction when effective population sizes are small (Zayed and Packer 2005). This is due to the ‘diploid male extinction vortex’ (Zayed and Packer 2005). The sex of a bee, and most other haplodiploid organisms, is determined by genotype at a single “sex locus”: hemizygotes (haploids) are males, heterozygotes are female, and homozygotes are sterile or non-viable males. The number of sex alleles in a subpopulation determines the proportion of diploids that are male and is itself determined primarily by the effective size of the population. Due to the production of sterile males when sex-determining locus heterozygosity is low (i.e. populations are small, and inbreeding occurs), bees are more vulnerable to habitat fragmentation than many other animal species (Packer and Owen 2001). This means that as bumble bee populations decrease in size, the frequency of diploid males will increase. Increases of diploid males in smaller populations increase the rate of population declines, causing a special case of the extinction vortex: “the diploid male extinction vortex”.

4) Lower genetic diversity in cuckoo bumble bees

Recent evidence also suggests that bumble bees with small populations have lower genetic diversity and increased susceptibility to parasites (e.g. Whitehorn et al. 2014), though this has not been studied in Suckley’s Cuckoo Bumble Bee. However, it is true of its hosts which are known to have low genetic diversity and higher than normal parasite loads (Cameron et al. 2011), supporting this pattern. These declines are likely to cause similar effects of small population size to the cuckoos that parasitize them.

5) Nectar and pollen availability to both Suckley’s Cuckoo Bumble Bee and hosts

Bumble bees are eusocial and require large inputs of floral resources (i.e. pollen and nectar) over the entire growing season to support colony growth and queen production in the fall.

6) Cuckoo bumble bees are more vulnerable to extinction than their hosts (Suhonen et al. 2015)

They are social parasites of nest-building bumble bees and depend on the distribution, abundance and colony health of the host species.

Number of locations

The term location defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present. It is not possible to calculate the number of locations for Suckley’s Cuckoo Bumble Bee in Canada. Since this species is wide ranging and the threats to this species are variable depending on the geographic area, the number of locations is more than 50 and likely in the hundreds.

Protection, status and ranks

Legal protection and status

Suckley’s Cuckoo Bumble Bee is not listed under any provincial or territorial acts in Canada. However, two of the host bumble bee species are listed under Schedule 1 of the federal Species at Risk Act (SARA): Rusty-patched Bumble Bee is listed as Endangered (June 2012) and Yellow-banded Bumble Bee is listed as Special Concern (May 2018). Western Bumble Bee occidentalis subspecies and mckayi subspecies are respectively assessed by COSEWIC as Threatened and Special Concern (2014); however, they are not listed under SARA (as of May 2019).

Non-legal status and ranks

Global Status Rank: G3 (Vulnerable) (NatureServe 2018).

Canada National Rank: N3 (Vulnerable) (last ranked June 2015) (Canadian Endangered Species Conservation Council 2016)

Provincial and territorial subnational ranks (Natureserve 2018):

BC, AB, SK, MB: S3S4 (Vulnerable/Apparently Secure) (June 2015)

YT: S2S3 (Imperiled to Vulnerable) (March 2016)

NF: SU (unknown)

ON, QC, NB, NS, PE – Not Ranked

International Union for the Conservation of Nature (IUCN) Red list Category: CR - Critically endangered

Suckley’s Cuckoo Bumble Bee has not been assessed under the United States Endangered Species Act. There was a petition to the State of California Fish and Game Commission to list Suckley’s Cuckoo Bumble Bee as Endangered under the California Endangered Species Act (Xerces Society for Invertebrate Conservation 2018). To date (August 2019) the species is not listed under this act.

Habitat protection and ownership

The Canadian range of Suckley’s Cuckoo Bumble Bee spans numerous provincial and national parks and protected areas. Records from protected areas include Cypress Hills Provincial Park (Saskatchewan), Banff National Park (Alberta), Birds Hill Provincial Park (Manitoba), Duck Mountain Provincial Park (Saskatchewan), Elk Island National Park (Alberta), Jasper National Park (Alberta), Kouchibouguac National Park (New Brunswick), Mount Revelstoke National Park (British Columbia), Riding Mountain National Park (Manitoba), Sandilands Provincial Forest (Manitoba), Prince Albert National Park (Saskatchewan), Waterton Lakes National Park (Alberta), Wood Mountain Provincial Park (Saskatchewan). There are localized efforts to ensure nectar and pollen resources are sustained across the landscape; however, most protected areas do not have initiatives to ensure pollinator habitat is sustained and climate change scenarios incorporated into planning measures.

It has been recorded on the Canadian Forces Base Shilo near Brandon, Manitoba.

Acknowledgements

British Columbia Ministry of Environment and Climate Change Strategy and Royal Saskatchewan Museum enabled time and resources for the respective authors to complete this report. Members of the Arthropods SSC provided review comments and ideas for assessment. Brett Hudson (BC Ministry of Forests, Lands, Natural Resource Operations and Rural Development) and Jenny Wu (COSEWIC Secretariat) completed the maps.

The following people provided field support and thoughts about the species, including the collection of specimens used as search effort in this report: Dawn Marks, Kyle Grant, Cara Dawson, Kirk Safford, Dennis St. John, Orville Dyer, Sara Bunge, Mark Weston, Brenda Costanzo, Josie Symonds, Katie Calon, Jamie Leatham, Darren Copley, Claudia Copley, Jeevan Sandhu, Lea Gelling, Rob Cannings, Paul Grant, Leah Ramsay, Dave Fraser, Lindsay Anderson, Lisa Tedesco, Janice Arndt, Natalie Stafl, Al Hansen, Erica McClaren, Derek Moore, Bonnie Zand, Andrew Fyson, Ryan Oram, Kristen Palmier, Joanne Neilson, Kendra Morgan, Maria Leung, Shannon Stotyn, and Syd Cannings.

Brian Sieben, Dawn Andrews, Nic Larter, Suzanne Carrière, Paul Catling, Brenda Kostiak, Alison Thompson, Rosemin Nathoo and the Wildlife Management Advisory Council (NWT), Kyle Wolki and the Community of Sachs Harbour, Sunje Aschbacher of South Nahanni Outfitters for trapping bees in the Mackenzie Mountains; Danny Allaire and J.C. Larter for assisting with trapping in the Fort Simpson area.

David Lee, Bert Dean and Rankin Inlet Hunters and Trappers Organization, Nunavut Tunngavik Inc. Department of Wildlife and Environment for facilitating permits and sampling bumble bees in Rankin Inlet, Nunavut.

Paul Grant (former Arthropods SSC Co-chair) and David McCorquodale (current Arthropods SSC Co-chair) provided editorial and review comments.

The cover photograph of Suckley’s Cuckoo Bumble Bee (photographed and collected along with four additional specimens, and deposited at the Royal Saskatchewan Museum) from Woodrow, Saskatchewan on June 30, 2019, by Cory S. Sheffield. This same photo has been shared in iNaturalist^©. Other photos of Suckley’s Cuckoo Bumble Bee by Sheila Dumesh, York University.

Authorities contacted

Anderson, Robert. Research Scientist, Canadian Museum of Nature, Ottawa, Ontario.

Bennett, Bruce. Botanist and Coordinator. Yukon Conservation Data Centre, Whitehorse, Yukon.

Boates, Sherman. Manager (retired). Biodiversity Wildlife Division Department of Natural Resources, Government of Nova Scotia, Kentville, Nova Scotia.

Cannings, Sydney. Canadian Wildlife Service, Whitehorse, Yukon.

Cardinal, Sophie. Canadian Collection of Insects, Arachnids and Nematodes, Ottawa, Ontario.

Carrière, Suzanne. Biologist, Department of Environment and Natural Resources (ENR), Government of the Northwest Territories, Yellowknife, Northwest Territories.

Copley, Claudia. Collections Manager. Royal British Columbia Museum, Victoria, British Columbia.

Cooper, Kaytlyn. Gwich'in Tribal Council, Inuvik, Northwest Territories.

Court, Gord. Provincial Wildlife Status Biologist, Dept. of Sustainable Resource Development, Edmonton, Alberta.

Desrosiers, Nathali. Biologiste en conservation, Ministère des Forêts, de la Faune et des Parcs, Québec City, Québec.

Fraser, David. Unit Head (retired). Species Conservation Science Unit. Conservation Science Section, Ecosystems Branch, British Columbia Ministry of Environment and Climate Change Strategy, Victoria, British Columbia.

Galpern, Paul. Associate Professor, University of Calgary, Faculty of Environmental Design, Calgary, Alberta.

Gauthier, Isabelle. Coordonnatrice provinciale des espèces fauniques menacées et vulnérables, Ministère des Forêts, de la Faune et des Parcs, Québec City, Québec.

Gelling, Lea. Zoologist. Conservation Data Centre, British Columbia Ministry of Environment and Climate Change Strategy, Victoria, British Columbia.

Govindarajulu, Purnima. Unit Head (Acting). Species Conservation Science Unit. Conservation Science Section, Ecosystems Branch, British Columbia Ministry of Environment and Climate Change Strategy, Victoria, British Columbia.

Hurlburt, Donna. Manager. Biodiversity Wildlife Division Department of Natural Resources, Government of Nova Scotia, Kentville, Nova Scotia.

Humber, Jessica. Ecosystem Management Ecologist, Endangered Species and Biodiversity Section, Wildlife Division, Department of Environment and Conservation, Government of Newfoundland and Labrador, Corner Brook, Newfoundland.

Jones, Colin. Provincial Arthropod Zoologist. Ontario Natural Heritage Information Centre, Ministry of Natural Resources and Forestry, Peterborough, Ontario.

Larter, Nic. Biologist, Department of Environment and Natural Resources (ENR), Government of the Northwest Territories, Fort Smith, Northwest Territories.

Needham, Karen. Spencer Entomological Collection, Beaty Biodiversity Museum at the University of British Columbia, Vancouver, British Columbia.

Pardy, Shelley. Senior Manager, Department of Environment and Conservation, Newfoundland.

Sabine, Mary. Biologist, Department of Natural Resources, Department of Environment and Conservation, Fredericton, New Brunswick.

Semmler, Sarah. Director. Living Prairie Museum, Municipality of Winnipeg, Winnipeg, Manitoba.

Stipec, Katrina. Data Management Specialist. British Columbia Conservation Data Centre, Victoria, British Columbia.

Watkins, William. Wildlife and Ecosystem Protection Branch, Manitoba Department of Conservation, Winnipeg, Manitoba.

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Biographical summary of report writers

Jennifer M. Heron is the provincial invertebrate conservation specialist with the British Columbia Ministry of Environment and Climate Change Strategy. She directs and manages the provincial approach to invertebrate conservation, including the development and implementation of provincial legislation, policy, procedures, and standards for the conservation, and recovery of invertebrate species at risk, their habitats and ecosystems, and to keep these species from becoming at risk. She has written/co-written eleven COSEWIC status reports. Her interests include the native bees of western Canada and thermal spring invertebrates.

Cory S. Sheffield has been studying bees and pollination since 1993, starting with his undergraduate thesis project at Acadia University in Nova Scotia. His Master’s research studied insect-plant interactions at Acadia followed by a PhD at the University of Guelph, Ontario. These studies focused on the bee fauna of Nova Scotia, including their diversity and contributions to crop pollination. Cory then worked on post-doctoral studies of bee taxonomy and DNA barcoding, followed by a research associate position in bee taxonomy with the Canadian Pollination Initiative (CANPOLIN) at York University, Ontario. Since 2012, Cory has been a research scientist and curator of invertebrate zoology at the Royal Saskatchewan Museum. His research continues to focus on bees: he has published on the taxonomy of Canadian/North American bees, the utility of DNA barcoding for bees, bee physiology, pollination contributions and diversity of the Canadian bee fauna.

Collections examined

The dataset used for this report comes primarily from a large dataset assembled for “An Identification Guide: Bumble Bees of North America” by Williams et al. (2014) and the same list of collections reported for other COSEWIC reports on bumble bees (COSEWIC 2010, 2014, 2015, 2018). Additional collections examined that were not in the original list are marked with an *. Recent data for Suckley’s Cuckoo Bumble Bee from Canada was also mined from Bumble Bee Watch^©, iNaturalist^© and other online sources, and the authors’ recent bumble bee collections that are not yet within museum collections.

• Academy of Natural Sciences, Philadelphia, Pennsylvania
• Algonquin Provincial Park, Ontario
• American Museum of Natural History, New York, New York
• André Francoeur Research Collection, Chicoutimi, Québec
• Atlantic Canada Conservation Data Centre, Sackville, New Brunswick
• B. Hicks Personal Collection, College of the North Atlantic, Carbonear, Newfoundland
• Bee Biology and Systematics Laboratory, Logan, Utah
• Bohart Museum, University of California, Davis, California
• Biodiversity Institute of Ontario, Guelph, Ontario
• B. Jacobsen, Greer Labs, Inc, Lenoir, North Carolina
• British Natural History Museum, London, United Kingdom
• C. Buidin/ Y. Rochepault Research Collection, Montréal, Québec
• C. Looney Research Collection, Olympia, Washington
• C. S. Sheffield Research Collection, Regina, Saskatchewan
• Canadian Forestry Service, Québec, Québec
• Canadian Museum of Nature, Ottawa, Ontario
• Canadian National Collection of Insects, Arachnids and Nematodes, Ottawa, Ontario
• College of the North Atlantic, Carbonear, Newfoundland and Labrador
• Connecticut Agricultural Extension Station, New Haven, Connecticut
• D.H. Miller private collection
• E. Nardone Research Collection, Guelph, Ontario
• Essig Museum of Entomology, Berkeley, California
• E. Normandin Research Collection, Laval, Québec
• Illinois Natural History Survey, Champaign, Illinois
• Insectarium Réne-Martineau, Québec, Québec
• Wallis-Roughley Museum of Entomology, University of Manitoba, Winnipeg, Manitoba
• K. Martins Research Collection, Montréal, Québec
• L. Richardson Research Collection, Hanover, New Hampshire
• LA County Museum, Los Angeles, California
• Laval University, Québec, Québec
• Lethbridge Agricultural Research Station, Lethbridge, Alberta
• Lyman Entomological Collection-McGill University, Montréal, Québec
• Madison-University of Wisconsin, Madison, Wisconsin
• M. Savard Research Collection, Saint-Fulgence, Québec
• Ministère des Ressources naturelles et de la Faune Québec, Various, Québec
• National Pollination Insect Collection (Logan), Logan Utah
• New York State Museum, Albany, New York
• North Carolina State University, Raleigh, North Carolina
• Nova Scotia Dept. Natural Resources, various, Nova Scotia
• Nova Scotia Museum, Halifax, Nova Scotia
• Ohio State University, Columbus, Ohio
• Oregon State Arthropod Collection, Corvallis, Oregon
• P.H. Williams Research Collection, London, United Kingdom
• Packer Collection York University, Toronto, Ontario
• Patuxent Wildlife Research Center, Laurel, Maryland
• P. Hallett Personal Collection, Toronto, Ontario
• R. Gegear Research Collection, Toronto, Ontario
• Royal British Columbia Museum, Victoria, British Columbia
• Royal Ontario Museum, Toronto, Ontario
• Royal Saskatchewan Museum, Regina, Saskatchewan
• S. Javorek Research Collection, Kentville, Nova Scotia
• S. Colla Research Collection, Toronto, Ontario
• Spencer Entomological Collection, Beaty Biodiversity Museum at the University of British Columbia, Vancouver, British Columbia
• *University of Calgary, Calgary, Alberta
• University of Colorado, Boulder, Colorado
• University of Massachusetts, Worchester, Massachusetts
• University of Minnesota, Minneapolis, Minnesota
• University of Michigan, Ann Arbor, Michigan
• University of Alaska, Fairbanks, Alaska
• University of Connecticut, Storrs, Connecticut
• University of Guelph, Guelph, Ontario
• University of Idaho, Moscow, Idaho
• University of New Hampshire, Durham, New Hampshire
• University of Prince Edward Island, Charlottetown, Prince Edward Island
• University of Nevada, Reno, Nevada
• V. Fournier Research Collection, Laval, Québec
• Yale Peabody Museum, New Haven, Connecticut