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Suckley's Cuckoo Bumble Bee
Bombus suckleyi
NatureServe conservation status
Global (G-rank): G2G3
State (S-rank): S1
External links
Phenology
Cuckoo bumble bees such as B. suckleyi have relatively unique life cycles compared to other bumble bee species. Unlike species that emerge in the spring to begin a new colony, B. suckleyi emerges slightly later in the year when colonies of other species have begun to be established. Though the exact phenology is poorly known, B. suckleyi is usually observed from late May to early October, with peaks in activity in June (CBD, 2020; Williams et al., 2014). After emerging, mated females will locate and enter the colonies of their host bumble bee species, where they kill or otherwise subdue the queen and take control of the colony workers through a mix of aggression and pheromones (CBD, 2020; Williams et al., 2014). They will then take advantage of the hive’s collected pollen and nectar to provision their own offspring, which are exclusively reproducing females and males (no queens or workers). As these offspring develop over the course of five weeks, they will leave the nest to mate, and mated females will overwinter to begin the cycle anew the following year (USFWS 2024).
Species range
B. suckleyi is historically scattered across the continent, with historic observations in 11 western states and 11 Canadian provinces; Colorado, Montana, Washington, and Utah make up the bulk of historic occurrences (CBD, 2020; Williams et al., 2014). It has undergone significant range reduction, with some estimates indicating it currently occupies less than 43% of its historic range; the same analysis estimates it exists at less than 10% of its previous relative abundance (Hatfield et al., 2015a). One of the more recent and extensive surveys of bumble bee habitat failed to find any B. suckleyi in the Wasatch and Uinta Mountains of Utah, in agricultural areas and adjacent habitat where the species historically occurred (CBD, 2020; Strange & Tripodi, 2019). The declines of this species are linked to declines of its host species, primarily B. occidentalis, which has seen a >70% reduction in relative abundance throughout its range from historic levels (Hatfield et al., 2015b). Other host species in decline include B. terricola, B. fervidus, B. nevadensis, and B. appositus (CBD, 2020). It should be noted that while B. suckleyi has been documented within colonies of all these species, it has only ever been recorded breeding in colonies of B. occidentalis and B. nevadensis (CBD, 2020; Thorp et al., 1983; USFWS, 2024). Declines in these host species have resulted in 0 observations of B. suckleyi within Utah between the years 2002-2019.
Habitat
Because they are obligate social parasites, the habitat needs of B. suckleyi are closely related to the needs of their host species. Specifically, B. suckleyi requires suitable nesting sites to locate host species colonies, access to diverse floral resources during the forage season, and sites that are suitable for overwintering females. Relatively little is known about overwintering sites compared to the other habitat needs of the species, in part because these sites are difficult to locate. That being said, B. suckleyi likely uses mulch and other decomposing vegetation for overwintering (USFWS, 2024). Host bumblebee nests are often located in abandoned underground burrows such as those left by rodents, in a diverse set of habitat types including woodlands, open meadows/agricultural fields, and even among man-made structures (USFWS, 2024).
One of the most important habitat requirements for B. suckleyi is the availability of suitable floral resources during the active forage season, as colonies subsist on pollen and nectar. Along with a variety of other factors, future bumblebee population size is determined by the amount of pollen available to foraging bees, as pollen provides the protein and nutrition required to produce new reproducing males and females; though the parasitic nature of these bees means they are not often the ones foraging, their host nests still depend upon the availability of floral resources. It would also be wrong to say that B. suckleyi never forages; though their foraging behavior differs from eusocial bees, males and females will still forage for pollen and nectar before infiltrating and after leaving a host colony (USFWS, 2024). B. suckleyi is a generalist forager that can be seen in flight in meadows and grasslands. Its host species, B. occidentalis, is a short-tongued species that requires small, open-faced flowers in grasslands, chaparral/shrublands, and open mountain meadows (CBD, 2020; Williams et al., 2014).
Food habits
As mentioned previously, B. suckleyi is a generalist forager with numerous floral associations. It has been observed that cuckoo bumble bee males visit more flowers for longer periods of time than eusocial bumble bees on their foraging flights; it has been suggested that this is the result of different energy needs, as cuckoo bumble bees do not collect pollen, and so balancing their energy intake with expenditure is less important (Fisogni et al., 2021; USFWS, 2024). The lack of pollen-collecting capability is a key difference between this species and eusocial bees; as mentioned above, they are dependent upon their host bees to collect pollen that will be used to provision B. suckleyi offspring.
Ecology
Bumble bees possess a number of traits that make them effective pollinators. Unlike many other bees, bumblebees vibrate their flight muscles to “buzz” flowers while pollinating, a behavior which helps shake pollen from flower anthers; furthermore, their hairy bodies provide a surface that pollen naturally sticks well to as bees travel between flowers (Williams et al., 2014). Individual bumblebee workers often visit just one species of plant at a time, which minimizes transfer of pollen between unreceptive species. Bumblebees are such effective pollinators that several wild flowers depend exclusively on bumblebees for pollination (Defenders of Wildlife, 2015). The plant species associated with B. suckleyi mimic that of its host, B. occidentalis: Ceanothus, Centaurea, Chrysothamnus, Cirsium, Eriogonum, Geranium, Grindellia, Lupinus, Melilotus, Monardella, Rubus, Solidago, Trifolium (RICP, n.d.).
Threats or limiting factors
Habitat loss due to human development in grassland and meadow ecosystems is one of the main drivers of decline in this species; the loss of sagebrush steppe habitat in the intermountain west has left much B. suckleyi’s remaining habitat in montane meadows, primarily in public lands where the species is threatened by livestock grazing and adjacent agricultural use (CBD, 2020). Grazing in particular can be harmful to bumble bees, as floral resources that bees rely upon are directly removed. Livestock may also trample nesting and overwintering sites, and reduce the number of suitable nesting locations available to bumble bees (Killingsworth et al., 2023; Xerces Society, 2022). In the long term, poorly managed grazing changes the makeup of plant communities on the landscape, with more unpalatable invasive species surviving and spreading, which in turn provide less benefit to pollinators such as B. suckleyi and its host species. Fire and bark beetle management, as well agricultural intensification, are also drivers of habitat loss for this species (CBD 2020).
The small population size of B. suckleyi relative to its host species makes it particularly susceptible to diseases. It has recently been shown that the commercial rearing of bumble bees was likely responsible for the spread of Vairimorpha bombi, a fungal pathogen, across North America; this pathogen has been found in B. suckleyi, and is also found in high levels within the wild B. occidentalis population (Cameron et al., 2011; Cameron et al., 2016). Introduced honey bees also pose a threat to B. occidentalis and other native bees: in addition to carrying diseases that can transmit to native pollinators, honey bees can competitively exclude native bees and force them to forage plants that provide less pollen and nectar.
Finally, climate change is a major threat to the continued conservation of this species. Habitat loss has reduced the existing habitat of B. suckleyi to montane areas in the intermountain west, in areas where climate models predict hotter and drier conditions as climate change progresses (CBD, 2020). Increased temperature and drought, increased variability in temperature and precipitation, early snow melt, and late frost events may lead to changes in pathogen pressure, the availability of floral habitat, and the availability of nesting sites suitable for B. suckleyi host species (Xerces Society, 2022). Phenological mismatch may also increase; that is, the timing of flower blooms and bee life cycles may no longer correspond to one another, leaving fewer food sources available for bumble bees during periods critical for hive persistence and reproduction. In the Rocky Mountains, a new mid-summer period of low floral resources has been linked to climate change; this provides an example of phenological mismatch that may disrupt the life cycle of bumble bee colonies (Alridge et al., 2011).
