SPECIES: Scientific [common] Bombus occidentalis [Western bumble ] Forest: Salmon-Challis National Forest Forest Reviewer: Mary Friberg Date of Review: 2/23/2018 Forest concurrence (or Yes recommendation if new) for inclusion of species on list of potential SCC: (Enter Yes or No)

FOREST REVIEW RESULTS:

1. The Forest concurs or recommends the species for inclusion on the list of potential SCC: Yes_X__ No___

2. Rationale for not concurring is based on (check all that apply): Species is not native to the plan area ______Species is not known to occur in the plan area ______Species persistence in the plan area is not of substantial concern ______

FOREST REVIEW INFORMATION:

1. Is the Species Native to the Plan Area? Yes_X__ No___

If no, provide explanation and stop assessment.

2. Is the Species Known to Occur within the Planning Area? Yes_X__ No___

If no, stop assessment. Table 1. All Known Occurrences, Years, and Frequency within the Planning Area Year Number of Location of Observations (USFS Source of Information Observed Individuals District, Town, River, Road Intersection, HUC etc.) 2017 1 Salmon-Cobalt Ranger District Personal communication, Katherine Baumann (November 6, 2017) 1993 1 Cobalt Ranger Station (Lemhi The Global Biodiversity County) Information Facility Occurrence Download (October 2017) 1967 26 8 miles south of Cobalt, ID The Global Biodiversity (Lemhi County) Information Facility Occurrence Download (October 2017) 2017 1 Donnelly Gulch (Lemhi County) Personal communication, Katherine Baumann (November 6, 2017) 1967 3 3 miles north of North Fork, ID The Global Biodiversity (Lemhi County) Information Facility Occurrence Download (October 2017) 1965 8 5 miles north of North Fork, ID The Global Biodiversity (Lemhi County) Information Facility Occurrence Download (October 2017) 1934, 1960 21 Gibbonsville, ID (Lemhi County) The Global Biodiversity Information Facility Occurrence Download (October 2017) 1940 6 ~12 miles SW of North Fork, ID The Global Biodiversity (Lemhi County) Information Facility Occurrence Download (October 2017) 1926 1 Middle Fork, Salmon River The Global Biodiversity (Lemhi County) Information Facility Occurrence Download (October 2017) 1966 1 Jordan Creek (Custer County) The Global Biodiversity Information Facility Occurrence Download (October 2017) 1974 1 Elk Creek Ranger Station (Valley The Global Biodiversity County) Information Facility Occurrence Download (October 2017)

a. Are all Species Occurrences Only Accidental or Transient?

Yes___ No_X__

If yes, document source for determination and stop assessment.

b. For species with known occurrences on the Forest since 1990, based on the number of observations and/or year of last observation, can the species be presumed to be established or becoming established in the plan area?

Yes_X__ No___

If no, provide explanation and stop assessment

c. For species with known occurrences on the Forest predating 1990, does the weight of evidence suggest the species still occurs in the plan area?

Yes___ No___

Provide explanation for determination

N/A – occurrences have been documented since 1990

If determination is no, stop assessment

d. Map 1, Western bumble bee range in (IDFG 2017a)

IDFG (Idaho Department of Fish and Game). 2017. Western bumble bee (Bombus occidentalis). Internet website: https://idfg.idaho.gov/species/taxa/ 78169. Accessed on October 23, 2017.

e. Map 2, Western bumble bee occurrences on the Salmon-Challis National Forest (Idaho Fish and Wildlife Information System [January 2017]

3. Is There Substantial Concern for the Species’ Capability to persist Over the Long-term in the Plan Area Based on Best Available Scientific Information?

Table 2. Status summary based on existing conservation assessments

Entity Status/Rank (include definition if Other) NatureServe G4- Apparently secure (Uncommon but not rare; some cause for long-term concern due to declines or other factors) Global Rank NatureServe S3- Vulnerable (due to a restricted range, relatively few populations (often 80 or fewer), recent and widespread declines, or State Rank other factors making it vulnerable to extirpation. State List IDAPA- Status SGCN- Tier 1 (Highest priority for the SWAP; represents species with the most critical conservation needs, i.e., an early-warning list of taxa that may be heading toward extirpation) USDA Forest No Region 1 Status Service No Region 4 Status

USDI FWS No species profile Other Not BLM Type 2 Xerces Society Red List Species – Imperiled (At high risk of extinction due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors.)

Table 3. Status summary based on best available scientific information.

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations 1 B2 The Western bumble bee is a year-round resident of the Salmon-Challis Distribution on National Forest (IDFG 2017a). Its estimated range encompasses all of the Salmon-Challis Forest (see Map 1). In historic surveys, it was widely distributed on the National Forest Forest (see Table 1). Since 1998, however, the Western bumble bee has experienced significant population (see Criterion 5) and range (see Criterion 2) declines, and using historic occurrence records to determine Cameron et al. 2011. Patterns of present distribution would likely overestimate its current presence widespread decline in North (Cameron et al. 2011). 2017 surveys found occurrences on the Salmon- American bumble . Cobalt Ranger District, confirming its presence on the Forest (see Table Proceedings of the National 1). Due to its mobility (see Criterion 3) and habitat requirements (see Academy of Sciences: Vol. 108: No. Criterion 6), potential habitat is widely distributed across the Forest, and 2. connectivity between habitat patches is good.

Based on distribution maps and habitat requirements, the Western IDFG (Idaho Department of Fish bumble bee is probably widely distributed across the Forest in patches of and Game). 2017a. Western suitable habitat, exhibiting metapopulation dynamics with some isolated bumble bee (Bombus occidentalis). subpopulations (Rank B2). Confidence in this assessment is low, however, Internet website: considering recent population trends and limited recent current https://idfg.idaho.gov/species/taxa occurrence data. /78169. Accessed on October 23, 2017. Confidence in Rank: High, Medium, or Low

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations 2 C The Western bumble bee is widely distributed in western North America Cameron et al. 2011. Patterns of Distribution in (NatureServe 2017). Its western distribution ranges from southern British widespread decline in North surrounding Columbia to central and extends eastward to the Dakotas, American bumble bees. geographic area , , and (NatureServe 2017). Additional Proceedings of the National populations of the subspecies Bombus occidentalis mckayi occur in Academy of Sciences: Vol. 108: No. northern , Yukon, and (NatureServe 2017). 2. Bombus occidentalis has been documented from sea level to over 2,000 m (Williams et al. 2014 in Hatfield et al. 2015). Recent surveys have Hatfield, R., S. Jepsen, R. Thorp, L. documented its presence in the , but the current range Richardson, S. Colla, and S. Foltz of the Western Bumble bee is estimated to be reduced by 28% from Jordan. Bombus occidentalis. The historic occurrences, particularly along the western margin of its range IUCN Red List if Threatened Species (Cameron 2011). Statewide conservation assessments indicate that the 2015: e.T44937492A46440201. Western bumble bee is critically imperiled along the western edge of its http://dx.doi.org/10.2305/IUCN.UK distribution. Recent surveys have confirmed its persistence in northern .2015- Idaho (Rhoades et al. 2016). 2.RLTS.T44937492A46440201.en NatureServe. 2017. NatureServe Despite recent range contractions, the Western bumble bee is still widely Explorer: An online encyclopedia of distributed outside the Salmon-Challis National Forest (Rank C). life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Confidence in Rank: High, Medium, or Low Internet website: http://explorer.natureserve.org. Accessed on August 29, 2017.

Rhoades et al. 2016. Evidence for Bombus occidentalis (: ) populations in the Olympic Peninsula, the Palouse Prairie, and Forests of Northern Idaho. Journal of Science: Vol. 16: No. 1.

3 C The Western bumble bee is a central place forager, meaning that Hatfield, R., S. Jepsen, R. Thorp, L. Dispersal foraging adults depart from and return to a fixed colony location. It is Richardson, S. Colla, and S. Foltz Capability also a social species, producing individuals that belong to either a non- Jordan. 2015. Bombus occidentalis. mating worker caste or a reproductive caste. Mated females (gynes) The IUCN Red List of Threatened leave their natal nest to establish new colonies; therefore movement of Species 2015: gynes, not workers, is the source of genetic and colonial dispersal. eT44937492A46440201. Temporally, dispersal is limited to the late summer or fall, when mated gynes leave their natal nest, and early spring, when mated gynes Lepais et al. 2010. Estimation of (queens) establish new colonies (Hatfield et al. 2015). Studies in the UK queen dispersal that used a conservative method to estimate dispersal distances of distances using sibship Bombus pascuorum and B. lapidarius queens indicate that queens reconstruction method. Molecular establishing new colonies typically disperse up to 3-5 km, and up to 8 km, Ecology: Vol. 19. from their natal nest (Lepais et al. 2010). Evidence suggests isolation between patches separated by 10 km or more (Lepais et al. 2010).

Based on studies of related taxa, the Western bumble bee appears to be able to disperse readily between habitat patches under 5 km apart (Rank C). Confidence in this rank is medium due to the lack of species-specific information.

Confidence in Rank: High, Medium, or Low 4 B No abundance estimates exist for the Western bumble bee on the Cameron et al. 2011. Patterns of Abundance on the Salmon-Challis National Forest, although suitable habitat is common on widespread decline in North Salmon-Challis the Forest (IDFG 2017). In nearby habitat in , the Western American bumble bees. National Forest bumble bee is one of the most abundant bumble bee species Proceedings of the National represented in historic (prior to 1999) collections. The relative Academy of Sciences: Vol. 108: No. abundance of the Western bumble bee to all bumble bee species has 2. sharply decreased from historic (prior to 1999) to present (2007-2009) surveys, however, with the Western bumble bee representing less than IDFG (Idaho Department of Fish 10% of specimens from current surveys. Decreases in relative abundance and Game). 2017. Idaho State in the Rocky Mountains are not as severe as in other parts of the Wildlife Action Plan, 2015. Boise, Western bumble bee’s historic range (see Criterion 3). ID.

Current occurrence records for the Forest (see Table 1) and survey data Rhoades et al. 2016. Evidence for from nearby areas, combined with broad habitat availability, suggest that Bombus occidentalis the Western bumble bee is probably uncommon on the Forest (Rank B). (Hymenoptera: Apidae) Confidence is low due to the lack of abundance data on the Forest, as populations in the Olympic well as the paucity of current occurrence records. Peninsula, the Palouse Prairie, and

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations Forests of Northern Idaho. Journal Confidence in Rank: High, Medium, or Low of Insect Science: Vol. 16: No. 1.

5 A There is no documentation of Western bumble bee population trends on Cameron et al. 2011. Patterns of Population Trend the Salmon-Challis National Forest (IDFG 2017; Rhoades et al. 2016). widespread decline in North on the Salmon- Current occurrence records are narrowly distributed compared to American bumble bees. Challis National historic occurrence records on the Forest (see Table 1), but this could be Proceedings of the National Forest a byproduct of sampling effort. Based on a dramatic decrease in relative Academy of Sciences: Vol. 108: No. abundance in historic records compared to current surveys (see Criterion 2. 4), it can be inferred that Western bumble bee populations are declining in the Intermountain West (Cameron et al. 2011). Recent surveys in Dolan, A. C., C. M. Delphia, K. M. Montana also provide some evidence of Western bumble bee population O’Neill, and M. A. Ivie. 2017. declines (Dolan et a. 2017). Over its entire range, it is estimated that the Bumble bees (Hymenoptera: Western bumble bee has declined by an average of 40% over the past 10 Apidae) of Montana. Annals of the years (Hatfield et al., 2015). If current trends continue, relative Entomological Society of America: abundance of the Western bumble bee is predicted to dip below 0% in Vol. 110: No. 2. 60-70 years. Hatfield, R., S. Jepsen, R. Thorp, L. Based on available evidence, it appears that Western bumble bee Richardson, S. Colla, and S. Foltz populations are declining on the Forest (Rank A). Confidence is low due Jordan. 2015. Bombus occidentalis. to the lack of documentation of population trends or current occurrence The IUCN Red List of Threatened records. Species 2015: eT44937492A46440201. Confidence in Rank: High, Medium, or Low IDFG (Idaho Department of Fish and Game). 2017. Idaho State Wildlife Action Plan, 2015. Boise, ID.

6 B The Western bumble bee is a generalist forager (Jepsen et al. 2014), and Bowers, M. A. Bumble bee Habitat Trend on so long as its three key habitat requirements are present [(1) colonization, extinction, and the Salmon-Challis overwintering sites- queens overwinter in small cavities on or just below reproduction in subalpine National Forest the soil; (2) nest sites- bumble bees typically nest in pre-existing meadows in northeastern . underground cavities, such as abandoned rodent burrows; and (3) floral Ecology: Vol. 55: No. 3. resources- bumble bees feed exclusively on pollen and nectar in all life stages and require access to sufficient flowering plants throughout the Cook, S. P., S. M. Birch, F. W. colony life cycle (early February to late November) (Goulson 2003; Jepsen Merickel, C. C. Lowe, and D. Page- et al. 2014)], it has broad habitat specificity (IDFG 2017). Dumroese. 2011. Bumble bee (Hymenoptera: Apidae) community The Western bumble bee can utilize subalpine meadows (Bower 1985), structure on two sagebrush steppe grasslands, and shrublands, including sagebrush steppe (Cook et al. sites in southern Idaho. The Pan- 2011), with sufficient and temporally stable floral resources (IDFG Pacific Entomologist: Vol. 87: No.3. 2017b). Bumble bee reproduction is particularly dependent on floral resource availability late in the season (see Criterion 8). Bumble bees Goulson, D. E. 2003. : have a high tolerance to cold and rainy conditions compared to other Their Behavior and Ecology. Oxford North American bee taxa (Jepsen et al. 2014). Colony performance University Press. appears to be associated with patch size (Bowers 1985). IDFG (Idaho Department of Fish It is difficult to assess habitat trends on the Forest, since pockets of and Game). 2017. Idaho State suitable habitat are likely to be present in a wide range of vegetation Wildlife Action Plan, 2015. Boise, types and at small scales. Based on Forest Service maps there are roughly ID. 111,000 acres of riparian herbaceous and 280,000 acres of upland herbaceous habitat on the SCNF. In general, riparian vegetation has seen Jepsen, S., S. F. Jordan, and R. Huff. past declines, but is largely trending towards the natural range of 2014. Species fact sheet: Western variability (USFS 2017). Trends are not known for upland herbaceous bumble bee (Bombus occidentalis). ecosystems including subalpine meadows. Grasslands across the Forest largely show very low to moderate departure from conditions pre- USFS (United States Department of European settlement. Sagebrush communities show largely low Agriculture, Forest Service). 2017. departures. Salmon Challis National Forest Data Assessment, Terrestrial Ecosystems Livestock grazing on the SCNF has a long history and is an important and Section (Draft). extensive land use there. For example, approximately 51% of the Forest is currently contained in grazing allotments, although roughly 1/3rd are vacant or closed. Sheep allotments have been reduced to five. Historic heavy grazing caused a shift in several plant communities (e.g. grassland extent increased) on the SCNF and resulted in shrubland and conifer species expansion into habitat supporting floral resources (USFS 2017). This has been further exacerbated by fire suppression policies of the 1950s through 1990s. Livestock operations have shifted to more intensively managed grazing systems to respond to concerns over impacts to riparian ecosystems and distribution of grazing effects across

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations landscape. In addition, grazing by sheep on the Forest, which can impact floral resources, has declined steeply since 1996 (USFS 2017).

The usurpation of native flowering plants by invasive grasses or other low-reward flowering plants can disrupt the amount and timing of floral resources for the Western bumble bee. The extent of invasive cheatgrass on the SCNF, which displaces native flowering plans, is not known, but the estimate within greater sage-grouse habitat is 106,955 acres or 23% (USFS 2017). Invasive thistles and knapweeds are a problem on the Forest, but provides an important late season food source for bumble bees (See Criterion 7).

Thus, the information on trends in floral resources is scarce, but broad habitat specificity, abundance of potentially suitable habitat, and trends in their quantity and quality suggest Western bumble bee habitat on the Forest as a whole is likely stable (Rank B). Confidence is low due lack of information on the habitat that meets all three requirements for the Western bumble bee.

Confidence in Rank: High, Medium, or Low 7 B Across its range, declines of the Western bumble bee appear to be Carson, B. D. 2013. The biological Vulnerability of influenced more by life history and demographics than by habitat loss control of spotted knapweed and Habitats on the (see Criterion 8). However, habitat fragmentation and loss also play a conservation of associated Salmon-Challis role in declines of several North American bumble bee species, including pollinator communities. Master’s National Forest the Western bumble bee (IDFG 2017). thesis.

Stable floral resources for the Western bumble bee are vulnerable to Hatfield, R., S. Jepsen, E. Mader, S. unrestricted grazing, the spread and control of noxious weeds, and global H. Black, and M. Shepherd. climate change. Conserving bumble bees: Guidelines for creating and Overgrazing can lead to soil compaction and subsequent destruction of managing habitat for America’s potential nest sites (Hatfield et al. 2012). Grazing effects on bumble bee declining pollinators. The Xerces forages depends on the timing, intensity, and livestock type (Hatfield et Society for Invertebrate al. 2012). Moderate grazing at the right time of year by cattle can benefit Conservation. floral resources for bumble bees, but grazing by sheep, overgrazing, and grazing in mid to late summer and early fall when floral resources are IAP (Intermountain Adaption scarce can negatively affect temporal floral resource stability for bumble Partnership). 2016. Intermountain bees (Hatfield et al. 2012). Improper livestock grazing can reduce native Adaption Partnership: Climate plant vigor and stressors such as climate change that manifest as Vulnerability Assessment prolonged drought and increased mean annual temperature is expected Summaries (Draft). to elevate the invasibility of grasslands (USFS 2017). IDFG (Idaho Department of Fish The usurpation of native flowering plants by invasive grasses or other and Game). 2017b. Idaho State low-reward flowering plants can disrupt the amount and timing of floral Wildlife Action Plan, 2015. Boise, resources for the Western bumble bee. However, bumble bees are ID. generalist foragers (e.g. Tepedino, Bradley, and Griswold 2008), and Kerr et al. 2015. Climate change although flowering invasive weeds such as thistles and knapweeds may impacts on bumblebees converge disrupt pollination networks for native plants, they are very attractive across continents. Science: Vol. floral resources for bumble bees and can actually increase native bee 349: Issue 6244. abundance, particularly later in the season and in the absence of native flowering plants (Carson 2013). Control of invasive weeds is a high IDFG (Idaho Department of Fish priority for both the State of Idaho and Federal land managers and and Game). 2017b. Idaho State aggressive control of high reward invasive flowering weeds on the SCNF Wildlife Action Plan, 2015. Boise, could be to the detriment of the Western bumble bee. But re-seeding ID. treated areas with native forbs and controlling invasive annual grasses while perennial seedlings re-establish can improve the quality of Western Tepedino, V. J., B. A. Brosey, and T. bumble bee habitat (USFS 2015). L. Griswold. 2008. Might flowers of invasive plants increase native bee Western bumble bee populations are expected to be vulnerable to the carrying capacity? Intimations from effects of global climate change (IAP 2016). Increasing mean Capitol Reef National Park, Utah. temperatures and a decrease in the snow to rain ratio of precipitation Natural Areas Journal: Vol. 28: No. are expected to lead to decreased soil water availability during seed 1. establishment and summer plant growth, as summer growth is positively correlated with winter precipitation and snow depth (IAP 2016). Late USFS (United States Forest Service) season floral resources are important for Western bumble bee Salmon-Challis National Forest. reproduction, so reduced summer plant growth could negatively impact 2015. Salmon-Challis National colonies’ reproductive capacities (see Criterion 8). In the Middle Rocky Forestwide Invasive Plant Mountain region, subalpine forblands are predicted to be highly sensitive Treatment. to climate change (IAP 2016). Subalpine forblands have a low to medium adaptive capacity due to historic high intensity grazing and the lack of USFS (United States Forest Service) developed soil at higher elevations (IAP 2016). In North America, bumble Salmon-Challis National Forest. bee species are not increasing the northern margin of their ranges, but 2016. Forest plan monitoring and

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations are shifting to higher elevations in their southern ranges (Kerr et al. evaluation report: Salmon-Challis 2015). This suggests that global climate change will result in range National Forest: Salmon and Challis constriction of bumble bees in North America. Forest plans FY 2012, 2013, 2014, and 2015. In summary, declines in Western bumble bee populations appear to be tied more to its life history and demographics than habitat loss. However, USFS (United States Department of they may be moderately vulnerable to modification of habitat by spread Agriculture, Forest Service). 2017. and control of invasive weeds, global climate change, and unrestricted Salmon Challis National Forest Data grazing (Rank B). Confidence is medium due to uncertainty inherent in Assessment, Rangeland models of global climate change. Management and Grazing Section (Draft). Confidence in Rank: High, Medium, or Low

8 A The Western bumble bee has an annual colony life cycle; overwintered Cameron et al. 2011. Patterns of Life History and queens establish nests in the spring and produce non-mating female widespread decline in North Demographics workers that assist in colony growth and development (Goulson 2003). In American bumble bees. the summer and fall, successful colonies begin to produce gynes (future Proceedings of the National queens) and males (Hatfield 2015; Goulson 2003). In the late fall, Academy of Sciences: Vol. 108: No. colonies die out and future queens overwinter until the next spring. This 2. annual life cycle makes the Western bumble bee vulnerable to temporal fluctuations in floral resources. Lack of early spring resources can result Cane, J.H. and V.J. Tepedino. in an inability for queens to establish colonies, lack of spring and early Gauging the effects of honey bee summer resources will hinder colony growth, and lack of summer and pollen collection on native bee early fall resources can result in a failure to create reproducing castes. communities. 2016. Conservation Letters 0(0):1-6. doi: The typical reproductive capacity of the Western bumble bee is 10.1111/conl.12263. unknown. In general, bumble bee reproduction is variable, and well over half of colonies fail to produce any gynes or only produce males (Goulson Furst, M.A., J.L. McMahon, R.J. 2003). A study observing 16 Western bumble bee colonies found that Paxton, and M.J.F. Brown. 2014. only one colony produced a single female gyne, indicating extremely low Disease associations between levels of reproduction (Gamboa et al. 1987). honeybees and bumblebees as a threat to wild pollinators. Nature The genetics of the Western bumble bee also increases its sensitivity to 506:364-373. low population size. Bumble bees are haplodiploid (males only have one doi:10.1038/nature12977. set of chromosomes), and sex is determined by heterozygosity at a single locus (Lepais et al. 2010). Decreased genetic variation increases the risk Gamboa, G. J., R. L. Foster, and K. of production of homozygous, sterile gynes. Heterozygosity is W. Richards. 1987. Intraspecific significantly lower in the Western bumble bee than in co-occurring, nest and brood recognition by stable populations of other bumble bee species (Cameron et al. 2011). queens of the bumble bee, Bombus This increases the risk of production of sterile gynes and decreases the occidentalis (Hymenoptera: effective population size of the Western bumble bee. Apidae). Canadian Journal of Zoology: Vol. 65. Some recent research suggests that competition between bumble bees and the introduced European honeybee can reduce bumble bee foraging Goulson, D. E. 2003. Bumblebees: efficiency, worker size, and reproduction (Hatfield et al. 2016) and can Their Behavior and Ecology. Oxford interact with climate to drive local declines of native pollinators University Press (Thomson 2016). Modeling indicates competition may be quite fierce with a 40-hive honey bee apiary collecting over 3 months the pollen Hatfield, R., S. Jepsen, R. Thorp, L. equivalent needed to produce four million wild bee progeny (Cane and Richardson, S. Colla, and S. Foltz Tepedino 2016). Furthermore, bumble bees can contract diseases and Jordan. 2015. Bombus occidentalis. pathogens from honey bees visiting the same floral resources (Furst, et The IUCN Red List of Threatened al. 2014; Hatfield et al. 2016). Pathogen and parasite infections, Species 2015: particularly by the microsporidian bombi, are strongly associated eT44937492A46440201. with Western bumble bee decline (Cameron et al. 2011; Shykoff and Schmid-Hempel 1991, Imhoof and Schmid-Hempel 1999, all cited in Rao Hatfield, R., S. Jepsen, E. Mader, S. and Stephen 2007). The spread of N. bombi is facilitated by the H. Black, and M. Shepherd. 2012. worldwide bumble bee trade to pollinate nursery plants. Infected bumble Conserving bumble bees: bees escape into wildlands where the disease is spread. Guidelines for creating and managing habitat for America’s Other significant pathogens and parasites affecting the Western bumble declining pollinators. The Xerces bee include the trypanosome Crithidia bombi and the tracheal mite Society for Invertebrate Locustacris buchneri (Shykoff and Schmid-Hempel 1991, Imhoof and Conservation. Schmid-Hempel 1999, all cited in Rao and Stephen 2007). The timing of Western bumble bee declines suggest that accidental escape of diseases Lepais et al. 2010. Estimation of from commercially reared Western bumble bee colonies may be the bumblebee queen dispersal major culprit of recent declines (NAS 2006, cited in Rao and Stephen distances using sibship 2007). A survey of western bumble bee species found that the Western reconstruction method. Molecular bumble bee had one of the highest rates of infection by N. bombi Ecology: Vol. 19. (Cameron et al. 2011), and pathogen load, rather than habitat loss or pesticide use, is strongly associated with bumble bee declines in North Rao, S., and W. P. Stephen. 2007. America (Cameron et al. 2011; Szabo et al. 2012). Bombus (Bombus) occidentalis (Hymenoptera: Apiformes): In

Species (Scientific and Common Name): Bombus occidentalis [Western bumble bee]

Criteria Rank Rationale Literature Citations In summary, the Western bumble bee has an annual life cycle, and decline or recovery. Pan-Pacific variability in reproductive success is high. Additionally, genetic factors, Entomologist: Vol. 83: No. 4. competition with honey bees, and parasite/pathogen relationships all contribute to the vulnerability of the species (Rank A). Confidence in this Szabo, N. D., S. R. Colla, D. L. rank is medium because the actual reproductive capacity of the Western Wagner, L. F. Gall, and J. T. Kerr. bumble bee is unknown. 2012. Do pathogen spillover, pesticide use, or habitat loss Confidence in Rank: High, Medium, or Low explain recent North American bumblebee declines? Conservation Letters: Vol. 5.

Thomson, D.M. 2016. Local bumble bee decline linked to recovery of honey bees, drought effects on floral resources. Ecology Letters 0(0): 1-8. doi: 10.1111/ele.12659. Summary and recommendations: Date: 2/23/2018

The Western bumble bee (Bombus occidentalis) is widely distributed across western North America and has experienced dramatic declines in the past 20 years. Conservation assessments in Idaho and by the Xerces Society suggest concern for this species, although it appears to be relatively stable in Idaho compared to other parts of its range. The Western bumble bee’s wide distribution off the Forest increases likelihood of recolonization along the borders of the Forest should it become extirpated. It is estimated to be widely distributed across the Forest, although based on patterns both rangewide and in neighboring Montana, it probably has low abundance and is experiencing population declines. The Western bumble bee has broad habitat specificity and is likely found in suitable habitat, areas with temporally stable and abundant floral resources, in a variety of vegetation types across the Forest. Habitat probably exists in patches on the Forest, but the Western bumble bee appears to be able to disperse several kilometers during nest establishment. Major threats to Western bumble bee habitat on the Forest include potential overgrazing, invasion of noxious weeds with low-reward flowers, and global climate change. Incidentally, some of the proposed plans for controlling flowering invasive weeds on the Forest may reduce resource availability for the Western bumble bee. High variability in reproductive success, low recorded numbers of reproduction, reduced genetic diversity, and competition with honey bees all increase the vulnerability of Western bumble bee populations. Infection by pathogens and parasites, presumably introduced by escaped bees from commercial colonies, appear to be a major driver of Western bumble bee declines.

Based on its low abundance, declining populations, susceptibility to disease, and numerous vulnerabilities related to its habitat requirements, life history, and demographics, there is substantial concern for the capability of the western bumble bee to persist over the long-term on the Salmon-Challis and it is recommended as an SCC.

Evaluator(s): Rei Scampavia and Mary Friberg