Andrena birtwelli

Justification:
Andrena birtwelli is a solitary bee known from over 650 observations. It occurs in the forested mountains of the western United States. It is likely a dietary specialist, collecting pollen from the genus Potentilla (Rosaceae). Using all observations of the species, the extent of occurrence is 1,761,808 km2. There have been limited observations of this species since 2013, although these records come from widely dispersed localities within the extent of occurrence. Across the range of the species, impacts from urbanization, habitat modification and loss, and climate change have been observed. These threats may be acting on the species at local levels. More information is needed to determine if this species is extant throughout its known range, and to describe a population trend. However, because the species is widespread, occurs in a variety of habitats, and uses a host plant that can be locally abundant, it is unlikely that this species is threatened with extinction at this time. As such, it is assessed as Least Concern for now.

Andrena birtwelli is a species that occurs in the shrublands and forests of the mountainous western United States. Nearly all observations of this species are from elevations higher than 1,000 m, with most observations from between the 2,000 and 3,000 m elevation range. The species is likely a dietary specialist, using pollen from the plant genus Potentilla (Rosaceae) (LaBerge 1980). This species likely nests underground like all other Andrena (Danforth et al. 2019), but nests from this species have not been described. Nest cells from other members of this genus are located at the ends of the lateral burrows, which are typically lined with a waxy Dufour’s gland secretion (Cane 1981) that serves to both isolate the provision from pathogens in the surrounding soil and to regulate water uptake from the soil atmosphere (Cane and Love 2021). Females provision each cell with a ball of pollen moistened with nectar on which they lay a single egg (Michener 2007). Records of this species come from May through August, with most observations from June and July (LaBerge 1980; Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena birtwelli is a ground nesting species, and nests may be harmed by certain agricultural practices such as tilling, which can kill bees nesting close to the surface (Williams et al. 2010). This species is likely a dietary specialist, which has been linked to higher risk of extinction due to reduced host plant availability, especially under climate change scenarios (Roberts et al. 2011) and reduced effective population sizes (Packer et al. 2005). Additionally, Andrena have been reported to have low reproductive output because of the short adult life span, and a low rate of brood cell provisioning (reviewed in Danforth et al. 2019). Other threats to bees generally include habitat loss or modification, climate change, pesticide use, exposure to pathogens from managed bee species, and competition with honey bees (Brown and Paxton 2009; Potts et al. 2010; Wojcik et al. 2018; Grab et al. 2019; Raven and Wagner 2021).

Conservation needs No known conservation actions are in place for this species. Observations of this species are known from Bureau of Land Management (BLM), United States Forest Service, and National Park Service land, but this does not confer any specific protections to the species. Further research is needed to determine the overall size of the population and to identify trends and existing threats. Specific conservation needs for this species have not been identified. Due to the importance of supporting wild bee populations for pollination services, general conservation practices are recommended including, restoring, creating, and preserving natural high-quality habitats to include suitable forage and nesting sites; limiting pesticide use on or near suitable habitat, particularly during the adult bee’s flight period; promoting farming and urban practices that increase pollinator-friendly plants in margin space; minimizing exposure of wild bees to diseases transferred from managed bees; and lastly, avoiding honey bee introduction to high-quality native bee habitat.

Surveys targeting this species are needed throughout its range to determine its persistence throughout its historic range. More information is needed about the population status, range limits, habitat, and ecology of this species.

(a. any taxonomic concerns about the validity of the species? b. any taxonomic revisions underway that would require a species reassessment.