Andrena subaustralis

Justification:
Andrena subaustralis is a pollen specialist solitary bee that visits plants from the genus Salix (Salicaceae) for pollen. It occurs in forests, shrublands, grasslands, agricultural areas, and urban areas of the western United States. Using all records for the species, the extent of occurrence is 1,666,048 km2. The species has not been observed since 2013, despite nearly seven times as much recent search effort as was required to detect the species prior to 2013. The persistence of the species throughout its range is not well understood. Lack of recent records may reflect limited sampling effort throughout its range, rather than a true absence. Across its range, impacts from climate change and drought, wildfire, and urbanization have been observed. These threats may be acting on the species at local levels. However, because the species is widespread and occurs in many habitat types, it is unlikely that it is threatened with extinction at this time. As such, it is ranked as Least Concern for now.

Andrena subaustralis is a dietary specialist that visits plants from the genus Salix (Salicaceae) for pollen (Laberge and Bouseman 1970). The species has been observed in forests, shrublands, grasslands, agricultural lands, and urban areas, 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 (J. H. Cane 1981) that serves to both isolate the provision from pathogens in the surrounding soil and to regulate water uptake from the soil atmosphere (James H. 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 February through July, but mostly from March, April, and May (Laberge and Bouseman 1970; Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

Since 2000, the Southwest U.S. has seen the worst drought in 1200 years (A. P. Williams, Cook, and Smerdon 2022). Drought can reduce the abundance of flowering plants on a landscape scale, and also reduce pollen and nectar quality (Wilson Rankin, Barney, and Lozano 2020). Drought conditions have been shown to reduce the diversity and abundance of native bees (Minckley, Roulston, and Williams 2013; Hung et al. 2021). Drought and climate change may be particularly impactful for this species, as it depends on riparian associated Salix species as its host plant. Due to dams, river diversions, drought, and groundwater pumping, the vegetation in these riparian areas has changed dramatically in the last century, especially in the west (Krueper 1993; Ohmart 1994). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena subaustralis 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 (N. M. Williams et al. 2010). This species is 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). This species has had its conservation status evaluated using NatureServe criteria at the U.S. state and Canadian province level in some areas where it occurs. It has been evaluated as Vulnerable in British Columbia, Canada.

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 better understand 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.