Andrena mesillae

Justification:
Andrena mesillae is a bee species known from limited records in the western United States, from central Texas and the Oklahoma panhandle west across Arizona and New Mexico, and north to Wyoming and southern British Columbia. It is reported to be a pollen specialist of plant species in the family Brassicaceae. Using all available records, the extent of occurrence for the species is 1,828,866 km2. This species has not been observed since 2013, despite over three times as much recent search effort as was historically required to detect it. The persistence of this species across its known range is not well understood. Threats, including climate change, drought, and urbanization have been reported from the range of the species, and may be acting on the species at a local level. However, because the species is widespread and occurs in several different habitat types, it is unlikely that it is threatened with extinction at this time. As such it is ranked as Least Concern.

There is little information about the habitat and ecology of this species. It is reported to be a pollen specialist of species in the plant family Brassicaceae, specifically from the genus Physaria (Laberge and Bouseman 1970). Records come from Sonoran Desert basin, the low hills and woodlands of the Madrean Archipelago, cold desert plateaus, tablelands of the southern Great Plains, and valleys of the Columbia Plateau. 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 before laying a single egg (Michener 2007). Records of this species come from March through June, with most records from March and April (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 (Hung et al. 2021). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena mesillae 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 pollen 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 and 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).

No known conservation actions are in place for this species. Observations of this species are known from the Bureau of Land Management (BLM), United States Fish and Wildlife 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.

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.