Andrena macswaini

Justification:
Andrena macswaini is a bee species known from limited observations in the Central Valley and surrounding foothills of California, as well as one observation from southeast Arizona. Using all known observations of this species, the extent of occurrence (EOO) is 77,207 km2. There have been no observations of this species since 1966, although this is likely due to limited recent search effort. It is unclear if the species is still extant throughout its entire range. Based on its occurrence in California’s Central Valley, this species may be subject to habitat loss due to agricultural intensification, as well as high levels of pesticide exposure. Additionally, the range of Andrena macswaini has been subject to a decades-long drought, which may limit the food plants available to the species. More information is needed to determine range extent, population trends, and threats to this species. As such it is listed as Data Deficient for now.

Little is known about the habitat and ecology of this species. There are two records of this species that are associated with the plant genus Oenothera (Onagraceae). Otherwise, information about the dietary breadth of this species is unavailable. If this night-blooming genus is indeed a dietary preference for Andrena macswaini, limited sampling effort during this time window may contribute to the paucity of records for the species. Limited records of this species are from California Central Valley and low Sierra Nevada foothills of California, with one record from the valleys and low hills of the Madrean Archipelago of Arizona. 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 March through May (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). This species may be impacted by agricultural intensification, including high exposure to pesticides where it occurs in California’s Central Valley. The Central Valley of California is one of the most intensively farmed regions in the U.S.(Katibah 1984). An estimated 71% of the Central California Valley ecoregion is devoted to agriculture (Soulard and Wilson 2015). This species may be impacted by high exposure to pesticides where it occurs in the Central Valley. This region has some of the nation’s highest pesticide inputs, which can be detected in non-target plant species, sometimes in insect-lethal concentrations, across all land use types (Halsch et al. 2020). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena macswaini 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). Dietary breadth of this species is not well established. If this species is a dietary specialist, it may be 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 level in some areas where it occurs. It has been evaluated as Imperiled in California.

No known conservation actions are in place for this species. Observations of this species are known from U.S Forest 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.