Andrena candida

Justification:
Andrena candida is a dietary generalist solitary bee that occurs in many habitat types in the western United States, northern Mexico, and southeast Canada. Using all known observations of the species, the extent of occurrence is 4,842,888 km2. Recent observations of the species come from widely distributed localities across its extent of occurrence. The species has been described as extremely abundant, and occurs in wildlands and disturbed landscapes alike. Across the range of the species, impacts such as climate change, exposure to pesticides, and habitat loss and degradation may be acting on the species. More information is needed to assess the impacts of these threats and to determine a population trend. However, because the species is abundant, widespread, occurs in many habitat types and uses pollen from many plant families, and has been recently detected in many parts of its range, it is unlikely that it is threatened with extinction at this time. As such, it is assessed as Least Concern.

Andrena candida is a dietary generalist bee that occurs in coastal, desert, mountain, and agricultural, and urban environments within its range (Youssef and Bohart 1968), including in forests, grasslands, shrublands. It has been observed foraging from within the following plant families: Asteraceae, Brassicaceae, Ericaceae, Rosaceae, Rhamnaceae, Apiaceae, Fabaceae, Boraginaceae, Ranunculaceae, Salicaceae and Polemoniaceae, among others (LaBerge 1977). This species nests underground like all other Andrena (Danforth et al. 2019). A comprehensive study from Youssef and Bohart (1968) of one nest site from Utah describes a small nest aggregation with one to five nest entrances per square meter. The nest site was located along a well-packed dirt road between the tire tracks over a distance of about 45 m. Nest entrances were surrounded by a tumulus that averaged 40 mm wide and 15 mm high. Burrows angled downward vertically 3 - 8 cm before angling 45° for another 3 - 5 cm. The nests of Andrena candida are unusually shallow compared to other Andrena species. The number of cells in each nest ranged from 2 - 4 cells. Nest cells were located at the ends of the lateral burrows, and lined with a waxy Dufour’s gland secretion (Youssef and Bohart 1968) 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). Field collections suggest that this species is bivoltine, flying for three to four weeks, and then reappearing approximately one month later for a second flight period, although this has not been confirmed in a nest site study (Youssef and Bohart 1968). Records of this species come from January through October, with most collections from February through July (LaBerge 1977; 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 (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). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena candida 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). Because this bee has exceptionally shallow nests (Youssef and Bohart 1968), tilling may be especially impactful for nest sites on agricultural lands. 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, 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.