Andrena calvata

Justification:
Andrena calvata is an apparently rare species of bee known from ten observations in the western United States and northern Mexico. Records for this species come from coastal California, Arizona, New Mexico and Chihuahua, Mexico. The extent of occurrence for this species is 616,853 km2, although there is a fair amount of uncertainty in this estimate due to the low number of records. There are no records of this species since 2013, so it is unclear whether the species persists in its entire range. Because of few records, it is presumed to be rare. Otherwise, population size and trend are unknown. Although direct threats to this species have not been described, climate change and low reproductive output may contribute to risk for this bee. More research is needed to determine the population trend, range, and threats to this species. As such, it is listed as Data Deficient.

There is little information about the habitat and ecology of A. calvata. Only one record includes a floral host from the plant family Asteraceae (Vigiuera sp.), so the dietary breadth of this species is undefined. This species likely nests underground like all other Andrena (Danforth et al. 2019), but nests of 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 before laying a single egg (Michener 2007). Records of A. calvata are from April (for California occurrences) and September (for Arizona, New Mexico, and Chihuahua occurrences) (Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

There are no specific threats identified for this species. Based on its range, it may be impacted by an ongoing drought in the Southwest US that is the worst 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). A recent study has shown reduced diversity and abundance of native bees under drought conditions (Hung et al. 2021). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena calvata 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). 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).

No known conservation actions are in place for this species. Observations of this species are known from United States 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.

Surveys targeting this species are needed throughout the range of this species are required to determine its persistence within its 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?)