Andrena bradleyi

Andrena bradleyi has been described as a dietary specialist that collects pollen from the plant genus Vaccinium (Ericaceae) to provision its nests (LaBerge 1985). However, the understanding of the dietary breadth within this genus may be biased by studies that focus on the pollination of blueberries (Vaccinium sp.) (Stubbs et al. 1992; Scott, Ginsberg, and Alm 2016; Watson, Bartel, and Gibbs 2024). This species has been found to collect pollen loads that are nearly 100% composed of low bush blueberry pollen (Stubbs et al. 1992; Scott, Ginsberg, and Alm 2016; Wood and Roberts 2018), making it an important part of blueberry agricultural systems. Andrena bradleyi also possesses the ability to sonicate Vaccinium flowers, which might otherwise be difficult for other bee species to access because of their shape (Scott, Ginsberg, and Alm 2016). However, in one recent observation of the species where the composition of the pollen load was examined, the specimen was found to be carrying nearly 100% Salix (Salicaceae) pollen (Watson, Bartel, and Gibbs 2024). This suggests that the bee may be able to use other pollen sources when its primary host plants are not available. The species has been recorded from deciduous, evergreen, and mixed forest, pastureland, and agricultural 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 (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 February to November, with most records from April and May (Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena bradleyi 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). 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).

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 Fish and Wildlife, National Park Service, and 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 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.