Andrena escondida

Justification:
Andrena escondida is a solitary bee that occurs in California, United States, and Baja California Norte, Mexico. Using all records of the species, the extent of occurrence is 73,239 km2. It is known from no more than 10 locations, if threats act on each locality separately. However, it has not been observed since 1975, and therefore, it is unclear if the species still persists throughout its known range. It is a dietary specialist, collecting pollen from the genus Layia (Asteraceae) to provision nests. It occurs in grasslands and vernal pools. California’s vernal pools face threats from climate change and drought, major habitat loss and fragmentation, and invasion of exotic species. These threats may decrease the quality and quantity of host plants available to Andrena escondida, and limit the suitable habitat for the bee. Across the range of this species, impacts from exposure to pesticides have also been observed, and these threats may be acting on the species at local levels. More information is needed to determine if this species is still extant, and if so, to determine its population size, and the degree to which threats are acting on the population. Because this species is known from no more than 10 locations, is facing continuing decline inferred in its quality of habitat, and is known from a relatively small range, it is assessed as Near Threatened for now.

Food habits comment: Andrena escondida is a dietary specialist that collects pollen from the genus Layia (Asteraceae) to provision nests (Thorp and LaBerge 2006). Habitat comment: This species has been recorded in grasslands and vernal pool systems (Spencer et al. 2006), between 5 and 2466 m. Reproduction comment: 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). Phenology comment: Records of this species come from March through June (Chesshire et al. 2023). Adults are assumed to emerge annually (Danforth et al. 2019).

Threat comments: Specific threats to this species have not been investigated. It may be impacted by drought and habitat loss, which have been documented in its range. 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). This species may be threatened by the loss of vernal pool habitat where its host plants occur. These habitats are made up of discrete, ephemeral wetlands, and are characterized by their impervious substrate that collects winter precipitation that dries out over the course of the summer (Holland 2009). In California, the Central Valley vernal pool ecosystem has declined 95% from its historic extent (Dahl 1990). Much of the remaining vernal pool habitat occurs in areas with high pressure from urbanization and agricultural expansion, which is expected to continue. Vernal pool habitat is also threatened by alteration of hydrology, which can happen either through droughts, which are predicted to become more frequent and severe across the range of the species (Franklin and MacDonald 2024), or via increased irrigation with reclaimed water. In some sites, increased water in vernal pool sites has occurred via agricultural runoff. Increased water or irrigation at the wrong time of year can kill host plants, and can contain pollutants such as herbicide and excess nutrients, which can kill host plants and encourage invasion of exotic species (USFWS 2016). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena escondida 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). Threats Threats:

Conservation needs No known conservation actions are in place for this species. Protected/Managed area comment: Observations of this species are known from United States Forest Service land, but this does not confer any specific protections to the species. Management comment: 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.

Research need comment: More information is needed about the population status, population trend, existing threats, range limits, habitat, and ecology of this species. Surveys targeting this species are needed throughout its range to determine its persistence throughout its historic range.

(a. any taxonomic concerns about the validity of the species? b. any taxonomic revisions underway that would require a species reassessment.