Andrena angusticrus

Justification:
Andrena angusticrus is a solitary bee species that occurs in the shrublands of the western coastal mountains in California, United States. This species is known from 14 records, all of which are female. There is currently no male description. Using all known records of the species, the extent of occurrence is 40,215 km2. The species has not been detected since 1976, therefore it is unclear if it is extant. Since 2013, here has only been 28% of the search effort as was required to detect this species historically, so lack of recent records may be due to insufficient search effort. In part of its range, Andrena angusticrus occurs in the California coastal sage scrub, a habitat type that is threatened by loss and modification due to urbanization, exotic species invasion, and altered fire regime. These factors, as well as pesticide exposure and climate change and drought may be acting on the species rangewide. Additionally, Andrena angusticrus may be a dietary specialist, using pollen from the plant Platystemon californicus, a resource that may become less common or predictable under climate change scenarios. Although this species does not meet the threshold for imperilment due to EOO size, it may be threatened by extinction, especially if it no longer occurs throughout its entire known range. For now, it is assessed as data deficient, but should be re-evaluated after surveys specifically targeting this species are carried out.

Because there are limited records of this species, little is known about its habitat or ecology. This species has been recorded from the shrublands and urban areas of the western coastal hills and mountains of California. Of the 14 records for this species, seven were collected from Platystemon californicus, a spring annual from the family Papaveraceae. In a study of the pollination biology of this plant species, four Andrena angusticrus pollen loads were examined, and found to contain 99% Platystemon californicus pollen (Hannan 1981). It is possible that this bee species is a dietary specialist, but more information is needed to determine the dietary breadth of this species. 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 (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 on which they lay a single egg (Michener 2007). Records of this species come from March through May, with most records from April (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 1,200 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 (Minckley, Roulston, and Williams 2013; Hung et al. 2021). Because Andrena angelesia has a short adult life stage, emerges in spring, and appears to be a narrow pollen specialist of the plant Platystemon which blooms in response to winter rain, it may be at further risk from winter droughts within its range. This species occurs in California coastal sage scrub habitat, one of California's most threatened vegetation types, which is globally imperiled (NatureServe 2024). This habitat is highly threatened by modification, fragmentation, and loss due to urban and suburban growth. Nearly half of California’s residents live in the coastal California counties where this habitat occurs, which is less than 10% of the state’s land area (Riordan and Rundel 2013). Less than 20% of this habitat remains (Davis, Stine, and Stoms 1994), and the existing fragments are considered some of most valuable real estate in California. This contributes to considerable development pressure on remaining fragments, and their high real estate value makes their conservation costly. Coastal sage scrub has also been modified significantly in the past 100 years due to exotic annual grass invasion, which has been facilitated by increased wildfire frequency (Minnich and Dezzani 1998; Talluto and Suding 2008). These factors also contribute to a feedback where increased grass cover increases fire frequency, leading to grassland conversion. This habitat conversion may limit the availability of host plants used by Andrena angusticrus and alter other abiotic factors important for this species. Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena angusticrus 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). This species may be 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) 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. Because this species occurs in a California coastal sage scrub, which has experienced loss, degradation, and fragmentation, conservation efforts geared toward protecting and restoring this threatened habitat type. Populations of Platystemon should be protected and included in restoration efforts. 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.

This species is only known from females - no males have been identified.