Andrena chaparralensis

Justification:
Andrena chaparralensis is a solitary bee that occurs in southern Texas, United States. Using all records of the species, the extent of occurrence is 12,359 km2. The species has only been recorded at six localities, which contribute to six locations at most, if threats act separately on each locality. Andrena chaparralensis is a dietary specialist, using pollen from the Asteraceae tribe Asterae. Specimens have only been recorded foraging on two spring blooming, annual species. Across the range of the species, impacts from climate change have been observed, and these impacts may contribute to variability in the quantity and quality of the host plants available to this species. More information is needed to understand the range limits of this species, its life history, and the degree to which threats are acting on the species. Because the species is known from a small EOO, few locations, and the habitat of the species has been impacted by climate change and solar development, the species meets the criteria for Vulnerable based on B1abiii.

Distribution

Population

Habitats and Ecology

Andrena chaparralensis is a dietary specialist, collecting pollen from plants in the family Asteraceae to provision nests. This species has been observed foraging from Astranthium integrifolium and Aphanostephus skirrhobasis, which are both members of the tribe Astereae (Neff and Larkin 2002). Andrena chaparralensis has not been observed forging at other Asteraceae flowers blooming concurrently, and scopal pollen loads examined as part of the description of this species found nearly exclusive Astereae pollen, suggesting dietary specialization within the tribe (Neff and Larkin 2002). Foraging takes place from about midday through the afternoon, to overlap with the anthesis of the species’ host plants. The species has been recorded in the shrubby vegetation and chaparral of the Rio Grande Plains of south Texas (Neff and Larkin 2002), and can be found in areas with compact, sandy soil (J. Neff, pers. comm. with S. Killingsworth 19 Feb 2025). This species nests underground like all other Andrena (Danforth et al. 2019). Two females were observed at their nests in deep sandy soil (Neff and Larkin 2002), but no other characteristics of the nest biology have been described. The species is hypothesized to nest in areas with sparse vegetation and packed sandy soil (Neff and Larkin 2002). 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 April (Chesshire et al. 2023; Neff and Larkin 2002). Adults are assumed to emerge annually (Danforth et al. 2019). The nests of this species have been observed to be parasitized by an undescribed Nomada species (J. Neff, pers. comm. with S. Killingsworth 19 Feb 2025), the impact of which has not been investigated.

Habitat Types

Use and Trade

Threats

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). The documented host plants of Andrena chaparralensis are spring annual plants, the populations of which fluctuate in response to winter rain. The variability of winter rain under climate change scenarios may limit the availability of these plant species, impacting population sizes of this bee species. Andrena chapparalensis also occurs in proximity to increasing solar development, which may contribute to some habitat loss and degradation for this species. Up to 700,000 acres of Texas lands were estimated to be converted to solar farms by the end of 2024 (Texas Real Estate Reseach Center 2024). Texas also ranks first in the country in projected solar growth, meaning impacts from solar development will continue to impact this bee species (Solar Energy Industries Association 2024). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena chaparralensis 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 Actions

Conservation needs No known conservation actions are in place for this species. Observations of this species are known from the Texas Parks and Wildlife land (Chaparral Wildlife Management Area), 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.

Code	Threat	Timing	Scope	Severity
1	Residential & commercial development - 1.2. Commercial & industrial areas	Ongoing	Majority (<50%)	Slow, significant declines
1.1	Residential & commercial development - Housing & urban areas	Ongoing	Majority (<50%)	Slow, significant declines
1.2	Residential & commercial development - Commercial & industrial areas	Ongoing	Majority (<50%)	Slow, significant declines
2	Agriculture and aquaculture - Livestock farming and ranching -> Agro-industry grazing, ranching, or farming	Ongoing	Majority (<50%)	Slow, significant declines
2.1.3	Agriculture & aquaculture - Agro-industry farming	Ongoing	Majority (<50%)	Slow, significant declines
3	Energy production and mining	Ongoing	Majority (<50%)	Slow, significant declines
3.3	Energy production and mining - Renewable energy	Ongoing	Majority (<50%)	Slow, significant declines
8	Invasive and other problematic species, genes, and diseases - 8.1 Invasive non-native/alien species/diseases -> 8.1.2. Named species	Ongoing	Majority (<50%)	Slow, significant declines
8.1.2	Invasive and other problematic species, genes, and diseases - Named species	Ongoing	Majority (<50%)	Slow, significant declines
9	Pollution - 9.3 Agricultural and forestry effluents -> 9.3.3 Herbicides and Pesticides	Ongoing	Majority (<50%)	Slow, significant declines
9.3.3	Pollution - Herbicides and Pesticides	Ongoing	Majority (<50%)	Slow, significant declines
11	Climate change & severe weather - 11.3.Temperature extremes	Ongoing	Majority (<50%)	Slow, significant declines
11.1	Climate change & severe weather - Habitat shifting & alteration	Ongoing	Majority (<50%)	Slow, significant declines
11.2	Climate change & severe weather - Droughts	Ongoing	Majority (<50%)	Slow, significant declines
11.3	Climate change & severe weather - Temperature extremes	Ongoing	Majority (<50%)	Slow, significant declines

Assessment