Andrena microchlora

Justification:
The small green miner bee (Andrena microchlora) is a widespread dietary specialist bee species that occurs in open habitat in the western United States and southern Canada. Using all known records for the species, the extent of occurrence is 2,887,314 km2. Although no range wide population trend has been observed, the species has been reported to be locally abundant in some studies. Across the range of the species, impacts from climate change and drought, urbanization, and exposure to pesticides have been reported, which may be acting on the species at local scales. Conversely, wildfire poses little risk to this subterranean bee and its Lomatium hosts, the bee found active at these flowers in the year following wildfire in the sage-brush steppe. In the sagebrush steppe, this bee’s floral hosts are fed by snowmelt, and the bloom is reliable. Because the species can be locally abundant, is widespread, and occurs across several habitat types, it is unlikely that it is threatened with extinction at this time. As such, it is assessed as Least Concern.

Andrena microchlora has been observed visiting plants from numerous families (Ribble 1968), but has been classified as a specialist within the plant family Apiaceae (Moldenke and Neff 1974). The species has also been described as a candidate Lomatium (Apiaceae) specialist based off of work done by Cane et al. (2020), where it was the most abundant bee species at 5 species of Lomatium in 11 of 30 Lomatium populations examined. Typically, the early spring bloom of Lomatium is abundant, as it is fed by snowmelt (J. Cane, pers. comm. with S. Killingsworth, 2023). The species is also recorded as a pollinator of globally vulnerable Lomatium insulare ((Buchmann, Adams, and Howell, A, Weiss, M 2010)). Andrena microchlora can be found across a wide range of open habitats including the grasslands and shrublands of Coast Range of California and Sierra Nevada foothills, oak savannahs of the Pacific Northwest, and the sagebrush steppe of the Great Basin. The species is generally not recorded from valley floors of central California, or the low desert of the southwest United States (Ribble 1968). 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 before laying a single egg (Michener 2007). Records of this species come from January through September, with the majority of records from March, April, and May (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 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). Although fire frequency has increased in some parts of the range of the species due to exotic annual grass invasion (Davies et al. 2011), this subterranean bee and its Lomatium hosts are not likely impacted by wildfire, as the bee has been found active at these flowers in the year following wildfire in the sage-brush steppe (J. Cane, pers. Comm. with S. Killingsworth 2023). Certain aspects of this species' biology may make it more vulnerable to some threats. Andrena microchlora 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 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).

No known conservation actions are in place for this species. Observations of this species are known from Bureau of Land Management (BLM), U.S. Fish and Wildlife, 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 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.

More information is needed about the population status, range limits, habitat, and ecology of this species.

Individuals often cannot be distinguished microscopically from A. chlorogaster by taxa experts, even when using the paratypes from the 1968 revision of the species (Ribble 1968).