Plant Selection by Bumble Bees (Hymenoptera: Apidae) in Montane Riparian Habitat of California

Abstract

Many bumble bee species (Bombus Latreille) have declined dramatically across North America and the globe, highlighting the need for a greater understanding of the habitat required to sustain or recover populations. Determining bumble bee species’ plant selection is important for retaining and promoting high-quality plant resources that will help populations persist. We used nonlethal methods to sample 413 plots within riparian corridors and meadows in the Sierra Nevada of California for bumble bees during two summers following extremely low and normal precipitation years, respectively. We assessed the five most abundant bumble bee species’ plant selection by comparing their floral use to availability. Additionally, we described the shift in plant selection between years for the most abundant species, Bombus vosnesenskii Radoszkowski. Bumble bee species richness was constant between years (13 species) but abundance nearly tripled from 2015 to 2016 (from 1243 to 3612 captures), driven largely by a dramatic increase in B. vosnesenskii. We captured bumble bees on 104 plant species or complexes, but only 14 were significantly selected by at least one bumble bee species. Each of the five most frequently captured bumble bee species selected at least one unique plant species. Plant blooming phenology, relative availability of flowers of individual plant species, and plant selection by B. vosnesenkii remained fairly constant between the two study years, suggesting that maintaining, seeding, or planting with these ‘bumble bee plants’ may benefit these five bumble bee species.

Floral resource selection by wild bees and honey bees in the Midwest United States: implications for designing pollinator habitat

“…Few studies have examined forb selection by bees in the Midwest United States, even though agro‐ecosystems in this region are critical habitat for honey bees and wild bees (see Williams et al 2011, Morandin & Kremen 2013, and Cole et al 2020 for studies from the Eastern United States and California). Furthermore, no studies compared bee preference data to seeding lists developed by natural resource managers that are often tasked with creating pollinator habitat.…”

A contemporary survey of bumble bee diversity across the state of California

“…Moreover, although we sampled in ecoregions where all of these species were historically present (Cole et al, 2020; Thorp et al, 1983; Table 2), we had a limited number of sites in the ecoregions where B . nevadensis (Cole et al, 2020; Thorp et al, 1983) and B . morrisoni have previously been collected (CDFW, 2019; Graves et al, 2020; Thorp et al, 1983; Table 2).…”

“…Bombus occidentalis was historically widespread across the state (Thorp et al, 1983), but is now restricted to high meadows (CDFW, 2019;Graves et al, 2020). This species appears to still be sporadically present in the Sierra Nevada (Cole et al, 2020;Hatfield & LeBuhn, 2007), and the Northern Coast Range (Graves et al, 2020), although we did not sample it in either of these ecoregions, which is consistent with another recent study in the Sierra Nevada (Loffland et al, 2017). The documented decline of B. occidentalis began in the mid-1990s in the most western parts of its range, including in California (Cameron, Jepsen, et al, 2011;Graves et al, 2020).…”

“…All of these species are present in recent California observations reported to online community science databases (e.g., iNaturalist, Bumble Bee Watch). Moreover, although we sampled in ecoregions where all of these species were historically present (Cole et al, 2020;Thorp et al, 1983; Table 2), we had a limited number of sites in the ecoregions where B. nevadensis (Cole et al, 2020;Thorp et al, 1983) and B. morrisoni have previously been collected (CDFW, 2019;Graves et al, 2020;Thorp et al, 1983; Table 2). Similarly, the current range of B. crotchii and B. pensylvanicus, which were previously abundant throughout California, appears to now be restricted to xeric and coastal sites in Southern California (Schweitzer et al, 2012;Thorp et al, 1983;Richardson & Woodard, unpublished data) and the Central Valley, where we did not sample.…”

“…Overcoming these barriers and protecting important species, like bumble bees, is necessary to prevent cascading negative impacts on agricultural and natural ecosystems (CDFA, 2018;Cole et al, 2020;Kremen et al, 2002;Macior, 1977;Thorp, 2014). Our study highlights the need to assess standardized monitoring frameworks for species assemblages with high habitat and species heterogeneity.…”