Study Species and Site Selection
The Yellow-Faced Bumble Bee (B. vosnesenskii ) is common along the west coast of North America, from British Columbia to Baja, California (Williams et al. 2014). We caught 99 wild, post-diapause B. vosnesenskii queens along roadsides in the vicinity of California State University Monterey Bay in California (N36 39 13.761, W121 48 6.109) during March 2017 (Permit # SC-13698). These queens were transported to the lab at the University of California Davis where they were placed in rearing boxes under controlled conditions (see Williams et al.2012; Malfi et al. 2019). The 28 colonies that first generated more than 15 workers (20.7 ± 3.0, mean ± SD, range 17-25) were placed in the field between 6 and 9 weeks after queen capture. The first colonies were deployed on 18-April and the last colonies on 10-May (see Supporting Information, Appendix 1 for set-up details). On average, colonies weighed 25.2 ± 5.4 g (mean + SD) at the time of placement.
We selected 14 sites in Colusa, Solano and Yolo Counties, California, USA (Fig 1) at which to deploy bumble bee colonies. Sites were selected based on proximity to riparian habitat, variation in proportion of semi-natural and agricultural land in the surrounding landscapes, and geographical separation of at least 1 km. The 1-km landscapes surrounding sites consisted of 2-63% semi-natural habitat (oak savannah, riparian, oak woodland, forbs plantings), 37-91% agricultural land (orchard, annual crops, pasture, vineyards) and <1-25% other land uses (urban, water). Land use was identified through in-field inspection, complemented with satellite images (Google Maps 2017) for non-accessible areas, and analyzed in QGIS (QGIS Development Team, 2009) (for details, see Supporting Information, Appendix 2 ).