Habitat loss due to agricultural intensification has negative implications for native bee communities throughout Western Canada. Wetland remnants are a common feature within the Prairie Pothole Region of Saskatchewan and are threatened due to continued conversion to agricultural land. Approximately sixty-one million acres of land are dedicated to agriculture in Saskatchewan. Wetlands and field margins in this region are embedded in these agricultural matrices and may act as important nesting and floral resources for many native bee taxa.

The purpose of this study was to determine whether conserved habitats, such as wetlands and field margins, in highly cultivated landscapes support native bee and pollinator diversity, which is expected to be ecologically and economically beneficial. I also examined differences in bee abundance and diversity across three crop types to explore the roles different crop types might play as a habitat or feeding resource for native bees. Bees were sampled from wetland and field margins into the surrounding cropland across two growing seasons in three crop types (canola, cereals and semi-natural re-seeded forage) to quantify the role that wetlands, field margins and crop types play in supporting native bee populations.

I found that the diversity and abundance of native bees collected from natural and semi-natural edge habitat was higher than that collected in-field. Areas with a higher availability of nesting resources tended to support a higher diversity of bee genera. Unmanaged semi-natural re-seeded forage sites supported a higher abundance and diversity of bees than canola and cereal crops. Finally, we found that bee community structure differed significantly between years, likely due to differences in temperature and precipitation.

Results of this study suggest that native bees may be using edge habitat for nesting and floral resources. Bees nesting in these areas may in turn provide pollination to agricultural crops through a “spill-over” effect. This project has improved our understanding of native bee communities and the value of management practices that promote sustainable agricultural production through pollination services. These results further support the need for management of agricultural cropland that preserves semi-natural habitat that is integral to native bee functional diversity.

Study Sites

This survey took place in the summers of 2018 and 2019. Across the two field seasons bees were sampled from fifteen field sites between Peterson and Humboldt, in the Prairie Pothole Region of Saskatchewan (Table 1.1). Each sample site was one quarter section of land, five of which were re-seeded semi-natural forage grassland, five were seeded with insect-pollinated crops and five contained wind-pollinated crops. Sites were chosen using the Ducks Unlimited Canada conservation program database based on a series of criteria (Figure 1.1). Criteria considered in site selection included: the estimated proportion of wetland habitat within 1km buffer of a point at the center of each quarter section, that the location was adjacent to the road, the intended crop for that field at time of selection, and finally farmer participation. Crops were classified by their pollination mechanism and crop rotation took place between the 2018 and 2019 field seasons, therefore site classification changed between years. Sites where canola was planted were classified as insect pollinated, wheat and barley were classified as wind pollinated and forage sites were classified as mixed pollination.

Table 1.1. Summary of crop type in each study site in 2018 and 2019. Site numbers correspond to the location of the study fields shown in Figure 1.1.

Field Sampling

To measure the diversity and abundance of native bees, field surveys were conducted with the use of passive sampling methods to collect bees present in re-seeded semi-natural forage grassland and agricultural settings containing a wetland habitat in the summers of 2018 and 2019. Passive sampling methods included standard blue, yellow and white pan traps painted with UV paint (New Horizons, Upper Marlboro, MD) (Droege et al. 2017) and blue/yellow vane traps (Springstar Inc., St. Louis, MO, USA) (Appendix 1). Pan traps were sampled using adapted versions of the standardized method of monitoring bee populations by LeBuhn et al. (2015), the National Protocol Framework for Inventory and Monitoring of Native Bees (Droege et al. 2017), and sampling protocols from the Canadian Pollination Initiative by Corey Sheffield (2009). As there are biases in the genera caught by each trap type, both trapping methods were used to ensure adequate sampling of genera present in the area (Mogren et al. 2016). Blue/yellow vane traps are known to be attractive to large and common bees such as Bombus spp. and are also known to capture a high volume (Hall 2018). Blue, yellow and white pan traps, on the other hand, are known to capture smaller and more rare species of bee (Joshi et al. 2015). Abundance was quantified as the sum of individuals collected in the three pans (one of each colour) and vane traps at each position of the transect.

Within each site, traps were spaced at 0m, 75m and 150m along two linear transects starting at the field or wetland margin extending into the field. During the 2018 field season, traps were also set at a 25m position, but were omitted in the 2019 season to minimize unnecessary destructive sampling. Transects extended into the field such that the 150m traps were positioned within the crop (see Appendix 2 for site diagram). Wetland margins were defined as the vegetation within a 10m radius of a permanent wetland (pothole) that was undisturbed by agricultural management. Field margins were defined as a strip of natural or semi-natural vegetation located adjacent to a field, between a field and a road or along a fence line.  Pan and vane traps were place on two separate poles space 1m adjacent to one another. Pan traps were secured to poles by custom 3D printed holders that allowed height adjustment with respect to the crop canopy (Appendix 3). Pan traps remained in the field for twenty-four hours, while vane traps remained in the field for a seven-day period prior to collection. Collection took place over five sample periods throughout the growing season: pre-seed, post-seed, flowering, post-flowering and post-harvest. Bees from the pre-seed (May) and flowering period (July) from both seasons were used in the analysis for this project. Timing of sample periods was dependent upon the growth of vegetation (percentage of crop flowering) and weather restrictions. The pre-seed period was defined as the period prior to a grower seeding the site and the flowering period was defined as the time in which >50% of flowers were present in sites with flowering crops. Only zero metre traps were sampled in canola and cereal crops in the pre-seed period of both years, as seeding had not yet taken place and traps could not be set along the rest of the transect until this was complete. Therefore, only bees captured in 0m traps across all crop types were used for pre-seed analysis. Differences in the community composition between bees found in edge habitat to those found in the field were the primary interest for the flowering period, therefore, only bees captured in 0m and 150m traps were used for this analysis (bees captured in 75m traps were omitted).

Native Bee Identification

Bees collected from pan and vane traps were washed and pinned using methods adapted from Sam Droege (Droege et al. 2010) and the CANPOLIN Survey of Pollinator Diversity in Canada by Corey Sheffield (2009). Bees were then rinsed and stored in 70% ethanol until they were ready to be processed and identified.  

Bees were identified to genus as very few species keys have been formed for western Canada, particularly for bees found in this study region. Additionally, most of the commonly used taxonomic keys for bees are to genus, and bee identification to genus is common for this type of study (Happe et al. 2018; Potts et al. 2005, O’Brien and Arathi 2018; Martins et al. 2018). Identification of bees to genus was performed using several taxonomic keys and identification tools including: The Bee Genera of Eastern Canada (Packer et al. 2007), The Bees of the World (Michener 2000), Bumble Bees of North American (Williams et al. 2014), The Bees in Your Backyard (Wilson and Carril 2016), The Bees of the Eastern United States (Mitchell 1960 and 1962), The Solitary Bees (Danforth, Minckley and Neff 2019), and Discoverlife (Ascher and Pickering 2015). All specimens are stored in either the Prager Lab at the University of Saskatchewan or the Rehan lab at York University.