Edge Effects and Pitfall Trap Design Influence Spider Diversity and Assemblages in Canola Agroecosystems on the Canadian Prairies

Spiders (Araneae) are generalist predators in agroecosystems and may contribute to biological control in canola (Brassica napus L. and B. rapa L.). However, their diversity and community structure remain understudied in the Canadian Prairies. To address this knowledge gap, we surveyed spider assemblages in the Aspen Parkland region of Alberta, Canada, using pitfall traps placed at field edges and interiors during the 2021 and 2022 growing seasons. We collected 968 spiders in 74 species across 14 families, with Lycosidae and Linyphiidae being the most abundant. Spider abundance was consistently greater at field edges, although family-level composition varied by year and location. In 2023, we tested pitfall trap modifications to improve spider retention, evaluating (1) trap diameter, (2) preservative substrate (glass beads vs. propylene glycol), and (3) polytetrafluoroethylene (PTFE; Fluon, a non-stick fluoropolymer) coated trap interiors (to reduce friction and prevent escape). This was tested across canola habitats adjacent to grass- or tree-dominated non-crop habitat edges. Trap catches, and resulting richness and diversity, were significantly higher in large PTFE-treated traps, particularly in treed-edge habitats compared to other treatment combinations. These results underscore the importance of non-crop field margins in maintaining spider diversity and highlight design improvements to optimize passive sampling. Our findings provide a regional baseline for spider assemblages in canola systems and offer methodological advances to support future ecological monitoring and conservation biological control efforts in Prairie agroecosystems.