Nesting of ground-nesting bees in arable fields is not associated with tillage system per se, but with distance to field edge, crop cover, soil and landscape context.
Ground-nesting wild bees are crucial for the pollination of wild plants and crops and thus human wellbeing. Arable land currently covers 14 million km2 globally, but little is known about the role of arable fields as potential nesting habitats and how agricultural management, such as tillage system, affects nesting. We quantified nest density and nesting incidence (plot-level nest presence/absence) of ground-nesting bees in 12 conventionally tilled and 13 no-till winter cereal fields in southwestern Switzerland. In each field, nests were quantified in eight belt transects at increasing distances from field edges within an area of 400 m2, and vegetation cover and soil properties were measured at nest sites and sites without nests. Nest density ranged from 0 (32% of fields) to 16 nests (mean: 4.0 nests) per 400 m2, corresponding to 0 to 400 nests ha-1 (mean: 101 nests ha-1). Fifteen nesting species were captured. Nest density was not significantly different between tillage systems. Nest density declined exponentially with distance from the field edge. Nest density and incidence were positively related to proportion of bare ground. Nests occurred across a wide range of soil textures and tended to increase with soil bulk density and sand content. Moreover, nest density tended to increase with the proportion of and proximity to areas under agri-environment scheme in the surrounding landscape. Synthesis and applications. Our study shows that arable fields, irrespective of tillage system, are used as nesting sites by various ground-nesting bee species, including important crop pollinators. The concentration of nests along field edges suggests that incentives to maintain small field sizes and to increase edge density have a great potential to support nesting of ground-nesting bees in agricultural landscapes. Moreover, measures to reduce crop cover, for example, through increased row spacing, offer a promising way to promote nesting opportunities in arable fields, in particular, if floral-rich agri-environment scheme areas are locally available. Further studies are needed to better understand to what extent tilled arable fields are suitable nesting habitats for ground-nesting bees or whether they act as ecological traps due to the adverse effects of tillage on bee offspring.