A review of soil tillage impacts on ground-nesting wild bees - mechanisms, implications, and future research perspectives

The key role of wild bees in providing pollination services is well recognized. Most wild bees nest in the ground and need suitable nesting habitat to thrive. Despite covering 14 million km2 of the world's surface, the potential of arable land as a nesting habitat has been largely neglected, although studies indicate that ground-nesting bees nest in arable soils. Therefore, it is important to understand the impact of tillage on bees' nesting and reproductive success. Here, we synthesize the existing knowledge of potential consequences and mechanisms underlying tillage effects on ground-nesting bees, identify knowledge gaps, and propose directions and approaches for future research. Our literature review has identified ten studies that directly or indirectly assessed impacts of tillage on ground-nesting wild bees in arable cropping systems, showing either no effect or a negative effect of tilled compared to no-till systems. Potential tillage effects include direct impacts related to physical injury of bees, offspring, and brood cells, destruction of nest burrow architecture, displacement of brood cells, and alteration of soil environmental conditions surrounding brood cells, as well as indirect effects related to soil cover, soil properties, and soil conditions. Our review highlights that we poorly understand how tillage influences bee nesting incidence, survival, emergence timing, offspring sex ratio, and, in the longer term, community composition. In particular, it remains unclear whether tilled arable soils are suitable nesting habitat or ecological traps for ground-nesting bees. To address these research gaps, we propose methods to directly quantify nesting and emergence of bees, and (semi-)field and laboratory experiments that allow to disentangle mechanisms driving tillage effects. Improved understanding of tillage effects and underlying mechanisms will help to develop more effective strategies to promote ground-nesting wild bees and the crop-pollinating and soil ecosystem services they provide through their foraging and nesting activities.