Mowing meadows without conditioner and the availability of bee-friendly habitats synergistically benefit wild bees in agricultural landscapes

Healthy populations of wild bees are essential for the pollination of wild plants and many crops. However, wild bees are under pressure, in particular due to the intensification of agriculture. To counteract negative effects, mitigation measures are being implemented via agri-environment schemes. While agricultural meadows offer valuable floral food resources for bees, the technique and timing of their harvesting are likely critical aspects determining their value for pollinators. Early mowing before or during peak flowering and the use of conditioners, devices commonly used to accelerate grass drying and increase feed quality, could negatively affect bees. Over a period of six years, we sampled wild bees in 74 landscapes in western Switzerland differing in conditioner use in ley- and permanent meadows as well as delayed mowing of ley meadows to investigate the individual and combined impacts of these measures on wild bees. We found that refraining from conditioner use in extensively managed permanent meadows in combination with increased proportions of bee-friendly habitats synergistically benefitted overall wild bee abundance and richness as well as bumblebee abundance. Moreover, to benefit bumblebees, the share of no conditioner use needed to be high in both, leys and intensively managed permanent meadows. There was, however, substantial variation of the effects of the measures between years, emphasizing the need for long-term monitoring of such measures to better understand their effectiveness.