Contrasting effects of past and present mass-flowering crop cultivation on bee pollinators shaping yield components in oilseed rape

The cultivation of mass-flowering crops (MFC) can promote pollinators by providing floral resources. However, there is missing knowledge about the effect of MFC cultivation history on bees and their pollination services in agricultural landscapes. We investigated how bee densities in oilseed rape (Brassica napus L.) (OSR) fields were affected by past (cultivation area of the preceding three years) and current MFC area coverages in the surrounding landscape. Moreover, we analyzed how insect pollination, its possible interaction with the plants` pod numbers and MFC covers influence yield components of individual OSR plants and calculated yields (t/ha). To test this, we conducted pollinator surveys and a pollinator exclusion experiment in one oilseed rape field in 17 agricultural landscapes in Germany. We found that wild bee densities were positively affected by past MFC covers and negatively impacted by current OSR covers, indicating enhanced pollinator populations due to previous MFC cultivation and contemporary pollinator dilution. In contrast, honeybees showed opposite responses to past and present MFC cultivation. Furthermore, seed weight per plant of open pollinated plants was positively correlated with past MFC covers. Pollinator exclusion decreased the seed number per pod and increased thousand-seed weight, while yields were unaffected. Pod number interacted with insect pollination in shaping yields, such that pollinator exclusion led to a steeper increase of yield with higher pod numbers. Insect pollination compensated for low pod numbers by increasing the plants` seed number per pod and ultimately yields. Our findings demonstrate a beneficial effect of high MFC covers in the past on bee densities and potentially yield components in the current year. Our study highlights the need for further research on how past and present landscape composition in terms of MFC cultivation interactively affect pollinator communities and their pollination services in agricultural landscapes.