Lately mass flowering crops (i.e. oilseed rape) have been shown to positively affect colony growth and densities of bumblebees. Consequently these highly rewarding crops have been suggested to be important for sustaining vital pollination services in agroecosystems. So far, however, no attention has been directed to potentially disproportionate benefits of high proportions of oilseed rape to short- and long-tongued bumblebees. Whereas the very common short-tongued bumblebees may benefit from nectar and pollen provided by the short-tubed flowers of oilseed rape the long-tongued and often more rare species may profit to a lesser extent due to their known competitive inferiority on short-tubed flowers. Once oilseed rape has stopped flowering, disproportionately built up numbers of short-tongued species may flood naturally occurring permanent resource in the surrounding landscape. For long-tongued species this may lead to competitive exclusion on generalist plants or resource depletion due to nectar robbing by short-tongued species on more specialized long-tubed plants. Here, we investigated the effects of the proportion of oilseed rape at the landscape scale on nectar robbing on red clover. Twelve landscape sectors (r=2km), covering a range of four to fifteen percent cover of oilseed rape, were chosen and phytometers of red clover placed next to a central field of oilseed rape in each sector. At four occasions from late July until mid of August 2008, we collected evenly aged flower heads (n=140) from the phytometers and determined the proportion of robbed and unrobed flower-heads per sector.

Results/Conclusions

We could show that the percentage of oilseed rape in the landscape had a marginally significant positive effect on the proportion of flower heads robbed for nectar (R²=0.27, p=0.057). Higher proportions of long-tubed flowers robbed for nectar in sectors with a high proportion of oilseed rape may not only negatively affect long-tongued bumblebees foraging on these flowers but may in turn lead to lower pollination and fruit set in these plants. We conclude that a more differentiated look on the potential effects of highly rewarding mass flowering crops is required in order to evaluate their contribution to sustaining a high diversity in pollinator species and the associated plants.