Landscapes with high amounts of mass-flowering fruit crops reduce the reproduction of two solitary bees

Abstract

The intensification of agriculture and the loss of natural habitat are key drivers of the observed declines in wild bee populations in recent decades. Mass-flowering crops are suggested to provide abundant resources for bees during flowering. However, mass-flowering crops could influence the offspring reproduction of bees after mass-flowering due to a lack of floral resources in the fields after mass-flowering. In our study we quantified the reproductive output of two solitary bee species (Osmia cornuta and O. bicornis) during and after mass-flowering of fruit crops (MFFC, i.e. apple, pear and sweet cherry) by means of trap nests. Ten trap nests were placed in landscapes with independent gradients of semi-natural habitat and MFFC in the surrounding landscapes. In addition, the composition of the collected pollen of both bee species was identified. Despite our relatively small sample size, we do detect a clear effect for both bee species, namely that the number of constructed brood cells after mass-flowering decreased significantly as the proportion of MFFC within 200 m of the surrounding landscape increased. During mass-flowering no effects were found, probably because O. cornuta collects significant amounts of Prunus spp., Malus spp. and Pyrus spp. pollen and because O. bicornis collects Malus spp. pollen to a certain extent. Yet, the negative effect after mass-flowering is clearly limiting the reproduction of O. bicornis, a solitary bee species with a later phenology compared to O. cornuta. The latter might also be a concern for other bee species that are active in late spring or summer. Landscape management needs to focus on providing additional floral resources to sustain viable populations of solitary bees and other pollinating insects after mass-flowering in landscapes with high amounts of MFFC.