Data from: Maize-dominated landscapes reduce bumble bee colony growth through pollen diversity loss

1. Bumble bees are important pollinators for a wide range of crops and wild plants. Performance of their colonies depends on pollen and nectar as food resources, but flowering plants are scarce in modern agricultural landscapes. It is well-known that semi-natural habitats can enhance floral resources and bumble bee abundance, but the impact of different crop types and their heterogeneity at the landscape scale remains unclear. 2. We tested the effect of two different crop types (oilseed rape (OSR) and maize) and of configurational (field border density) and compositional heterogeneity (crop diversity) on weight gain of buff-tailed bumble bee colonies (Bombus terrestris) colonies and the pollen diversity collected by them in 20 landscapes in Central Germany. 3. We found that augmenting maize cover had a detrimental effect on pollen diversity collected by the bumble bees, probably due to intensive management resulting in low plant diversity. This low pollen diversity translated into reduced colony growth, since colonies with high pollen diversity gained more weight than colonies with low pollen diversity. 4. In contrast, OSR cover, configurational and compositional heterogeneity did neither affect colony growth nor pollen diversity. However, for OSR the timing of the flowering period was important. When OSR fields had a high flower cover at the end of the OSR blooming period, colonies showed increased growth rates. 5. Synthesis and applications. Our results complement previous laboratory studies by showing that high pollen diversity leads to better colony performance under field conditions. Therefore, the maintenance of floral diversity in agricultural landscapes is crucial to ensure that bumble bees can fulfil their nutritional needs. However, the heterogeneity of crops, at least under the currently very low levels of crop rotation, does not contribute to this aim. In contrast, crop identity and timing of mass-flowering crops turned out to be important factors, as maize reduced pollen resources, while late blooming OSR oilseed rape was beneficial for bumble bee colonies. Hence, maize cover per landscape should be reduced and strategies to enhance landscape wide flower diversity, especially towards and after the end of oilseed rape bloom should be promoted to support bumble bee colonies that provide important pollination services