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.

1. 大黄蜂是多种作物和野生植物的重要传粉者。其蜂群的表现依赖于花粉和花蜜作为食物资源，但现代农业景观中开花植物稀缺。已知半自然生境可增加花卉资源和大黄蜂数量，但不同作物类型及其在景观尺度上的异质性影响仍不明确。
2. 我们在德国中部20个景观中，测试了两种不同作物类型（油菜（OSR）和玉米）以及构型异质性（田埂密度）和组成异质性（作物多样性）对buff-tailed大黄蜂（Bombus terrestris）蜂群增重及所采集花粉多样性的影响。
3. 我们发现，增加玉米覆盖度对大黄蜂采集的花粉多样性具有不利影响，这可能是由于集约化管理导致植物多样性较低所致。这种低花粉多样性会导致蜂群生长减缓，因为花粉多样性高的蜂群比多样性低的蜂群增重更多。
4. 相比之下，油菜覆盖度、构型异质性和组成异质性既不影响蜂群生长，也不影响花粉多样性。然而，油菜的花期时间很重要：当油菜田在花期结束时具有较高的花覆盖度时，蜂群生长速率会提高。
5. 综合与应用。我们的结果补充了先前的实验室研究，表明在田间条件下，高花粉多样性可提升蜂群表现。因此，维持农业景观中的花卉多样性对于确保大黄蜂满足其营养需求至关重要。然而，作物异质性（至少在当前作物轮作水平极低的情况下）对这一目标并无助益。相反，作物种类和大宗开花作物的花期时间是重要因素：玉米会减少花粉资源，而晚花油菜则对大黄蜂蜂群有益。因此，应降低每个景观中的玉米覆盖度，并推广提升景观尺度花卉多样性的策略（尤其是在油菜花期结束前后），以支持提供重要传粉服务的大黄蜂蜂群。