Data from: Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control

1. In modern agricultural landscapes many organisms providing ecosystem services such as pollination and natural pest control are likely constrained by shortage of nectar and/or pollen required for adult nutrition. More and more flower-rich field margin strips and other habitats are created to eliminate these constraints. For most target organisms, however, it is not well known which (types of) flowers are effective in providing suitable pollen and nectar. 2. We studied the suitability of a wide range of flowers as a food source for zoophagous hoverflies (hoverflies with predatory larvae) at five different levels: nectar accessibility (based on flower morphology), realized adult fitness, flower choice (both based on cage experiments), flower visitation, and hoverfly abundance in mixed vegetation (both based on field observations). 3. Realized survival of Episyrphus balteatus is related to effective flower depth by a sigmoid function. The critical flower depth is 1.6 mm, which is less than the proboscis size of the hoverfly. For Asteraceae the critical floret depth is even less than 1.0 mm, which – in contrast to common knowledge – rules out most species within this family. 4. Both flower choice in the laboratory and flower visitation rates in the field are well correlated with nectar accessibility and realized adult survival. 5. In mixed floral vegetation the number of zoophagous hoverflies is highly correlated with the abundance of only those flowers that have accessible nectar for these hoverflies. 6. Synthesis and applications. This comparative study demonstrates that nectar (and not pollen) accessibility is the main driver determining flower resource suitability, flower choice and abundance of zoophagous hoverflies in arable field margins. The study identifies the limited range of plant species that can effectively support these beneficial insects. Preserving the right flowers in and around agricultural fields could enhance local populations and the pest control and pollination services they provide.

1. 在现代农业景观中，诸多提供授粉（pollination）与自然虫害防治（natural pest control）等生态系统服务的生物，其生存很可能受到成虫营养所需的花蜜与/或花粉短缺的限制。当前，人们正越来越多地营建富含花卉的田埂带及其他生境，以缓解这类限制。然而，对于多数目标生物而言，究竟哪些（类群的）花卉能够有效提供适宜的花粉与花蜜，目前仍不甚明晰。2. 本研究从五个不同维度，探究了多样花卉作为捕食性食蚜蝇（zoophagous hoverflies，即幼虫营捕食性生活的食蚜蝇类）食物源的适宜性：分别为基于花部形态的花蜜可及性（nectar accessibility）、成虫实际适合度、基于笼养实验的花卉选择偏好、花卉访花率，以及基于野外观测的混合植被中食蚜蝇的种群丰度。3. 黑带食蚜蝇（Episyrphus balteatus）的实际存活率与有效花深呈S型函数（sigmoid function）相关。其临界花深为1.6 mm，小于该食蚜蝇的喙（proboscis）长度。对于菊科（Asteraceae）植物而言，其临界小花深度甚至不足1.0 mm——这与普遍认知相悖，意味着该科的多数物种均无法为这类食蚜蝇提供适宜花蜜。4. 实验室中的花卉选择偏好与野外的访花率，均与花蜜可及性及成虫实际存活率呈现显著相关性。5. 在混合花卉植被中，捕食性食蚜蝇的种群丰度仅与那些其花蜜可被该类食蚜蝇获取的花卉的丰度高度相关。6. 综合与应用。本对比研究表明，花蜜（而非花粉）的可及性是决定农田田埂边缘（arable field margins）捕食性食蚜蝇的花卉资源适宜性、花卉选择偏好与种群丰度的核心驱动因素。本研究明确了能够有效支撑这类益虫的植物类群范围十分有限。在农田内部及周边保留适宜的花卉，可提升当地这类益虫的种群规模，进而增强其提供的虫害防治与授粉服务。