Riparian buffer strips promote biomass, species richness and abundance of flying insects in agricultural landscapes

Agricultural intensification is debated as one of the major drivers for the decline of insect biodiversity. Agri-environmental schemes (AES) are a common measure to promote biodiversity in agriculture by granting compensational payments to farmers for environmentally friendly practices. In this study we examined the effect of buffer strips of at least 5 m width, adjacent to small watercourses and drainage ditches, on insect biomass and insect species richness in agricultural landscapes. We selected ten arable fields in each of four regions in lower and upper Bavaria, Southern Germany. 25 out of 40 sites had a buffer strip between arable crops and watercourse. Insects were sampled at three time periods (May/June, June/July and August/September) for two weeks each. In each period two samples were collected (one per week). On each site Malaise traps were set up in 5 and 80 m distance to the embankment of the watercourse. Half of the samples was then subjected to metabarcoding and the other half was classified into different insect groups by morphological identification and the number of the individuals for each group was counted. For hoverflies (Syrphidae), individuals were identified at species-level. Data on vegetation structure (cover of grasses and herbs) in the studied riparian buffer strips was collected and correlated with number of species, abundances and biomass of flying insects. The five taxonomic orders with the highest species richness and individual numbers were: Diptera, Hymenoptera, Coleoptera, Lepidoptera and Hemiptera. Diptera dominated hereby with 34% of all species and 81% of all individuals. On average, mixed models indicated 31% higher insect biomass, 15% higher species richness and 29% higher individual numbers of flying insects in buffer strips at 5 m distance to the watercourse compared to sites with no buffer strip. The effect was even stronger for butterflies (32% higher species species richness, 70% more individuals) and hoverflies (24% higher species richness, 51% more individuals). In the presence of a buffer strip significantly higher numbers were found for total individuals, Diptera, Hymenoptera and Coleoptera. In 80 m distance to the watercourse, the samples of flying insects were not significantly influenced by a riparian buffer strip. This study highlights the importance of buffer strips in agricultural regions and their multifunctional potential in fostering biodiversity additionally to their acknowledged use for water protection. Ideally, buffer strips are rich in herbs and inflorescences and are therefore beneficial for the insect fauna by serving as valuable habitat with high potential connectivity at landscape level.