Microhymenoptera in Roadside Verges and the Potential of Arthropod-Friendly Mowing for Their Preservation

The worldwide decline of insects is one of the major challenges for humankind. One of its main drivers is intensive farming, which reduces habitats and food resources for insect populations and causes direct mortality by pesticides. In addition, mowing of grassland poses another threat to insects, especially when it is done frequently, such as in roadside verges. Roadside verges comprise large areas worldwide and can serve as habitats for animals and plants and as corridors to connect populations when they are maintained ecologically, for example when mowing is done arthropod-friendly and with low frequency. Microhymenoptera are highly diverse, mostly show a parasitoid lifestyle and occur in large numbers in meadows and grasslands. Although they have an important ecological function in regulating their arthropod host populations, they were mostly neglected in studies on the influence of mowing on insects. Therefore, we assessed which families of microhymenoptera occur in roadside verges and meadows, examined if they are affected by the mowing process with a conventional mulching mower, and studied which groups benefit from arthropod-friendly mowing, using the mowing head Eco 1200 plus from MULAG, and a flushing bar made of track tarpaulin. In our experimental grassland plots, we found specimens of 18 families from the six microhymenoptera superfamilies Chalcidoidea, Ceraphronoidea, Diaprioidea, Ichneumonoidea, Platygastroidea and Proctotrupoidea. Mowing with a conventional mulching mower caused a significant loss of up to 64% for parasitoid Hymenoptera. The Eco 1200 plus showed an arthropod-friendly effect only on the number of individuals of Chalcidoidea, saving 38% of individuals compared with the conventional mower. The flushing bar showed a significant effect on total number of individuals with a reduction only on Chalcidoidea and a tendency for Ichneumonoidea with 30% and 47%, respectively. Our study demonstrates the detrimental effect of mowing with a conventional mulching mower on microhymenoptera and shows that this effect can be partly reversed for Chalcidoidea and Ichneumonoidea through the arthropod-friendly mower and a flushing bar. Our study highlights the importance to consider microhymenoptera in the context of insect decline.