The role of flower-rich field margin strips for pollinators, natural enemies and pest control in arable fields

Abstract: Flower strips in field margins of arable crops can have various functions. They may create a buffer between crop and ditch thereby limiting the run-off of nutrients and pesticides into the water. Depending on the plants they harbour, the strips can also support insect biodiversity, including pollinators and natural enemies of pests and the ecosystem services they provide to the farmers. In a two-year project all these functions have been quantified in a collaboration of different organisations, including farmers, agronomists, conservationists and scientists. In the arable landscape of the Hoeksche Waard more than 30 fields, cultivated with potato or winter wheat and with a variety of different field margin strips, have been selected each year. While annual flower strips are generally rich in flowers, in perennial strips the amount of flowering forbs decreases with age due to the growing dominance of grasses. Within these field margin strips insect diversity clearly increases with the amount of forbs. Also the number of wild bees (especially bumblebees), hoverflies and other natural enemies such as lacewings increases with the amount of forbs in the flower strips, whereas the number of butterflies and dragonflies are not affected. In wheat fields only low numbers of aphids were found. In potato fields the numbers were initially higher, but the numbers generally decreased from late June onwards, and in a faster rate where more natural enemies (mainly lacewings and hoverflies) were observed in these fields. This indicates the importance of natural enemies for the regulation of these pests. The rate of aphid population decline was higher where the field margin strips contained more flowering forbs. The farmers, immediately informed on the results of the monitoring in their fields, largely refrained from using pesticides, in contrast to common practice. The results indicate that flowering field margin in combination with pest monitoring can contribute to agrobiodiversity in two ways: by providing resources (nectar and pollen) for a range of insect species and by reducing the use of pesticides.