Sown wildflower strips offer promising long term results for butterfly conservation

Flower-feeding insects are declining due to the loss of floral resources. The EU Grassland Butterfly Index underlined a decrease in abundance of 39% between 1990 and 2017. In Northwest Europe, land use intensification is the main driver of grassland butterflies' decline. Among the agri-environment schemes in Wallonia (Belgium), wildflower strips are implemented along crops in order to prevent further loss. They provide nectar sources and host plants to butterflies. For more than ten years, butterflies have been monitored in the Walloon flower strips. The strips vegetation was also surveyed to evaluate the larval food potential. In addition, flower visits for nectar were studied in 2020. Between 2010 and 2020, 56 butterfly species (more than 84,000 individuals) were recorded inside the strips and their population showed an overall increase of 82%. We suggest that this increase was due to the strips temporal continuity, combined with a local climate change. Most recorded butterflies belonged to common generalist species but species that are rare, threatened or protected in the region were also observed. Most species had at least one potential host plant inside the strips and we hypothesize that some of the abundant food generalists used them as breeding sites. In 2020, butterflies mainly visited Centaurea jacea, Lotus corniculatus and Medicago sativa for nectar collection. Before the flowering of C. jacea in June, few nectar was available and only a few flower visits were recorded. Thus, flower strips mostly benefit butterflies in summer in terms of nectar supply. IMPLICATIONS FOR INSECT CONSERVATION: Our results highlight that the long term application of wildflower strips can be a good instrument to contribute to butterfly conservation in Wallonia. The study of the potential host plants and effective nectar sources provide additional understanding on how butterflies interact with the flower strips.