Sown wildflower fields are an efficient measure to reduce visitation rates of honeybees and other pollinating insects on Jacobaea vulgaris

Abstract

Local mass occurrences of Tansy ragwort (Jacobaea vulgaris, Asteraceae), a native plant usually sparsely interspersed within extensive grassland, have been suggested to cause health hazards for livestock and humans. While current management options, particularly herbicide application and mechanical eradication, undermine conservation targets of extensive grasslands, sown wildflowers are a popular agri-environment scheme that may reduce pollinator visitation on J. vulgaris and thus, mitigate the plant’s dominance. Hence, with this study we aimed to explore whether the availability of sown wildflower patches reduces pollinator visitation rates on J. vulgaris in species-rich and species-poor grassland, respectively, and whether patterns depend on wildflower cover. To do so, we monitored pollinator visitation rates on J. vulgaris when wildflower fields were available to flower visitors or temporarily covered by mesh nets in a field experiment. The availability of wildflower patches significantly reduced visitation rates of honeybees, bumblebees, solitary bees and hoverflies on J. vulgaris, while increasing wildflower cover promoted this effect for J. vulgaris in species-poor grassland. Moreover, species-specific flower preferences and seasonal shifts in the quantitative availability of wildflower species determined the efficacy of wildflower patches. To conclude, our findings suggest sown wildflower fields as an alternative management option to mitigate the dominance of J. vulgaris in plant-pollinator networks given that wildflower mixtures are tailored to species-specific requirements of flower visitors. Future research, however, should explore whether wildflower fields ultimately mitigate the negative consequences associated with the local mass occurrences of J. vulgaris without undermining conservation targets on extensive grassland.