Hoverfly happiness: Floral attributes and abundance influence the activity of Toxomerus marginatus in a Nutrient Network site

Pollinators are clearly important for plant communities, facilitating reproduction for 90% of wild plants and contributing to a third of the food we eat. Pollinator declines are increasingly reported, but attention has traditionally focused on bees. Non-bee species such as hoverflies and beetles, however, are gaining increasing recognition. In this study, we investigate the pollinator community in a Nutrient Network site in an old field in Maryland; as part of this distributed ecological experiment, we installed 8-foot herbivore exclusion fences in May 2019. We address relationships between the abundance and diversity of flowering plants and their floral visitors, the relative importance of bee and non-bee visitors, and the impacts of large vertebrates. Using repeated surveys of eighteen 25 m² field plots, we recorded flowering plant species richness, floral abundance, and floral visitation. We observed a total of 26 species of plants flowering during the study period, as well as 29 insect morphotypes and 854 floral visits. Of these visits, 48.5% involved the hoverfly Toxomerus marginatus. In general, floral visitation depended on plant characteristics such as floral height and abundance and varied greatly among plant species. For example, hoverfly visits were positively related to floral abundance, and fruit set was positively related to overall floral visitation, but the short-term effects of fencing on floral abundance, floral visits, and fruit set were non-significant. In this and perhaps other herbaceous plant communities, non-bee floral visitors merit additional research and public attention.