Family-Level Diversity of Hymenopteran Parasitoid Communities in Agricultural Drainage Ditches and Implications for Biological Control.

Agricultural drainage ditches contain a variety of non-crop vegetation, including potential sources of alternate hosts and food for hymenopteran parasitoids that provide conservation biological control on adjacent farm fields. To assess the patterns of family-level diversity of hymenopteran parasitoids, we surveyed ditch and adjacent crop habitats during June, July, and August 2021-2023, using yellow sticky traps over one week. We sampled two agricultural drainage ditches on each of five farms on the Delmarva Peninsula, eastern USA. We collected 36,725 specimens and identified 29 families across 738 sticky traps. Parasitoid diversity was greater in agricultural ditches than in adjacent fields. While parasitoid family diversity and abundance varied across the farms, ditches within a farm were similar. Within crop fields, diversity was greater at 1.5 m from agricultural ditches than at 9.1 m from the ditches. For several well-sampled families, greater abundance on one farm relative to others extended to both ditches and adjacent crops. Our findings indicate that agricultural drainage ditches serve as an existing beneficial semi-natural habitat for parasitoids on farms. Further research into ditch management practices may reveal methods of enhancing parasitoid abundance and conservation biological control while requiring relatively little investment from farm managers.