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

## TL;DR

Agricultural drainage ditches support diverse parasitoid wasp communities, which can help control pests in nearby crops, offering a low-cost strategy for sustainable farming.

## Contribution

This study demonstrates that drainage ditches serve as semi-natural habitats for parasitoid wasps, enhancing biological control in adjacent crops.

## Key findings

- Parasitoid wasp diversity was higher in agricultural ditches than in adjacent crop fields.
- Diversity in crop fields increased closer to drainage ditches, suggesting a spillover effect.
- Family-level diversity and abundance varied across farms but was consistent within farms.

## Abstract

Conservation biological control consists of practices that often use semi-natural habitats to increase beneficial natural enemies on farms. While agricultural drainage ditches are designed to control the water table on farm fields, natural enemies may exploit their diverse, natural vegetation as a source of food, hosts, and/or shelter adjacent to cropland. Among those natural enemies are parasitoid wasps, a diverse group of insects that develop and kill invertebrate hosts, including potential pests in crop fields. These natural enemies serve as the foundation for the sustainable management of arthropod pests in agroecosystems. To evaluate the value of drainage ditches for supporting natural enemies that are important to conservation biological control on farms, we surveyed parasitoid wasps using sticky traps in ditches and adjacent crop fields on five different farms. Wasps were more diverse within agricultural ditches than adjacent crops and diversity within the crops was greater in samples taken closer to agricultural ditches. Families differed in abundance between ditches and crops of different farms. Our results indicate that parasitoid wasps inhabit agricultural ditches for food or shelter, laying the groundwork for their development for conservation biological control of pests.

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.