# In-Field Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) Parasitism Rates of Plutella xylostella (L.) (Lepidoptera: Plutellidae) in Virginia Cole Crops

## TL;DR

This study found that Diadegma insulare wasps are effectively parasitizing diamondback moth larvae in Virginia, offering a potential biological control method for this pest.

## Contribution

The study provides the first recent data on D. insulare parasitism rates of P. xylostella in Virginia over a four-year period.

## Key findings

- Parasitism rates by D. insulare ranged from 15% to 100% across Virginia locations and years.
- D. insulare was the only parasitoid found, indicating its dominance in the region.
- Results suggest D. insulare could be a viable biological control agent when used with other strategies.

## Abstract

The diamondback moth, Plutella xylostella (L.), is a pest of brassica crops that is found around the world and is challenging to manage due to insecticide resistance. To manage this pest, alternative integrated pest management techniques are being explored. Biological control of the diamondback moth by parasitoid wasps, such as Diadegma insulare, Oomyzus sokolowskii, and Microplites plutellae, has been previously studied in the literature. However, there are no current records of parasitism rate activity on P. xylostella in Virginia in the past 15 years. This information is crucial to determine the role of biological control agents in managing P. xylostella and whether it could potentially be used as a control strategy. We sampled locations across Virginia from 2022 to 2025 to survey the current parasitism rates of P. xylostella on brassica farms. The percentage rates of parasitism averaged between 30.1 and 65% each year. The lowest rate of parasitism was 15% in 2025, with the highest at 100% in 2022. Overall, our findings suggest that D. insulare is currently playing a role in P. xylostella management in Virginia and has the potential to be successful as a biological control agent when paired with other management strategies. No other parasitoids were found during the study.

The diamondback moth, Plutella xylostella (L.), is a significant pest of brassica crops that is found across the globe. Due to the development of insecticide resistance, control tactics have shifted focus towards integrating pest management techniques such as biological control. Diadegma insulare (C.), Oomyzus sokolowskii (K.), and Microplites plutellae (M.) are parasitoids of P. xylostella found in the Eastern United States. From 2022 to 2025, we surveyed P. xylostella larvae and pupae in locations across Virginia to assess the current rates of parasitism in brassica fields. Specimens were brought to the laboratory and reared to assess parasitoid emergence rates. Only D. insulare specimens were found during the study. Adult P. xylostella, larvae and pupae, adult D. insulare, D. insulare pupae, unknown parasitoids, and unknown deaths were recorded and used to calculate the rates of parasitism at each location. We concluded that the parasitism rate varied by location and year, which was expected due to regional conditions and seasonality. Rates averaged between 30.1 and 65% by year, with the lowest individual rate being 15% in 2025 and the highest at 100% in 2022. This suggests that D. insulare is actively present in Virginia and could be a successful biological control agent when paired with other integrated pest management techniques to reduce P. xylostella populations.