Revealing parasitoid-pest interactions using multi-marker metabarcoding: a nationwide assessment of parasitism in cabbage stem flea beetle larvae

Management of insect pests of oilseed rape (Brassica napus L.) such as cabbage stem flea beetle (CSFB; Psylliodes chrysocephala L.) is increasingly challenging given a lack of effective permitted insecticides. Conservation biocontrol, including through naturally-occurring parasitoids of CSFB, is one of the first lines of defense and the baseline for integrated pest management (IPM) strategies, but we lack a comprehensive understanding of the parasitoids involved and how parasitism rates vary regionally. Such data have traditionally required rearing of parasitoids from their pest hosts, which is highly error-prone, laborious and overlooks instances of multiple parasitism and hyperparasitism. Using multi-marker DNA metabarcoding, including a novel host-exclusion parasitoid PCR primer pair, we assessed parasitism of 1,040 CSFB larvae from 13 counties in England, UK. We confidently identified seven parasitoids of CSFB larvae, with ten taxa present in unfiltered data. Parasitism rates differed between regions, with the north of England having significantly lower parasitism rates than southern regions. Our novel beetle parasitoid PCR primers demonstrate a high congruence with visual detections of parasitoids and a high rate of novel interaction detections. Ultimately, by determining both rates of parasitism and natural enemy identities, these data significantly extend our understanding of in-field CSFB parasitoid community dynamics. This information could guide the enhancement of farm management, especially using network science in decision support systems related to insecticide application, but also more broadly in the context of crop and habitat management for improved conservation biocontrol and IPM.