Application of a novel molecular diagnostic method to examine the spatio‐temporal trends of <scp> <i>Carcelia iliaca</i> </scp> , a larval parasitoid of oak processionary moth ( <scp> <i>Thaumetopoea processionea</i> </scp> )

Abstract The oak processionary moth (OPM; Thaumetopoea processionea ) (Linnaeus, 1778) (Lepidoptera: Notodontidae) is an invasive pest across northern Europe. In the United Kingdom, it threatens biodiversity and public health through defoliation caused by numerical dominance of infested trees and the shedding of harmful urticating hairs. Its spread from the current established zone in and around Greater London combined with the rising cost of chemical control show an urgent need for alternative sustainable management. Current alternative strategies revolve around conservation biocontrol using the larval parasitoid Carcelia iliaca (Ratzeburg, 1840) (Diptera: Tachinidae), which is a successful parasitoid reaching parasitism rates of ~80%. However, fundamental biological information regarding C. iliaca is still missing, meaning that creating evidence‐based policies is impossible. Here, we apply a recently developed diagnostic assay to detect C. iliaca from OPM larval tissue to assess parasitism rates across the Greater London area. Samples were collected from 12 sites across two field seasons with 1624 caterpillars collected in 2021 and 2239 caterpillars collected in 2022. Parasitism rates showed a higher average in Year 1 (60.8% [±2]) than Year 2 (34.0% [±2%]), with wide range across sites in both years (Year 1, 50%–69%, Year 2, 26%–53%). However, these data show no spatial trend in parasitism rate despite the sites surveyed being composed of a range of habitat types. We evaluate the effectiveness of the diagnostic assay and discuss the wider implications of long‐term monitoring of parasitoids and the need to identify interactions to better inform conservation biocontrol management strategies.