Validation of a molecular tool for detecting <i>Diadegma</i> spp. (Hymenoptera: Ichneumonidae) parasitism in diamondback moth, <i>Plutella xylostella</i> (Lepidoptera: Plutellidae) larvae in North America

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is a major pest affecting Brassica crops worldwide. In North America, P. xylostella outbreaks have caused substantial economic losses in canola (Brassica napus L. and Brassica rapa L.) and brassicaceous vegetable (Brassica oleracea L.) crops. Natural enemies, including the parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae), play a key role in controlling P. xylostella populations. To better understand the distribution and impact of Diadegma spp., we adopted Diadegma-specific primers, developed originally for haplotyping, and used a molecular screening approach to detect parasitism of P. xylostella larvae collected across Canada and the United States. The primer set was first tested on D. insulare adults and laboratory parasitized P. xylostella to confirm amplification and specificity and determine sensitivity. Screening was then conducted on 421 field-collected larvae from 45 localities to detect and estimate parasitism rates and assess geographic variability. The results revealed notable variation across collection sites in parasitism of P. xylostella larvae by Diadegma spp., with more parasitized larvae collected from field sites in Canada than from field sites in the United States. Variation in parasitism across collection sites could be the result of differences in climate, agricultural practices, and insecticide use between agricultural systems. This research demonstrates the utility of molecular screening to detect parasitism of P. xylostella larvae and highlights potential differences in the impact of Diadegma spp. on the population dynamics of P. xylostella in Canada and the United States.