One of the parasitoids you should often find on P. xylostella is the ichneumonid Diadegma. That genus is rahter puzzling with many close species to differentiate. Look for this key:
Title: Identification of the Diadegma species (Hyemnoptera: Ichneumonidae, Campopleginae) attacking the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae).
Source: Bulletin of Entomological Research. 90(5):375-389.