Molecular markers for Diadegma (Hymenoptera: Ichneumonidae) species distinction and their use to study the effects of companion plants on biocontrol of the diamondback moth

Molecular markers facilitate the quantification of parasitization of pest species and the distinction of related parasitoid species. We designed new markers that allow the distinction of often misidentified Diadegma semiclausum and D. fenestrale parasitization in field-collected diamondback moth (Plutella xylostella) larvae. The markers were applied to study if cornflower (Centaurea cyanus) companion plants increase parasitization of the diamondback moth by Diadegma parasitoids in cabbage fields, as they do in the Mamestra brassicae-Microplitis mediator pest-parasitoid pair. Among 1708 P. xylostella larvae analyzed, we found a high parasitization rate (72.2 % total, 41.7 % by D. semiclausum, 24.9 % by D. fenestrale, 5.6 % by both), but no significant effects of cornflower presence. Our results highlight the need for species-specific markers and the specific action of companion plants. To increase natural control in crop fields, habitat management needs to be tailored to-and acts on-specific target species.