The potential for reducing non-target effects on parasitoids of Ceutorhynchus obstrictus Marsham (Coleoptera: Curculionidae) through spatially targeted insecticide applications

Insecticide applications against the cabbage seedpod weevil, Ceutorhynchus obstrictus Marsham (Coleoptera: Curculionidae), in oilseed rape are conducted during the flowering of the crop. During the flowering period, many parasitoids of pest species are abundant in the crop, and therefore, the preservation of natural pest regulation mechanism is especially important. Parasitism often has a substantial mortality factor in pest populations and as far as possible, parasitoids need to be protected from harmful non-target effects of insecticides. In the present study, the effects of three different insecticidal products on the parasitism of cabbage seedpod weevil larvae were investigated. Additionally, the effects of a spatial targeting of insecticides, using conventional and dropleg application technique, were compared. This was done in field trials within a completely randomized block design and four replications at one trial site in 2019 and two sites in 2020. The parasitoid species Trichomalus perfectus Walker and Mesopolobus morys Walker (both Hymenoptera: Pteromalidae) were abundant in almost equal shares in 2019, whereas M. morys was the most important species in 2020. Only in 2019 were harmful effects of the insecticide Biscaya (a.i. thiacloprid) on parasitism rates of C. obstrictus larvae observed. Clear benefits, due to spatial targeting of insecticides using the dropleg technique on parasitoids, were not found. The harmfulness of insecticides is probably affected by properties of different insecticidal products and the temporal coincidence of parasitoid occurrence and insecticide application.