Effects of insecticide treatment against blue stem weevil (Ceutorhynchus sulcicollis) and cabbage stem weevil (C. pallidactylus) on crop injury and yield in winter oilseed rape (Brassica napus)

Blue stem weevil (Ceutorhynchus sulcicollis) and cabbage stem weevil (C. pallidactylus) can occur in high numbers in winter oilseed rape (Brassica napus) fields in Sweden, but their impact on crop yield is poorly known, hindering the development of evidence-based pest management recommendations. We conducted five field experiments in two cropping seasons, where we tested pyrethroid insecticide application in autumn targeting C. sulcicollis, in spring targeting C. pallidactylus, as well as combined applications in autumn and spring targeting both species. We evaluated stem injury, emergence of the new generation of weevils and crop yield and oil content. Insecticide treatment in autumn reduced stem injury length and severity by C. sulcicollis, whereas insecticide treatment in spring did not affect stem injury. Emergence of the two species was similar across all treatments, which possibly was an artefact of limited sampling effort and small-scale spatial heterogeneity in emergence. Insecticide treatment in autumn, spring or a combined treatment all increased yield by 8–10 % (185–245 kg ha⁻¹) with minor effects on oil content. Our results show that C. sulcicollis cause significant yield loss that can be reduced by insecticide treatment in autumn, which therefore can be economically motivated, whereas the results for insecticide treatment in spring targeting C. pallidactylus are inconclusive. Stem mining weevils are just some of several insect pests that can be economically important in oilseed rape and the availability of insecticides is declining. This calls for research into preventative pest management and well-motivated use of the few insecticides that remain in use.