| Pieris rapae is an insect pest the feeds exclusively on species of the Brassicaceae. It is a serious economic pest in commercial horticulture as larvae chew large holes in leaves, can destroy developing heads of broccoli, cauliflower, and cabbage, and otherwise contaminate or stain produce, rendering it unmarketable. P. rapae is notoriously difficult to manage in organic production systems, and organic brassica growers traditionally rely upon floating row covers or biopesticides to prevent feeding damage. Natural pest resistance is important when making cultivar recommendations for production systems with limited pest management options, and our research evaluates natural and induced resistance in broccoli (Brassica oleracea var. italica) against P. rapae with a focus on the roles that glucosinolates (GSLs) play in resistance. GSLs are constitutive and inducible secondary metabolites in members of the Brassicaceae and are toxic to generalist insect pests. However, the relationship between GSL and P. rapae, a brassica specialist, is complex and not well understood. A controlled greenhouse study evaluated the effects of P. rapae herbivory on GSL induction and subsequent larval growth and performance on two broccoli cultivars ('Beneforte' and 'Green Magic') that varied in concentration of two GSLs, glucobrassicin and neoglucobrassicin. Our results show that neoglucobrassicin and glucobrassicin concentrations did not significantly increase as a result of herbivory, but the overall growth and performance of P. rapae was influenced by cultivar. Larvae consumed less total leaf area and weighed less as pupae when feeding on 'Beneforte', which also had a significantly higher glucobrassicin concentration, indicating that cultivar may be important in resistance against P. rapae. A field study performed at two certified organic farms in Minnesota in 2015 and 2016 evaluated six broccoli cultivars ('Belstar,' 'Fiesta,' 'Green Magic,' 'Marathon,' 'Packman,' and 'Thompson') for natural resistance to P. rapae and for yield performance. The cultivars in this study did not significantly differ in susceptibility to P. rapae infestation, and neoglucobrassicin and glucobrassicin concentrations did not explain P. rapae egg and larval abundance. We conclude that cultivar selection should not be based on natural resistance to pests in this case but rather based on cultivar performance. 'Belstar,' 'Fiesta', 'Green Magic,' and 'Packman' were top performers and produced a consistent and high quality product, and therefore we would recommended these cultivars as acceptable selections for organic systems in Minnesota. |
