- Vegetables: broccoli
- Crop Production: tissue analysis
- Education and Training: on-farm/ranch research
- Pest Management: field monitoring/scouting, integrated pest management
- Production Systems: organic agriculture
A controlled greenhouse study evaluated the effects of imported cabbageworm herbivory on induction of glucosinolates (secondary plant metabolites) and subsequent imported cabbageworm growth and performance on two broccoli (Brassica oleracea var. italica) cultivars (‘Beneforte’ and ‘Green Magic’). A field study was performed in 2015 and 2016 at two sites to evaluate six broccoli cultivars (‘Belstar,’ ‘Fiesta,’ ‘Green Magic,’ ‘Marathon,’ ‘Packman,’ and ‘Thompson’) for natural resistance to imported cabbageworms and for performance in yield and crown quality. Our research goals were two-fold: 1) to advance the understanding of the roles that glucosinolates play in natural and induced plant resistance, especially on cultivated crops and 2) to make broccoli cultivar recommendations to organic growers, thereby contributing to Minnesota’s ability to produce high-quality, organic broccoli.
The imported cabbageworm (Pieris rapae Linneaus) 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. Imported cabbageworms are 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 glucosinolates (GSLs), secondary metabolites in brassicas, may play a role in resistance to insect pests. GSLs are constitutive (naturally present) and inducible (activated) in the plant, and they are toxic to generalist insect pests. However, the relationship between GSLs and imported cabbageworms, a brassica specialist, is complex and not well understood, and more research must be done to better understand their roles in natural and induced resistance against insect pests, especially on cultivated crops.
Moreover, demand for local produce is on the rise in Minnesota and elsewhere in the Upper Midwest. While the demand for local produce has increased at the consumer level, there has not been a concurrent increase in research to meet those demands. In surveys done in 2009, 2011, and 2014 by the Minnesota Department of Agriculture, organic fruit and vegetable growers ranked "organic variety trials", "insect pests and management", and "variety development" as several of the top five research priorities. Though there are recent broccoli cultivar recommendations for the Upper Midwest, these recommendations assume conventional management. In organic systems, crops need to yield a consistent and high-quality product over diverse, sometimes sub-optimal, climatic conditions and endure various biotic and abiotic stressors, and therefore cultivar trials in organic systems should be performed to determine adaptability to local growing conditions and performance in systems with restricted agronomical practices.
Induced herbivory greenhouse study
i) Determine if feeding by imported cabbageworms influences broccoli defense by inducing two glucosinolates (glucobrassicin and neoglucobrassicin)
ii) Determine if previous feeding affects subsequent imported cabbageworm growth and performance
Broccoli cultivar trial
i) Determine natural imported cabbageworm egg and larval populations on six broccoli cultivars
ii) Determine concentrations of glucobrassicin and neoglucobrassicin of six broccoli cultivars and correlate to insect pressure
iii) Determine yield and crown grading of six broccoli cultivars