Effects of Glucosinolates in Broccoli on Imported Cabbageworms and Implications for Organic Pest Management

The imported cabbageworm (Pieris rapae L.) is a specialist herbivore that feeds on species in the Brassicaceae (mustard or cruciferous) family. It is a serious economic pest in commercial horticulture because larvae chew large holes in leaves, can destroy developing heads of broccoli, cauliflower, and cabbage, and otherwise contaminate or stain produce, rendering the produce unmarketable. In conventional systems, this pest is controlled with broad-spectrum insecticides, but P. rapae is notoriously difficult to manage in organic systems. While there is conflicting evidence on the effects of the naturally-occurring defensive compounds (glucosinolates) found in brassicas on specialist herbivores such as P. rapae, understanding the roles that they play in mediating plant-insect interactions may provide alternative pest management strategies. Working on two certified organic sites in the upper Midwest (St. Paul, MN and Ramsey, MN), as well as in the greenhouse, replicated experiments were conducted to determine 1) the glucosinolate content in six field grown broccoli (Brassica oleraceae var. italica) varieties and correlate glucosinolate concentration to P. rapae egg and larval populations; 2) if previous herbivore damage influences broccoli defense against lepidopteran pests by inducing glucosinolates; and 3) if previous herbivore damage influences feeding by lepidopteran pests. Results show that broccoli varieties vary widely in glucosinolate content, and we will report these results. Implications of this research could lead to selection of cultivars with specific glucosinolate profiles to reduce pest pressure in organic systems.