Competition, herbivory and genetics interact to determine the accumulation and fitness consequences of a defence metabolite

1. Plant defence theory predicts that chemical defences should be optimally deployed to protect against herbivore damage, but avoid production costs when herbivores are rare. However, many chemicals involved in herbivore defence are also involved in other interactions, such as competition. 2. We studied how the accumulation of sinigrin, the dominant glucosinolate defence metabolite of Brassica nigra, responded to field manipulations of herbivory and competition in groups artificially selected for high or low constitutive sinigrin levels. Additionally, we quantified the fitness costs and benefits of sinigrin production in these treatments. 3. The induction response to molluscan herbivores was independent of a genotype's constitutive sinigrin level. In contrast, genotypes selected for low constitutive sinigrin levels were significantly more inducible in response to plant competitors as compared to genotypes with high constitutive levels. 4. Accordingly, the fitness costs and benefits of sinigrin production agreed with optimal defence theory in the absence, but not in the presence, of competition. 5. Thus, predictions for the optimal investment to defensive traits may be misleading if they do not incorporate all of the costs and benefits of traits involved in multiple interactions.