Chemical diversity in Brassica oleracea affects biodiversity of insect herbivores

Intraspecific variation in plants plays a major role in the composition and diversity of the associated insect community. Resistance traits of plants are likely candidates mediating community composition. However, it is debated whether total concentrations of chemical compounds or specific compounds determine herbivore resistance, and how chemical diversity among plant genotypes in turn affects the composition of the associated herbivore community. To study the role of specific chemical compounds in affecting the herbivore community, we used cultivated Brassica oleracea. The cultivars differ qualitatively in glucosinolate profile, i.e., foliar composition of different glucosinolate compounds, and only a little in total concentration of glucosinolates, the secondary metabolites specific for the Brassicaceae family. In field and laboratory experiments, we tested whether individual compounds explained differences in herbivore community composition, and whether herbivores with a similar degree of host plant specialization responded in a similar way to variation in glucosinolate profiles. In the field B. oleracea cultivars differed widely in species richness and composition of the herbivore community, as well as in the density of insects they harbored. Plants with high concentrations of the short side chain alkenyl glucosinolate, glucoiberin, harbored low herbivore diversity. Higher biodiversity was found when plants had glucosinolate profiles containing high concentrations of glucosinolates with elongated side chains, which are biosynthetically linked to glucoiberin. Although glucosinolates are known to have differential effects on generalist and specialist herbivores, all herbivore species exhibited similar responses to the intraspecific variation in foliar glucosinolate profiles of the B. oleracea cultivars. This observation is supported by the correspondence between oviposition preferences of the specialist herbivore Pieris rapae and the generalist Mamestra brassicae in the field and the laboratory, using the same cultivars, and may be due to the relatively low concentrations of glucosinolates in cultivars. Our results show that variation in the concentration of short side-chain glucosinolates affects the composition of the herbivore community associated with brassicaceous plants.