Plant secondary metabolite diversity as a resistance trait against insects: a test with Sitophilus granarius (Coleoptera: Curculionidae) and seed secondary metabolites

A hypothesis is tested about the functioning of plant secondary metabolite diversity taking into account the role of concentration of mixtures or single compounds: a high diversity secondary metabolite mixture in high concentration provides a more effective protection against herbivores than single compounds or low diversity mixtures in both low and high concentrations. Additionally the role of two unusual secondary metabolites for a weevil when they are incorporated in its diet is explored. Number of eggs laid and diet consumption are measured in the granary weevil Sitophilus granarius exposed to cereal and non-cereal secondary metabolites incorporated in artificial diets to test the hypothesis. Artificial diets had one, two, four or six compounds. The mixtures or single compounds were offered simultaneously to the weevils in concentrations ranging from 0.05 to 5mg g?1 in a multiple choice experiment. Low oviposition in high concentration six compounds mixture supported the hypothesis. However, low oviposition in a low concentration mixture of four compounds suggests that a moderate diversity may be equally functional in low and high concentrations. Diet consumption by the weevil was not affected by concentration but an increasing number of compounds in the diet reduced it slightly but significantly. The unusual secondary metabolites for the weevil were less or equally effective reducing oviposition and consumption as usual compounds did.