Metabolic profiling: a new tool in the prediction of host-specificity in classical biological control of weeds?

Current host-specificity testing for the selection of environmentally safe weed biological control agents is based on the molecular phylogeny of the weed. According to the centrifugal phylogenetic theory, non-target species closely related to a target weed should be at greater risk of attack by a biological control agent than distantly related plant species, as they are biochemically and morphologically more similar and therefore more likely to share the cues used by specialists to select their host. However, whether a molecular phylogeny is a suitable surrogate for phenotypic traits at lower taxonomic levels remains poorly tested. In the model of the potential weed biological control agent Ceutorhynchus cardariae Korotyaev (Coleoptera: Curculionidae) for the invasive Brassicaceae plant Lepidium draba L., several distantly related plant species were attacked by C. cardariae under no-choice conditions, revealing a disjunct host range. The present study compared the feeding preferences of a specialist (C. cardariae) and for comparison an oligophagous (Plutella xylostella L., Lepidoptera: Plutellidae) insect testing 47 plant species. Their respective host choices were assessed using different phylograms based either on molecular data, physical attributes (leaf dry matter content, trichome shape and density) and plant chemistry. We hypothesized that the feeding preferences of these two species would be influenced by different sets of cues and that the host choice of both insects does not follow the molecular phylogeny at lower taxonomic levels. Furthermore, we suggest that a phylogram based on phenotypic traits may be a better predictor of host use for both insects.