Adaptations of an insect to a novel host plant: a phylogenetic approach

1. The importance of behavioural vs physiological adaptations in the evolution of host associations by herbivorous insects is largely unknown. 2. We compared sister species of beetles, one of which, Ophraella slobodkini, feeds on the lineage's ancestral host, Ambrosia artemisiifolia, while O. notulata has shifted to a novel host, Iva frutescens. Assuming O. slobodkini represents the features of the Ambrosia-feeding ancestor, we asked if behavioural and physiological barriers to utilizing Iva existed and if adaptation to these barriers occurred. We also tested for trade-offs between use of novel and ancestral hosts by O. notulata. 3. We found evidence that the ancestor of O. notulata would have been deterred from feeding on Iva and suffered lower conversion efficiency. 4. Ophraella notulata appears to have adapted behaviourally by increasing consumption of Iva, but we did not detect a significant increase in its physiological capacity to use Iva. Additionally, the switch to Iva by O. notulata did not reduce its physiological capacity to use the ancestral host, Ambrosia. 5. Our results suggest that novel host associations may arise from behavioural adaptations, with physiological adaptations a secondary result of behavioural changes. We discuss implications for hypotheses of host shifts and the evolution of specialization.