Institut de Zoologie, University of Neuchâtel, Case Postale 2, 2007 Neuchâtel, Switzerland.
Wind tunnel experiments were performed in order to determine whether Oreina cacaliae (Coleoptera, Chrysomelidae) can discriminate between plant volatiles emitted by host and non-host plants. In addition, we tested if already attacked host plants are more attractive than unattacked ones. All plants offered in the dual choice tests are closely related taxonomically belonging to the same subtribe (Senecioninae) of the family Asteraceae. In early spring, the flowering host Petasites paradoxus was preferred over the flowering non-host Tussilago farfara. P. paradoxus were more attractive to the beetles when conspecifics have been feeding overnight on the plants, but not when the plants had been attacked during only half an hour before the experiments. Later in the season O. cacaliae changes its host and feeds on leaves of Adenostyles alliariae, a plant that contains pyrrolizidine alkaloids the beetles sequester for their own defence. Contrary to P. paradoxus the host plant A. alliariae became more attractive after a short attack by conspecifics (half an hour before the experiment) but lost its increased attractivity when it was attacked overnight. Gas chromatographic analyses of the emitted odours showed quantitative differences between attacked and unattacked host plants. Results indicate that O. cacaliae is able to discriminate between odours emitted by host and non-host plants as well as by attacked and healthy plants.