Chemo-ecological responses of apple plants to insect herbivory

Plants respond to insect herbivory by emitting a specific subset of volatile compounds called herbivore-induced-plant-volatiles (HIPVs), which do not only mediate plant-insect interactions, but are also involved in plant-plant signaling and priming of neighboring plants. This dissertation investigated the volatiles involved in the chemo-ecological interactions between apple (Malus domestica Borkh.) and two of its main tortricid defoliators, namely the European species Pandemis heparana Denis and Schiffermüller and the North American species Pandemis pyrusana Kearfott. Studies were conducted to characterize the volatiles emitted from apple foliage that were either intact, mechanically-damaged or exposed to larval feeding by P. heparana. Sixteen of the characterized volatiles elicited consistent antennal responses in P. heparana adults. When the most promising volatiles were tested in the flight tunnel, only lures containing acetic acid in combination with either phenylacetonitrile or 2-phenylethanol were significantly attractive to P. heparana adults. A field trial conducted in Sweden demonstrated that traps baited with the same lure combinations were highly attractive to P. heparana also in field conditions. This work was replicated on P. pyrusana. Nine aromatic compounds and other 12 volatiles characterized in P. pyrusana-infested apple foliage were tested in the field in combination with acetic acid, but only phenylacetonitrile and 2-phenylethanol caught significantly more adult P. pyrusana than acetic acid alone. These two independent studies clearly showed that the combinations of acetic acid with either phenylacetonitrile or 2-phenylethanol are highly attractive to both P. heparana and P. pyrusana. The following studies aimed to prove the occurrence of plant-plant signaling and priming phenomena in apple. Indeed, apple plants that were exposed to HIPVs of neighboring conspecifics infested with P. heparana larvae responded to a subsequent infestation with a significantly higher emission of several induced volatiles with respect to non-exposed, infested plants, thus indicating a priming effect. Similar experiments performed with the root-feeding beetle Melolontha melolontha L. showed that its feeding damage on apple plants is able to trigger a systemic response, which leads to volatile-mediated plant-to plant signaling similarly to what observed in response to an aboveground damage. Knowledge gained from this work is essential for the development of new environmentally friendly techniques of insect pest control based of natural volatile organic compounds.