Abstract: During the last few years my coworkers and me started to elucidate the chemicalecology of jumping plant lice (Hemiptera: Psyllidae) and their interactions with vectoredphytoplasma and different host plants. We investigated the olfactory reactions of these insects tohost plants used for reproduction or overwintering, and elucidated chemically mediatedinteractions between all players in this multitrophic system. While e.g. the behaviour of thehawthorn psyllid C. melanoneura was not influenced by phytoplasmas, complex interactionsbetween Malus domestica, the apple psyllid C. picta, and Ca. P. mali were investigated inlaboratory and field. Results from Y-tube olfactometer trials showed that immature adults ofC. picta are able to distinguish the odours of healthy and infected apple trees and preferred theodour of infected trees. GC-MS analysis of the headspaces revealed the induction of asesquiterpene in infected trees, which was proofed to be attractive for C. picta in olfactometertrials. Finally, we investigated the influence of the psyllids’ nymphal development on infectedtrees by measuring several performance parameters. In total, the fitness of the offspring ofC. picta was influenced negatively, but the females developed a strategy to cope with it. Thismeans that the phytoplasma directly manipulates plant physiology and indirectly psyllidbehaviour, leading to a better spread within its host plant population. In contrast, its vectorC. picta evolved mechanisms to minimize harmful effects emanated by the phytoplasma.Currently we started fundamental research projects on several phytoplasma – vector – plantsystems with the aim to develop species specific traps for monitoring and mass trapping of thesevector species of fruit tree phytoplasmas.