Specificity and regulation of chemical defenses in black poplar (Populus nigra L.) against insect herbivores The interactions of plants and their herbivores are complex in part because of the specificity of plant defense responses. The specificity of anti herbivore defense helps plants reduce their physiological and ecological costs while at the same increasing the effectiveness of defenses. This thesis investigated the specificity of phytochemical anti herbivore defenses in a boreal tree species the black poplar (Populus nigra L.) using up to five herbivore species belonging to the same feeding guild but with different levels of specialization. It mainly focused on defense specificity based on herbivore identity but also investigated induction profiles in response to different herbivore developmental stages (adult stage versus egg stage). All in all the main defense of black poplar the salicinoids showed no herbivore specific induction and little induction in general. In contrast Kunitz type trypsin protease inhibitors (KTIs) as well as two groups of volatile organic compounds responded in an herbivore specific fashion. Interestingly the greatest differences in black poplar defense specificity were observed at a higher taxonomic level between coleopteran and lepidopteran herbivores. Differences in the regulation of defense responses were also observed within different developmental stages of a single herbivore species. Herbivory and oviposition on black poplar leaf foliage by C. populi resulted in different phytohormone responses although none of the other measured metabolites showed significant changes caused by C. populi eggs. The data obtained highlight the complexity of plant herbivore interactions and show that both herbivore specific defenses as well as non specific defenses have the potential to affect the composition of poplar associated herbivore communities.