Plants and their herbivores constitute more than half of the organisms on earth. Therefore a better understanding
of the evolution of plant defenses against their herbivores is central for our understanding of biodiversity and
species interaction, as well as better managing insect pests in agro-ecosystems. When under above- or
belowground attack, plants resist herbivores by different means, including the attraction of predators or
parasitoids of the herbivores near the site of damage by producing volatile organic compounds (VOCs). This
phenomenon has been referred to as ‘indirect plant defense’. The term defense here implies a fitness benefit for
the plant, and consequently, an adaptive value for volatile production. However, whether the production of
herbivore-induced VOCs has evolved and being maintained as a defense over evolutionary times, and whether
the attraction of carnivores benefits plant fitness has not yet been proven. I therefore here propose to 1) study the
macro-evolution of VOCs production and the subsequent carnivore attraction using a phylogenetic comparative
approach, 2) test the fitness benefits of VOCs production and nematode attraction at the genotypic (within
species) level, and 3) test whether there is local adaptation in the tripartite interaction between plants, herbivores
and predators along the ecological gradients. I will use 40 species of grasses and measure root volatile
production and subsequent predatory nematode attraction using a custom-designed belowground olfactometer. I
will then test different models of evolution to address macro-evolutionary patterns of trait evolution. Second, I
will focus on one grass species that has a broad distribution in the Alps to measure selection for increased VOCs
production and subsequent nematode attraction. Third, I will perform reciprocal transplant experiments to test
for local adaptation of tritrophic interactions. Overall, the combined approach of behavioral assays, chemical
analysis, phylogenetic modeling, and field experiments that span macro- and micro-evolutionary times will
answer and greatly contribute the debate around the adaptive role of herbivore-induced volatile organic
compounds as plant indirect defenses.