Micro-organisms, in particular symbiotic bacteria, can affect the way that herbivorous insects utilise their host plants. The best known examples are the obligate endosymbionts that enable their hosts to feed on diets impoverished in essential nutrients. But recent research has identified new ways in which bacteria can influence insect-plant interactions, and which often involve facultative rather than obligate associations. In some species facultative symbionts are associated to host plant use, the best known example coming from pea aphids (Acyrthosiphon pisum) where surveys demonstrate that particular bacteria are strongly associated with aphids on certain food plants. The associations might be explained if symbionts benefit their insect hosts through the manipulation of plant-induced defenses (i.e. the systemic mobilization of plant defenses after insect attack, for example through the emission of volatile organic compounds that promote the effectiveness of natural enemies). I propose to explore systematically the effects of facultative insect bacterial symbionts on induced plant defences from plant gene expression to wider community effects using the pea aphid model system. This species has by far the best characterised symbiont community. My approach will bring together ideas and techniques from the evolutionary theory of symbiosis, community ecology, plant physiology and modern molecular plant biology. This project will be developed during two years at the Wageningen University in the laboratory of Prof. Marcel Dicke, a leading expert in plant induced defenses and multitrophic interactions. After my previous experience studying insect ecology in Spain and the United Kingdom, this fellowship is the ideal next step for reaching my scientific and professional maturity, to build up lasting cooperation and collaboration and finally to establish my own group afterwards.