The ecological implications of tyrosine supplementing symbioses in grain pest beetles Symbiosis has repeatedly enabled insects to tap into the rich metabolic repertoire of microorganisms thereby enabling them to gain access to improved nutritional and defensive capabilities. In herbivorous beetles nutritional symbionts that supplement their hosts’ diet with tyrosine precursors via the shikimate pathway used for cuticle biosynthesis are especially pervasive. In this doctoral thesis I elucidate how tyrosine supplementing symbionts mediate life history traits and ecological interactions in diverse grain pest beetles. I first show that while the intracellular symbiont Shikimatogenerans is present in all life stages of O. surinamensis there is a life stage specific increase in symbiont density during metamorphosis and early adulthood. This dramatic symbiont increase coincides with an increased demand for symbiont provisioned tyrosine precursors used to accelerate cuticle biosynthesis. A time series analysis of cuticle development further showed that symbiotic beetles form their cuticles more rapidly than do their experimentally symbiont depleted (aposymbiotic) counterparts. However host investment into symbiont proliferation is metabolically costly and this cost was reflected in a delayed onset of reproduction in symbiotic females. Furthermore I demonstrated an ecological benefit of these associations by showing that survival against predators and pathogens increases rapidly in young symbiotic beetles when compared to aposymbiotic beetles thus correlating with accelerated symbiont mediated cuticle formation. Finally I demonstrated that shikimate pathway encoding symbionts are generally widespread in beetles. Symbiont mediated tyrosine provisioning thus transcends specialised intracellular symbionts and is a function that may also be fulfilled by loosely associated general gut microbial communities. The work of this thesis yields insights into the broader (and often understudied) benefits of nutritional symbionts.