Previous work on pea aphid (Acyrthosiphon pisum) host-plant associated races has attributed their divergence to genes involved in chemosensory functions and metabolism of chemicals. In this study the host plant metabolic processes that drive A. pisum host plant race formation were investigated. First, profiles of aphid acceptance of plants were developed using the electrical penetration graph (EPG) technique. The acceptance of four A. pisum clones from two host races, associated with Medicago sativa or Trifolium pratense, was profiled across nine Medicago and ten Trifolium plant species. Acceptance profiles correlated strongly with aphid performance on plants. Aphid acceptance profiles were then compared with untargeted metabolomic profiles of plants, using random forest regression. Analysis revealed a small number of compounds that explained a large proportion of the variation in the A. pisum races differential acceptance of plant species. Two of these compounds were identified using tandem mass spectroscopy as L-phenylalanine and L-tyrosine, suggesting a possible link to the expression of a specific plant metabolic pathway. M. sativa and T. pratense plants were then pre-exposed to two divergent A. pisum clones. Aphid responses to pre-exposed and control plants were then profiled using EPG. The results suggested that M. sativa and T. pratense plants differ in their fixed (constitutive) and dynamic (induced or suppressed) responses to aphid attack. Exposing M. sativa plants to A. pisum clones appeared to also cause a change in the concentration of L-tyrosine, further suggesting a role of plant metabolic pathways in A. pisum divergent acceptance behaviour. The same two aphid clones were tested to see if they responded positively or negatively to diets containing varied concentrations of L-phenylalanine or L-tyrosine, but no conclusive evidence of aphid repulsion or attraction was found. This project identified that elements of plant chemical ecology could underlie divergent selection among A. pisum host races.