New Zealand has several insect pasture pests which cause economic losses of between $1.7 B and $2.3 B annually in the dairy, and ‘sheep and beef’ industries. Research on grass species defence provides insights into methods to reduce this economic impact. Epichloë endophytes and silicon accumulation are two well-studied mechanisms of reducing phytophagous insect damage in grass species, but their potential synergies have not been investigated. Previous research has hypothesised that plants infected with endophyte may accumulate more silicon than their non endophyte infected counterparts, suggesting multi-tiered defences. Of specific interest to this thesis is the potential impact of silicon supplementation on the alkaloid profiles of novel grass-endophyte associations. This research aimed to investigate changes to silicon concentration over time, endophyte growth and alkaloid concentration in two cool season grass species from the sub-family Pooideae and subsequent effects of these variables on major New Zealand insect pests. This interaction was investigated through the use of whole plant glasshouse trials, excised root bioassays, and artificial diet experiments using a range of above- and below ground phytophagous pasture pests (Listronotus bonariensis, Wiseana copularis and Costelytra giveni) as well as a generalist herbivore model study organism (Epiphyas postvittana). The two grass species included; Lolium perenne infected with a novel association with an endophyte naturally found in Festuca arundinacea (Epichloë coenophiala), and Festuca pratensis infected with its naturally occurring endophyte (Epichloë unicatum). Results from this research do not indicate that, in the grass endophyte associations studied, endophyte infection is linked to an increase in plant silicon content. There is evidence to suggest that the herbage material of L. perenne endophyte-infected plants has less silicon than endophyte free plants. There were no direct correlations between plant silicon content and the production of bioactive secondary metabolite alkaloids (lolines) or endophytic mycelial mass in F. pratensis and L. perenne. Although silicon supplementation was not linked to a direct increase in plant silicon content, there was an increase in loline production in the herbage of L. perenne, indicating a potential role of silicon in either modulating the soil environment or influencing plant biochemical reactions potentially leading to a change in production of lolines. This study also found negative effects of endophyte alkaloids on insect performance and feeding which correlate to previous literature but was not able to accept nor reject the initial hypothesis of synergistic effects of the two defences studied. Interestingly, results suggested differential allocation of silicon between plant species and lolines within individual plant tissues. Silicon was higher in the root material of L. perenne than F. pratensis and the opposite was true for the herbage material. Also, the proportion of each loline in the root and herbage of F. pratensis differed. In summary, this research has provided insights into the temporal interactions between silicon and endophyte infection. However, further research is required to investigate the multi tiered effects of these two plant defences on economically important phytophagous insects.