Ryegrass infected with the endophyte AR37 is of significant value to New Zealand’s pastoral sector. It provides superior protection against a range of ryegrass pest insects, and added an estimated $42 million dollars to New Zealand’s economy between 2007 and 2011. Epoxy-janthitrem alkaloids, produced by AR37, are thought to provide this bioactivity, but limited experiments investigating this have been conducted to date. A porina larval bioassay was undertaken to investigate the bioactivity of epoxy-janthitrem I, the major epoxy-janthitrem compound expressed by AR37. The alkaloid was found to be a feeding deterrent at all three concentrations tested (1, 2.5 and 5 μg/g wet weight), with stronger deterrence observed at the two higher concentrations. Some evidence of toxicity of epoxy-janthitrem I was also identified; future work to resolve this is discussed. Temperature and ryegrass cultivar are two factors known to affect the concentration of alkaloids in ryegrass. How these factors affect epoxy-janthitrem concentrations in AR37-infetced ryegrass has never before been examined. Perennial and Italian ryegrass were grown in high (20°C) and low (7°C) temperature controlled environment rooms for 10-12 weeks, ryegrass was then freeze dried, ground and incorporated in a semi-synthetic diet which was fed to porina larvae to examine how these variations in alkaloid concentration would affect larvae. AR37-infected ryegrass grown at high temperature was found to contain higher concentrations of epoxy-janthitrem, which when fed to porina larvae had a strong anti-feedant effect and reduced larval survival. In comparison, AR37-infected ryegrass grown at low temperature had a small anti-feedant effect on larvae and only when in perennial ryegrass. Epoxy-janthitrem concentrations were slightly affected by plant cultivar and concentrations were identified to be higher in the pseudostems when compared to the leaves of ryegrass plants. Findings from this thesis improve on the understanding of the bioactivity of epoxy-janthitrem and explain reports from colder areas of the country, that AR37 is not able to adequately control porina populations. Further research should clarify the limitations of the AR37 endophyte in cooler regions by first identifying whether these results are replicated in the field and then by identifying the temperature ranges the AR37 endophyte may not be as effective in.