Shirin’s PhD aims to understand the physiological and molecular interactions between fruit fly larvae and their host fruit, and the influence of these interactions on larval performance and mortality of the Queensland fruit fly, Bactrocera tryoni. The study started with a focus on evaluating the performance of B. tryoni larvae in three ripening stages of the fruit of two tomato cultivars, and also the effect of fruit being on or off the plant. The tomato cultivars (‘cherry’ and ‘Roma’) were chosen based on a previously published study which identified that fully ripe fruit of these two cultivars differed significantly in their suitability to B. tryoni larvae (Roma was a good host, cherry a poor host). Following the ‘whole organism’ study, the work continues at the molecular level by doing transcriptomic and metabolomic analyses to determine molecular interactions between the fly larvae and the fruit. Shirin believes this study is significant for two reasons. Firstly, B. tryoni is a major pest in Australia and more basic information is needed for its long term control. Secondly, there is a general lack of detailed study on the interactions between fruit feeding insects and the fruit they feed upon, and so this thesis will significantly contribute to the larger field of insect-plant herbivory studies.