Graduation date: 2008The cinnabar moth (Tyria jacobaeae (L.), Lepidoptera: Arctiidae) is an icon in population ecology and biological control that has recently lost its shine based on evidence that (1) it is less effective than alternatives (such as the ragwort flea beetle Longitarsus jacobaeae (Waterhouse) (Coleoptera: Chrysomelidae) for controlling ragwort Senecio jacobaea L. (Asteraceae), (2) it eats nontarget plant species (including arrowleaf ragwort Senecio triangularis Hook. (Asteraceae), a native North American wildflower), and potentially harms the animals that depend on these native plant species, and (3) it carries a disease (caused by a host-specific microsporidian Nosema tyriae). This presents us with an opportunity to study whether poor nutrition and disease might constrain colonization of new hosts by this phytophagous insect and thereby mitigate risk of biological control to nontarget plant species. We evaluated the interactions within a tritrophic system composed of the cinnabar moth (herbivorous insect), its Old and New World Host plant species, and its entomopathogen (Nosema tyriae) both at the individual and population levels in a controlled environment. Chapter one concentrates on the two trophic (herbivore-host plant) interactions, addresses the importance of the preference and performance relationship, and the reasons why we might observe a weak relationship between preference and performance. Performance was measured both by vital rates and by population dynamic parameters, and we conclude that the projected population growth rate of the cinnabar moth population is the best indicator of host suitability. We found a positive correlation between preference and performance in the cinnabar moth (Tyria jacobaea) on Old World and New World host plants. The second chapter incorporates the third trophic level, the pathogen Nosema tyriae, and measures the individual and interacting effects of pathogen dose and host plant species on the performance of the cinnabar moth. It concludes that all cinnabar moth vital rates (rates of growth, development, survival, and reproduction) decrease with the increasing dose of pathogen (Nosema) spores. Vital rates generally were lower on the New World host S. triangularis compared to Old World host S. jacobaea. The projected population growth rates of cinnabar moth populations were more sensitive to low infection dose in cinnabar moth populations on the New World host S. triangularis compared to the Old World host S. jacobaea. At high pathogen doses, the effect of the pathogen was so overwhelming that no effect of host could be expressed. In conclusion, we observed a strong positive correlation between preference and performance of the cinnabar moth on the New World and Old World test plants. In the most successful new host-herbivore association, the cinnabar moth was more vulnerable to the impact of the natural enemy on New compared to Old host plant species.