The ability of non-native plants to successfully invade and dominate ecological communities is a central area of ecological study. Many important hypotheses such as the evolution of increased competitive ability and the novel weapons hypothesis suggest that the reason for non-native success is the ability of non-native plants to have reduced herbivore attack, allowing them a competitive advantage over native plants that are attacked by insect herbivores. The insects that "choose" the "winners and losers" of plant-plant interactions are also dealing with their own antagonistic relationships, particularly parasitoid predators. Many herbivores have been found to deal with these parasitoids by medicating themselves through their diets. We tested to see if this self-medication may help explain herbivore preference for native or non-native plant species by testing the immune responses and survivability of Trichoplusia ni fed on the native Mimulus guttatus and the non-native Nasturtium officinale. We also tested the plant consumption rates for caterpillars with and without parasitoid interaction when presented with Nasturtium officinale and Mimulus guttatus together. Immune responses of 5th star T. ni were elevated while feeding on N. officionale compared to those feeding on M. guttatus. Further, we found that T. ni neonates were not able to survive on N. officionale while neonates survived on M. guttatus. Our results suggest that herbivores may alter plant-plant competition when they are faced with their own top-down pressures, while still having significant restraints on their ability to consume.