Ecological immunology and host plant adaptation in lepidopteran herbivores Herbivorous insects live in complex environments in which they intimately interact with a broad range of pathogens. Therefore herbivores require a range of efficient immune defense mechanisms that can cope with all potential pathogens as specifically and immediately as possible. Environments differ in the threats they pose: the risk of exposure to pathogens varies as well as the pathogenesis of those microorganisms. Therefore heterogeneous environments of herbivores will generate a complex natural selective pressure on their immune system possibly causing variations in immune system functions among herbivorous insects. Variation in immune system functions will also impose different costs of defense upon herbivores leading to complex patterns of trade offs against other life history traits such as sexual traits. The complexity of environments wherein herbivorous insects and pathogens interact is also impacted by plants. The evolutionary arms race between plants and their herbivores provides plants with a highly sophisticated defense system. However the relationship between plants and herbivores cannot be fully understood without incorporating the impact of herbivorous pathogens. We are only beginning to understand the complexity of herbivore pathogen and herbivore plant interactions that occur in complex environments. General explanations for both immune system variation and host plant adaptation in herbivorous insects are rare. Most immunological studies on herbivorous insects have almost exclusively focused on physiological and molecular mechanisms of the innate immune system generally neglecting the fact that herbivores interact with their environment. The aim of this thesis was to examine the importance of insect ecology on immune system variation and host plant adaptation in lepidopteran herbivores.