Water and light are two essential factors for plant growth and development. While increasing evidence underscores their significance in plant biotic interactions, the ways in which these factors shape plant-insect interactions remain unclear. In the first part of my work, I investigated the impact of water availability on plant-insect interactions. We discovered that drought stress leads to alterations in the defense strategies of plants, including enhanced resistance and reduced tolerance to herbivory. Drought stress also compromised the plant's indirect defenses, as evidenced by decreased numbers of natural enemies and reduced emission of repellent volatiles under such conditions. Furthermore, we documented the capacity of insect herbivores to induce drought-like responses (e.g., stomatal closure) through their salivary proteins. These stomatal closures were closely tied to the inhibition of important defense-related HIPVs, mirroring the HIPV changes induced by water deficits. Our findings suggest that herbivores may exploit drought-like responses of plants to their advantage, revealing the connection between stomatal behavior and HIPV emission. The second part of my work delves into the role of light in the host range evolution of lepidopteran herbivores. We discovered that higher plant volatile emissions correlate with host plant specialization within the Lepidoptera lineage. In contrast, reduced levels of volatile emission at night are associated with dietary generalization. Drawing from these results, we proposed the "Salient Aroma Hypothesis" that anticipates the evolutionary patterns of host plant specialization based on diel variations in plant volatile signals. Overall, our work enhances our understanding of how water and light impact the ecology and evolution of plant-insect interactions.