Abstract <p>Alien plants, when introduced into an ecosystem, can become new food sources for native phytophagous insects. However, in the early stages of invasion, insects are often not adapted to utilizing these plants. This paper examines the adaptive mechanisms of phytophagous insects during their host shift to new plants. Morphological, physiological, and behavioral changes necessary for exploiting a new food resource are described. Physiological adaptations can include changes in gene expression, particularly detoxification and digestive enzymes, mutations in detoxifying enzymes increasing metabolic efficiency, mutations in toxin target sites, increasing resistance through horizontal gene transfer or associations with microbiota. Possible scenarios for phytophagous insect transition to alien host plants are discussed, depending on the relative rates of acquiring of physiological and behavioral adaptations in the insect population.</p>