Populations have to keep their ability to survive and reproduce to be maintained upon environmental changes. This is challenging especially when fast changes occur and require adoption of new features, either metabolic, developmental, behavioral, physiological or morphological providing an enhanced fitness, through the process of adaptation. Two main mechanisms of adaptation have been described “phenotypic plasticity” and “adaptive evolution”. Phenotypic plasticity is defined as the ability of organisms to change of phenotype without changing genotype in response to environmental conditions [1]. Since they do not involve mutations, these different phenotypes are expected to involve distinct transcriptional programs. In the case of adaptive evolution, populations can acquire a better fitted phenotype to the environment thanks to the spread in the population of pre-existing genetic variants or of new mutations conferring enhanced capability (For review, [2] and [3]). Furthermore, adaptation to a novel environment can lead to evolution of reproductive barriers between populations and to the process of ecological speciation [4]. Phytophagous insects are particularly relevant models for the study of plasticity, adaptive evolution and speciation. Their dependence on their host plants for food, oviposition site, refuge against predators and sometimes for reproduction in addition to the parallel evolution of plants in response to this herbivorous pressure lead to constant adaptive changes in their physiology and behavior. Fidelity to their host-plants can promote spatial or temporal isolation of insect populations leading to new species [5]. Our main insect model, the moth Spodoptera frugiperda, or “fall armyworm (FAW)” is a pest of crops. It exists as two variants, one found on corn (C strain), the other found on rice or pasture grass (R strain). Both variants coexist in the same geographical areas, initially America, and are morphologically indistinguishable but possible to identify genetically. Since 2016, it has become invasive in Africa, India, China, Australia and threats European agricultures. Despite of this apparent preference for different host-plant ranges, the two strains remain highly polyphagous. By a combination of life history traits measurements and omics approaches, we explored the molecular basis of this plasticity. Using comparative transcriptomics and genomics approaches, we also analyzed the molecular basis of adaptive evolution to different host-plant ranges in lab and natural populations. By population genomics approaches, we have started to characterize the genomic signatures of adaptive evolution and strain differentiation, both in native and invasive populations. I will present our current hypotheses on the evolutionary status of the Fall armyworm based on this integrative study. 1.West-Eberhard MJ: Phenotypic plasticity and the origins of diversity. Annual Review of Ecology and Systematics, 1989, 20(1):249-278. 2.Orr HA: The genetic theory of adaptation: a brief history. Nat Rev Genet 2005, 6(2):119-127. 3.Fisher RA: The Genetical Theory of Natural Selection. Oxford: Oxford University Press; 1930. 4.Nosil P: Ecological speciation. Oxford: Oxford University Press; 2012. 5.Nosil P, Crespi BJ, Sandoval CP: Host-plant adaptation drives the parallel evolution of reproductive isolation. Nature 2002, 417(6887):440-443.