Abstract Herbivorous insects and their host plants comprise most known species on Earth. Illuminating how herbivory repeatedly evolved in insects from nonherbivorous lineages is critical to understanding how this biodiversity is created and maintained. We characterized the trophic niche of Scaptomyza flava, a representative of a lineage nested within the Drosophila that transitioned to herbivory ~10–15 million years ago. We used natural history studies to determine whether S. flava is a true herbivore or a cryptic microbe‐feeder, given that the ancestral character state for the family Drosophilidae is likely microbe‐feeding. Specifically, we quantified oviposition substrate choice and larval viability across food types, trophic‐related morphological traits, and nitrogen isotope and sterol profiles across putatively herbivorous and nonherbivorous drosophilids. The results of these studies show that S. flava is an obligate herbivore of living plants. Paired with its genetic model host, Arabidopsis thaliana, S. flava is a novel and powerful system for exploring mechanisms underlying the evolution of herbivory, a complex trait that enabled the exceptional diversification of insects.
