Abstract <p>Understanding the ecological mechanisms limiting population persistence is essential for the conservation of highly specialised and endangered insects. <i>Tomares nogelii dobrogensis</i> is one of the rarest butterflies in Europe and is strictly monophagous on <i>Astragalus ponticus</i> in Romania. Despite its high conservation relevance, quantitative data on its ecology and life history have been largely lacking. During the 2025 flight season, an extensive field study in northern Dobrogea, Romania, investigated phenology, adult lifespan, behaviour, habitat characteristics and host-plant synchrony across six colonies. Patch-level analyses were used to assess how habitat structure, light availability and host plant phenology influenced adult abundance. Oviposition patterns and preadult stages were documented. Adult flight activity closely overlapped with the reproductive phenology of <i>A. ponticus</i>, with peak butterfly abundance coinciding with bud formation and flowering. Observed adult lifespan was short, resulting in a narrow reproductive window. At the patch scale, butterfly abundance increased strongly with host-plant patch area and light availability, while host plant density alone was not a significant predictor. Oviposition was highly aggregated among host plants, occasionally leading to high larval densities and cannibalism.</p> Implications for insect conservation <p>Our results indicate that population performance of <i>T. n. dobrogensis</i> is primarily constrained by the spatial and temporal availability of suitable host plant stages. Conservation management should therefore prioritise the maintenance of large, open <i>A. ponticus</i> patches and minimise disturbances by livestock during the flight and larval feeding periods. In addition, illegal collecting, which represents a significant threat to the species, must be prevented, e.g. by strict admission controls during flight and larval time.</p>