The gall-wasp Aulacidea hieracii L., 1758 (Hymenoptera: Cynipidae) forms a stem gall on the hawkweed Hieracium × robustum Fries, 1848 (Asteraceae), a weedy herb that grows in the steppe biotopes of Eurasia. In its turn, gall-former serves as food for a large number of parasitoids and predators, which also live and develop inside and outside the gall. Moreover, the inhabitants of the galls are consumed by birds. In addition, a specific microbiota develops inside the gall, which includes representatives of gram-negative bacteria Pseudomonas rhizosphaerae, Curtobacterium flaccumfaciens, Pantoea agglomerans and fungi Alternaria alternata. The authors found out the composition of parasitoids, predators, inquilines, and microorganisms for a gall on the hawkweed. For the first time, studies were carried out on the development of the moth caterpillars Oxyptilus chrysodactyla ([Denis & Schiffermüller], 1775) on the hawkweed H. robustum. We found that the caterpillars of the moths cohabit on their fodder plant on the surface of the gall, together with the larvae of gall-wasps. Notably, the gall-wasp larvae are the first to inhabit the plant. A parasitoid from the Pimpilinae family was also bred from the moth caterpillars. Thus, the gall is a complex ecosystem, the study of which is not only of fundamental importance, but can be useful for solving the problems of the practical application of biological agents for weed control or for combating harmful species of gall-formers.