The vast majority of methods for insect biodiversity assessment are invasive and require insect specimen collection and fixation. Specifically, current methods are unsuitable for studying insect communities that reside in plant galls, which serve as intricate habitats for various organisms, including hosts, predators, parasitoids, and inquilines. In this research, we propose new non-invasive methods for the quantitative and qualitative determination of insect biodiversity in plant galls. A gall is a mini ecosystem consisting of gall-formers, their parasitoids, predators, and inquilines. The main challenge in the determination of insect species in a gall is the need to dissect a gall, destroying a hidden ecosystem. The development of non-invasive methods will allow for the study of endangered insect species inside galls. Using regression analysis, we established a precise correlation between the volume of Hieracium robustum Fr. galls and the number of its inhabitants, as well as the ratio of hosts to parasitoids. Moreover, we found that each of the nine insect species in these galls has a unique morphological pattern of the juvenile feces. Therefore, the species composition can be determined using these morphological observations. Importantly, this method is non-invasive, as the feces observation inside the galls occurs after the emergence of gall inhabitants into their natural environment and seasonal plant death. Our research can pave the way for new methods in biodiversity measurements, specifically in plant galls.