Classical biological control is a practice to control alien invasive weeds, but many introduced biological control agents exhibit only a weak negative impact on their targets. One reason is that prerelease impact studies in the natural environment are often difficult to carry out. Heracleum mantegazzianum Sommier and Levier (Apiaceae), which is native to the Caucasus, is a perennial noxious weed introduced into Europe and North America. We examined the impact and host size preference of different endophagous insect guilds in the weed’s native range. Instead of the commonly used insect exclosure approach, we estimated plant vigor before and after herbivore attack under natural conditions. Endophagous herbivores were dominated by the weevil species Lixus iridis Olivier, Nastus fausti Reitter, and Otiorhynchus tatarchani Reitter (Coleoptera: Curculionidae), the fly Melanagromyza heracleana Zlobin (Diptera: Agromyzidae), and an unidentified root-boring agromyzid fly species. Most observed insect species exhibited a strong preference for either big or small plants, but none of them caused serious damage within the study period. Occurrence of root-feeding weevils was associated with weak plants, but because of their long larval development, it was not possible to assign this relationship clearly to either feeding damage or host size preference. A comparison with other studies indicated that mature H. mantegazzianum plants are quite tolerant to herbivory. Insects belonging to the feeding guilds studied here will probably not guarantee successful biological control. Further research should focus on earlier stages in the weed’s life cycle.