Giant knotweeds (Reynoutria spp.) are highly productive and aggressive invaders in riparian wetlands of Europe and North America. We sampled ground-dwelling beetles by pitfall traps from six sites comparing monotypic Reynoutria stands with the invaded native Urtica-dominated stands. Three sites were located in a semi-natural softwood forest and three sites were on a ruderal embankment. The analyses are based on a total of 13,244 individuals from 218 species. Location and site significantly influenced beetle assemblages. Moreover, there were pronounced differences between vegetation stands. The monotypic Reynoutria stands exhibited lower beetle abundance, species richness and rarefaction diversity irrespective of location. However, the negative effect on species richness, abundance and assemblage similarities were stronger on the transformed ruderal embankment than in the semi-natural softwood forest. Reynoutria invasion seems to influence microclimatic conditions. We found a higher abundance of silvicolous and a lower abundance of xerophilous ground beetles in the Reyountria stands than in the Urtica-dominated stands. Feeding guilds reacted differently to Reynoutria invasion that reduced the abundance of predators and herbivores but enhanced that of detritivores. Detritivores assumingly profit from the perennial presence of the large quantities of Reynoutria litter. We conclude that highly productive invaders pauperise the arthropod fauna and alter link strengths in trophic cascades shifting primary producer-based food webs to detritus-based food webs.