Augmentative biological control offers a potentially effective, but largely unexplored, opportunity to control native weeds with native phytophagous insects. Herbivore effects on the target weed may further be enhanced by interspecific competition with other plant species. We assessed the impact of root-boring larvae of the Sesiid moth Pyropteron chrysidiforme on the target weed Rumex obtusifolius for two groups of initially small and large plants, with or without competition from a Lolium perenne sward. In a field experiment, 106 Rumex roots were planted into plots with either pure L. perenne or bare soil, and R. obtusifolius plant performance was measured after one year. Overall, competition from the grass sward strongly reduced aboveground biomass and root mass of R. obtusifolius. Herbivory alone had little impact on R. obtusifolius growth. However, in combination with grass competition, herbivory negatively affected above- and belowground biomass of R. obtusifolius plants, but only when growing from initially smaller roots (agent × competition interaction: P < 0.05 for each). Our results indicate that joint effects between augmentative biological control and plant competition can reduce the growth of a major grassland weed.