Technical Abstract: Consumer communities are being re-arranged through unprecedented rates of human-mediated invasions and extinctions. Such changes in consumer composition and diversity potentially alter pressure and impact on resource populations. Although insect herbivore invasions are common, and exotic herbivores continue to be deliberately introduced in weed biological control programs, the influence such “species additions” on native resident herbivore guilds, along with their individual and combined effects on native plant resource species, are rarely investigated. Here, we used natural patterns of site-to-site and plant-to-plant variation in herbivore composition to examine how the addition of an invasive weevil, Rhinocyllus conicus, combines with a guild of native floral herbivores (tephritid flies, pyralid moths) to influence aggregate herbivore densities and rates of seed loss on a native thistle species, Cirsium canescens, across its range in midgrass prairie. We found that invasion by R. conicus more than doubled aggregate floral herbivore density on native plants, resulting in significantly higher rates of seed damage and reduced seed production by native thistles at invaded sites. Individuals attacked by R. conicus showed similar patterns of significantly higher aggregate insect densities and higher rates of seed damage compared to plants attacked only by native herbivores. Thus, adding a novel herbivore to the floral guild significantly increased the cumulative impact of floral herbivores on plant reproductive success. Insect densities and seed damage on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in aggregate density and resource consumption with invasion resulted from the inclusion of a particular functionally dominant insect herbivore rather than any facilitation or complementarity effects among herbivore taxa. In fact, some evidence for interference between insect species emerged, with a trend towards reduced herbivore impacts on seed production when all three taxa attacked a given plant compared with plants attacked by R. conicus alone. Overall, the addition of a highly abundant and functionally dominant invasive herbivore dramatically altered community function, increasing both aggregate insect densities and rates of resource depletion, leading to strong negative impacts on seed production by native thistles.