Predicted changes in global atmospheric CO₂ levels have the potential to alter plant productivity and nitrogen content and therefore lead to differences in plant communities and plant-insect interactions. Most elevated CO₂ research has focused on the direct effects of elevated CO₂ on mesocosms containing one plant species, or one plant and one herbivore. However elevated CO₂ will potentially affect reticulate food web interactions and naturally complex community assemblages. I investigated the interactions between Scirpus olneyi (a C₃ sedge), Spartina patens (a C₄ grass), and Delphacodes detecta (an herbivorous planthopper) under elevated and ambient CO₂ conditions in a field cage experiment on a Mid-Atlantic salt marsh. Previous research has shown that elevated CO₂ alters the growth rate and nitrogen content of Scirpus olneyi, but not Spartina patens. Thus under elevated CO₂ conditions rapidly growing Scirpus often shades and alters the microclimate of Spartina patens, the only host plant of the monophagous phloem feeding planthopper Delphacodes detecta. In the presence of non-host plant Scirpus olneyi, there were significant decreases in Delphacodes fitness as indexed by development time, survivorship, and body size. Moreover, there was a significant interaction between plant community composition and CO₂ treatment whereby the combination of elevated CO2 and the presence of Scirpus olneyi led to a larger decrease in Delphacodes performance than could be attributed to either factor alone. Results of this study emphasize the importance of examining indirect trophic interactions and multi-species assemblages when considering the potential impacts of global climate change on natural communities.