The mismatch in C:N stoichiometry between insect herbivores and their host plants strongly suggests that herbivores are nitrogen limited. Omnivory has evolved in some herbivore lineages, and the occasional consumption of nitrogen-rich prey may help offset the inherent penalty many herbivores pay for consuming only plant food. We tested the consequences of diet on the growth and mortality of an omnivorous katydid, Conocephalus spartinae that feeds on Spartina cordgrass and on other insects, which inhabit the marsh. In the laboratory katydids were fed one of eight different dead or alive insects: four predators and four herbivores. Both the predators and the herbivores included two more actively searching insects and two more sedentary insects. Fed dead herbivores, katydids grew slowly and survived poorly compared to when they were offered a diet of dead predators. Conversely when katydids were fed live predators they had a lower survival and growth rate than when they were fed live herbivores. In the field, katydids occur most abundantly in low-marsh habitats where cordgrass is most nutritious and there are higher insect densities. Moreover, the frequency of invertebrate parts in the gut contents of Conocephalus is positively related to the density of insects across marsh habitats. These data suggest that by tracking insect populations and selectively colonizing nutritious Spartina, katydids can more easily meet their nitrogen demands. Thus, dietary supplementation with prey and the occurrence of omnivory may have been promoted in certain groups of “herbivores” as means of increasing nitrogen intake in a carbon-dominated world.