Elevated atmospheric CO2 has generally been found to have negative impacts on the performance of insect herbivores via negative effects on plant quality. Yet, reduced food quality may lead to an increase in crop damage when pests compensate by consuming more plant tissue. The flow-on effects of elevated CO2 level on higher trophic levels have rarely been investigated. We tested the effects of elevated CO2 on the behavior and performance of the omnivorous bug Oechalia schellenbergii (Heteroptera: Pentatomidae) and its prey, a polyphagous chewing herbivorous pest (Helicoverpa armigera; Lepidoptera: Noctuidae), feeding on pea (Pisum sativum) foliage. We hypothesized that elevated CO2 would impose negative nutritional effects on the omnivore by lowering the quality of plants, the prey, or both. Plants grown at elevated CO2 were significantly larger, with reduced N content, than plants grown at ambient CO2. H. armigera larvae feeding on elevated CO2-grown plants were significantly smaller than those grown on ambient-grown plants, but prey N content did not differ between CO2 treatments. The omnivore required prey to complete its development, and performed best on a mixed plant-prey diet, regardless of CO2 level. Bugs that failed to reach adulthood were slower to develop on the elevated CO2-prey diet; those that successfully eclosed were not affected by CO2 treatment in terms of development time, adult weight, adult longevity, hatch rate or pre-oviposition time. The bugs did not display compensatory feeding when offered prey of similar size from different CO2 treatments. The bugs performed best when fed larvae from the elevated-CO2 treatment apparently because these prey were smaller and thus easier to subdue. Taken together, results indicate that elevated CO2 may benefit generalist predators through increased prey vulnerability, which would put pest species under higher risk of predation.