Ascorbate, or vitamin C, is a key antioxidant in both plant and animal tissues. Under normal conditions, ascorbic acid works in conjunction with glutathione and antioxidant enzymes to scavenge reactive oxygen species (ROS) from cells and extracellular fluids. Oxidative stress results when these low molecular weight antioxidants are depleted or the oxidized:reduced ratio becomes too large to reduce ROS proliferation. To better understand the importance of daily ascorbate intake rates on the maintenance of oxidative balance in insects, we fed Manduca sexta caterpillar artificial diet containing 100%, 30% and 0% normal ascorbate concentration. Within 72 hours, the radical quenching ability of gut fluid and hemolymph (using a stable ABTS radical), were reduced. The effect on growth was more modest. These results suggest that dietary availability of ascorbate can mediate the ability of insects to tolerate ROS, whether they arise from endogenous sources (respiration, pathogen attack), ingested toxicants, or UV exposure.