We considered the effects of plant secondary metabolites on the immune response, a key physiological defense of herbivores against pathogens and parasitoids. We tested the effect of host plant species and ingested iridoid glycosides on the immune response of the grazing, polyphagous caterpillar, Grammia incorrupta (Arctiidae). Individuals of G. incorrupta were fed either one of three plant diets with varying secondary metabolites, or an artificial diet with high or low concentrations of iridoid glycosides. An immune challenge was presented, followed by measurement of the encapsulation response. We failed to detect a significant difference in the immune response of G. incorrupta feeding on diets with varying concentrations of iridoid glycosides, or feeding on different host plants. However, the immune response was lower in caterpillars consuming the artificial diet compared to those consuming the plant diets. When caterpillar performance was measured, pupal weights were lower when caterpillars ingested high concentrations of iridoid glycosides due to a decrease in feeding efficiency. Overall, individuals of G. incorrupta that consumed different plant diets exhibited a high immune response with low variation. We conclude that the immune response of G. incorrupta is adapted to feeding on a variety of plants, which may contribute to the maintenance of this caterpillar's polyphagous habit.