We sought to explain patterns in parasitism frequency of two specialist herbivores (Geometridae) by investigating the influence of plant diet as a source of variation in immune response variables important for defense against parasitism. Field collected caterpillars (Eois apyraria and Eois nympha) were assigned to one of two species in the plant genus Piper (Piperaceae): 1) a host species with high diversity of defensive chemistry, P. cenocladum C.DC., or 2) a host species with lower investment in chemical defense, P. imperiale C.DC. Hemolymph was extracted from fifth instar larvae, and immune strength measured unidimensionally using a phenoloxidase (PO) enzyme assay. Parasitism data came from 19 years of accumulated host plant-caterpillar-parasitoid associations from a long-term rearing project at La Selva Biological Station in Costa Rica, where the experiment took place. We found that immunity was significantly weakened when caterpillars were reared on the host plant with higher phytochemical diversity (P. cenocladum). Moreover, host plants inducing a weak immune response hosted caterpillars with higher parasitism rates. We conclude that patterns in parasitism frequency can be partially explained by cascading effects of host plant traits.