Mixed mating is thought to be adaptive in many species of flowering plants because self-fertilization provides reproductive assurance when pollinator densities are low. In species with dimorphic flowers, automatic self-fertilization by small cleistogamous (CL) flowers might also provide reproductive assurance by avoiding floral herbivory. We tested this hypothesis by quantifying flower production, seed production, and seed predation across an entire reproductive season within a Missouri population of Ruellia humilis, a species with mixed mating enforced by dimorphic flowers. The production of CL flowers was delayed relative to the production of open-pollinated chasmogamous (CH) flowers, consistent with the hypothesis of reproductive assurance. The rate of seed predation by larvae of the Noctuid moth Tripudia rectangula was much higher for fruits produced by CH flowers (69%) than by CL flowers (18%). Thus, although CH flowers self-pollinate in the absence of outcross pollen, CL flowers are a much more economical source of selfed seeds, not only because they require fewer resources but also because they experience much lower levels of seed predation. Seed predation by Tripudia is common in many other cleistogamous species of Ruellia native to the southern United States and Mexico and might have played a significant role in the evolution of CL flowers in this large genus.