Insect herbivory has been shown to have substantial impacts on plant growth and reproduction in many systems. However, the effect of insect herbivores on rare and endangered plants has not been widely studied, even though natural limitations on growth and reproduction, in combination with anthropogenic changes in habitat quality and quantity, likely contribute to population decline. In the current study, an insect exclusion experiment was used to assess the effects of insect herbivory on a federally listed endangered annual plant: Chorizanthe robusta var. robusta (Polygonaceae). The primary insect herbivore was a microlepidopteran larva, an undescribed species of Aroga (Gelechiidae). Excluding Aroga larvae led to a 30% increase in seed output compared to water control. No change in plant survivorship or seed/flower ratios was found with insect exclusion. Control plants given supplemental water had increased seed production, but only in the absence of Aroga infestation, an indication that infested plants could not fully exploit additional resources. These results show that rare annuals are potential host plants for insect herbivores, and herbivory is an important factor to consider when evaluating endangered plant populations.