Herbivory is common on the federally endangered Clematis morefieldii (Ranunculaceae), but little is known about its effect on reproductive output. Over 3 yr, we applied the insecticide Sevin (0.26% carbaryl) or water (as a control) to 40 plants (20 per treatment) every 3 wk during the growing season. Herbivores destroyed fewer flower buds on insecticide-treated plants (18.4% fewer in 2010, 45.6% fewer in 2011, 39.7% fewer in 2012), but fruit production increased 3.7-fold on insecticide-treated plants only in 2011. High rainfall during peak flowering/fruiting (April–June) influenced the effectiveness of treatments by boosting achene production in 2011. We compared the amounts of vegetative herbivory (vertebrate browse damage, invertebrate browse damage, piercing-sucking herbivore damage, or leaf miner damage) within each year. Both vertebrate and invertebrate browse damage were significantly greater (1.2- to 2.2-fold) on control plants for all three years. Piercing-sucking herbivory was greater (25–40% more) on control plants during most of the growing season for all three years. Vertebrate damage was more frequent on insecticide-treated plants (25–40% greater) during 2012. During high rainfall years, invertebrate herbivory reduced reproductive output, either directly by attacking flower buds/flowers or indirectly by vegetative herbivory that reduces resources available for reproduction. Insecticide use during years of high insect infestation could be a viable option for managers seeking to increase seed output by this rare plant.