Insect exclusion experiments have demonstrated that insect herbivores can reduce host plant fitness through both direct and indirect mechanisms. I did an experiment on Cirsium altissimum (tall thistle), whose apical meristems are attacked by the larvae of Platyptilia carduidactyla (artichoke plume moth), during 2012 to determine whether apical meristem mining affects C. altissimum fitness and to determine whether these effects arise indirectly through plant-mediated effects on floral visitation. In a restored tall grass prairie, 180 tall thistle adult plants were randomly selected and assigned randomly to treat with insecticide, water and unmanipulated control. On these plants, I quantified effects of apical meristem mining on plant architecture, flowering phenology, flower visitors’ activity and seed production. Apical meristem miners affected several aspects of plant architecture, including reducing plant height and increasing the proportion of axial flower heads, and many aspects of plant flowering phenology, including delaying flowering and date of maximum floral display. Apical meristem miners significantly decreased C. altissimum lifetime seed production, showing their strong effects on plant fitness. Bombus pensylvanicus and Melissodes desponsa were the most common visitors on C. altissimum flower heads. No strong effect of apical meristem miners was reported on the behavior of bee (Apidae) species, which may have resulted from the availability of the major visitors of C. altissimum flower heads throughout the flowering season. Overall, apical meristem mining strongly affected the plant reproduction success but no evidence was found to suggest that these effects on fitness of C. altissimum arose through changes in floral visitation. Being a monocarpic plant with little seed bank, reduced seed production by C. altissimum may translate into smaller population sizes.