In the study of biological control of weeds, effects of insect herbivores that attack different plant organs have long been assumed to be independent events. Emerging research indicates stronger interactions between herbivore guilds than was expected historically. My research focuses on damage to apical meristems of tall thistles (Cirsium altissimum) by stem mining insects and the indirect, plant-mediated effects of this damage on flower head feeding insects. Two questions were addressed: 1) how does mining of the apical meristem affect tall thistle architecture, including number of branches and flower heads produced? 2) what influence does apical meristem mining have on the intensity of damage to the plant by flower head feeding herbivores? Forty adult tall thistles at two different sites (80 plants total) were assigned to levels of a "meristem-herbivore exclusion treatment" in April, 2008. Insecticide was applied to the apical meristem of treatment plants. Control plants were sprayed with water or were not sprayed. Apical meristem mining had significant affects on; flower heads (F2,2=45.97, p=0.021),primary stems (F=11,p <0.001) and plant height over-all (F2,4=26.39, p=0.004). There was a marginally significant effect on flower head damage, further work will be needed to understand the biological significance of this finding.