In the study of biological control of weeds, effects of insect herbivores that attack different plant organs often are assumed to be independent of one another. Emerging research indicates stronger interactions between herbivore guilds than was expected historically. Our 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? and 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 three study sites (120 plants total) in Sedgwick County KS were assigned to levels of a “meristem-herbivore exclusion treatment” in April, 2008. For plants on which the apical meristem was protected, insecticide was applied to the apical meristem only, bi-weekly. Control plants were sprayed with water (water control) or were not sprayed (true control).

Results/Conclusions

Applying insecticide to the apical meristem of tall thistle was successful in significantly reducing the occurrence of apical meristem damage (χ² = 17.58, p = 0.001). Bolting tall thistles with protected apical meristems produced fewer flower heads (F_2,2=45.97, p=0.021), had longer primary stems (p<0.023 f=11.01) and had greater plant height over-all (F_2,4=26.39, p=0.004) than plants whose apical meristems were not protected. Number of lateral branches per plant did not differ significantly among levels of the apical meristem protection treatment. Preliminary analyses show a weak trend (χ² = 6.63, p = 0.157) toward a difference in the severity of damage to flower heads among levels of the apical meristem protection treatment. This study provides experimental evidence that natural insect herbivore damage to apical meristems changes plant architecture and quantity of resources available to flower head feeding insects. These changes provide a mechanism for indirect interactions between meristem- and flower head feeding insect guilds on thistles.