MANY plant characters, including toxic secondary compounds, trichomes, spines and tough, nutrient-poor leaves have evolved at least in part as defences against pathogens and herbivores, including phytophagous insects(1-6). Models of the evolution of resistance(7) (9) predict that allocation to defence is determined by a tradeoff between the benefits of resistance, such as reduction in herbivore damage, and costs of resistance, generally envisaged as reduction in fitness in an environment in which herbivores are absent(9). However, despite attempts to determine the tests of resistance, there is little convincing evidence that they exist and constrain the evolution of defences(10,11). Here me report the existence of such a cost in the tall morning glory, Ipomoea purpurea : genotypes that exhibit relatively high levels of resistance to insects that cause damage to apical meristems exhibit relatively low tolerence to this form of damage. We also show how this type of tradeoff constrains the evolution of resistance.