Genetically engineered crops with novel forms of pest resistance are under scrutiny by researchers and regulatory agencies for risks associated with increased fitness and competitive ability that can lead to invasiveness of the crop itself or of wild crop relatives that obtain insect-resistance traits through gene flow.  Risk assessment research on the impacts of insect herbivores on plant population dynamics is vital for informing regulatory decisions on transgenic insect-resistant crops. If transgenes flow from crops to weedy wild relatives, the resulting protection from herbivory could make a difference in fitness and in rates of population spread.  To estimate the effect of damage by Bt-susceptible lepidopteran herbivores on the rate of population spread of weeds, we compared Bt-spray protected wild radish (Raphanus sativus) and wild mustard (Brassica rapa) plants to plants inoculated with cabbage butterfly larvae (Pieris rapae) on the central coast of California. The contribution of lepidopteran herbivory at each stage of the life cycle to the overall decrease in population growth was explored using Life Table Response Experiments. We showed that the effect of insect-resistance can convert stable population trajectories into increasing ones, thereby leading to an increase in invasiveness.  However, damage by Bt-susceptible herbivores affected wild radish and mustard populations differently, with unequal responses at some plant growth stages and by plant community (agricultural vs. natural vegetation). Although wild radish and mustard are closely related plants, the mechanisms by which decreased herbivory contributes to population growth are different.  Seedling survivorship in wild mustard contributes to increased rates of spread, while survivorship of young plants and increased maternal fitness contribute more to rates of spread for wild radish. We discuss the opportunities and constraints of the invasive species model for risk assessment of transgenic crops.