Costs and Benefits of Plant Resistance to Herbivory

The cost-benefit theory of the evolution of plant resistance to herbivory assumes that the allocation of plant resources to defense against herbivores is costly. We present a graphical model, which states that allocation of plant resources to defense against herbivores evolves so as to maximize the difference between benefits and costs associated with resistance. A method for quantification of such costs, using genetic regression coefficients between fitness and defense, is described. This technique was applied to measure the cost to tall morning glory. (Ipomoea purpurea) of resistance to the sweet potato flea beetle (Chaetocnema confinis). The morning glory population studied exhibited significant amounts of heritable variation in resistance in flea beetles. However, there was no evidence for costs of resistance to plants in this population.