One of the most common patterns of phenotypic plasticity in biology is the Temperature-Size Rule: many insects, reptiles, amphibians and other organisms raised at higher environmental temperatures are smaller as adults. Is this pattern an adaptation?is smaller better at higher temperatures?or an inevitable consequence of the effects of temperature on physiology and development? Recent studies with two agricultural pests, Tobacco Hornworms and Imported Cabbageworms, show that both larval (caterpillar) nutrition and rapid evolution can reverse the Temperature-Size Rule, leading to larger adults at higher rearing temperatures. These exceptions to the rule provide a unique opportunity to understand the developmental mechanisms and selective causes of this widespread pattern of phenotypic plasticity. The proposed studies integrate laboratory and field experiments with mathematical models to explore how larval nutrition, oxygen limitation, hormonal regulation, and natural enemies (predators and parasitoids) interact to generate the Temperature-Size Rule and its exceptions. These studies will provide a new mechanistic and evolutionary understanding of how plant quality, climate and seasonal weather interact to determine body size and ecological success in Tobacco Hornworms and Imported Cabbageworms, two important agricultural pests in the US. Research and training opportunities for undergraduate and graduate students at UNC and Duke, and outreach via Public Television, the North Carolina Museum of Life and Science, and the San Francisco Exploratorium, are integral components of the project. In addition, a citizen science program for Imported Cabbageworms (i.e., Cabbage White Butterflies) will be developed that involves community gardeners and organic farmers in North Carolina and other states.