DISSERTATION RESEARCH: Exploiting Seasonality and Differences in Herbivory to Create a Novel Framework for Testing Optimal Defense Theory

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Plants form the basis of most terrestrial food webs, and insect pests consume more plant tissue than all other animals combined. Thus, understanding how plants defend against insect pests is crucial to maintaining important ecosystem services and food production. Compared to old leaves, young leaves are often more valuable to future plant success as well as more nutritious and desirable to insects, but younger tissue may have more defenses against such attack. The mechanisms of plant growth alone might explain this pattern, or it could be an evolutionary adaptation to protect against pests. A series of experiments will test the alternate explanations of growth physiology alone or with defense response using populations of plants that have evolved separately under either low or high levels of insect attack for the last 400 years. The work will incorporate the contributions of undergraduate students.This research will examine a cornerstone of research on plant-insect interactions, the optimal defense theory, which predicts that herbivory selects for young tissues to be better defended than old tissue. However, physiological constraints, as opposed to adaptive responses to selection imposed by herbivory, could lead to the same patterns. In this proposal, I aim to advance understanding of what drives young leaves to be so highly defended by disentangling whether adaptive evolution or physiological constraints drives differences in allocation in defense. To test the alternate hypotheses of physiological constraints versus adaptive evolution, a rigorous new framework of optimal defense will be developed that requires tests of defense to: (1) include root tissue, (2) directly measure relative tissue values, and (3) evaluate how these values change through a growing season. The framework will be tested using an intraspecific comparison between plant populations that predictably vary in their risk of attack (by using European and North American maternal lines of Verbascum thapsis) for specific tissues using pot experiments and measures of tissue value (tissue removal and seed output) and chemical defense allocations.