Date of Award

Spring 1-1-2016

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ecology & Evolutionary Biology

First Advisor

Deane Bowers

Second Advisor

Michael Breed

Third Advisor

Rebecca Safran

Fourth Advisor

Timothy Seastedt

Fifth Advisor

Shannon Murphy


Although there are multiple hypotheses regarding the top-down and bottom-up controls of herbivore populations, the Tri-Trophic Interactions (TTI) Hypothesis is the first to make predictions regarding the simultaneous effects of plant quality, herbivore diet breadth, and natural enemies on herbivore performance. My dissertation research tests several predictions of the TTI hypothesis to determine how plant secondary metabolites, herbivore diet breadth (specialist or generalist) and natural enemies interactively determine the performance of insect herbivores. I study plants in the genus Penstemon (Plantaginaceae), which are chemically defended by bitter iridoid glycosides (IGs), and their associated lepidopteran herbivores. First, I examined the bi-trophic interactions between two different Penstemon species and generalist and specialist lepidopterans. The fitness of the generalist species varied greatly between the two plants, whereas the specialist performed consistently well on both host plant species. Second, I determined the level of variation in Penstemon IGs to which herbivores are naturally exposed. I focused on three potential sources of variation: inter-annual (across years), location (among geographically distinct populations) and plant tissue type. Finally, I examined the role of IGs in tri-trophic interactions by examining the ability of IGs to protect a specialist caterpillar from predators and parasitoids. Ant bioassays indicated that even low amounts of IGs are effect deterrents against invertebrate predators. However, measurements of phenoloxidase activity, an important component of the insect immune response cascade, suggested that caterpillars reared on certain diets were more vulnerable to parasitoid attack. This research provides important contributions to the recently proposed TTI hypothesis and increases our understanding of the chemical mediation of multi-trophic interactions.