Dissertation Research: Influence of Resource Limitation on Plant Responses to Herbivory: Responses of Silphium integrifolium to Gall Insect Attack

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Insect herbivores can have large effects on the performance, life histories, and demography of plants, and often exert strong natural selection on plant populations. In addition, plants are often exposed simultaneously to stresses from biotic (i.e. herbivory, parasitism) and abiotic (i.e. resource limitation, heat) sources. This project will use the tallgrass prairie perennial Silphium integrifolium (Asteraceae) and one of its major herbivores, the gall wasp Antistrophus silphii (Hymenoptera:Cynipidae), as a model system to examine how herbivory, nutrient stresses, and water stress interact to affect life history and physiological responses of plants. Experiments will address two specific questions. 1. How does gall infestation affect life-history traits and fitness components of Silphium intergrifolium? 2. How do nutrient limitation and water stress influence the physiological and growth responses of plants to gall insect attack? To answer the first question, plant physiology (i.e. photosynthesis, water potential), growth, biomass allocation, and reproduction will be measured over a two-year period in experimentally galled and gall-free plants growing in natural field populations. The second question will be addressed in a common garden experiment in which gall frequencies and nutrient/water availabilities will be manipulated in a factorial design, and physiology, growth, and reproductive output will again be measured.