Characterization of Cereal Aphid Resistance in Tetraploid Switchgrass Populations (Panicum virgatum L.)

Switchgrass, Panicum virgatum L., is a perennial warm-season grass that has been identified as a model species for the development of bioenergy crops in the United States. The objectives of this research were to evaluate selected switchgrass populations for host suitability and differential resistance to potential aphid pests, determine the categories (antibiosis, antixenosis, and/or tolerance) of resistance among selected switchgrass populations, and elucidate Schizaphis graminum (Rondani) feeding behavior on resistant and susceptible switchgrasses. Screens for host suitability of two switchgrass populations, Summer and Kanlow, and two experimental strains, KxS and SxK, revealed all switchgrasses were unsuitable feeding and reproductive hosts to Rhopalosiphum padi (L.), and Diuraphis noxia (Mordvilko). Both Sipha flava (Forbes) and BLOCKIN BLOCKIN S. graminum were able to establish on all switchgrasses tested with differential levels of resistance among the switchgrasses. Two no-choice studies, performed to characterize the categories of resistance (antibiosis and tolerance) to S. flava and BLOCKIN BLOCKIN S. graminum, demonstrated that Kanlow possesses high levels of antibiosis to both aphids, while KxS possesses low-to-moderate levels of antibiosis to S. flava. Functional plant loss indices indicated that tolerance is an important category of resistance for Summer to S. graminum. Two choice studies evaluated S. graminum and S. flava preference for switchgrass populations, with a third study to assess S. graminum feeding behavior using the electrical penetration graph (EPG) technique. Choice studies for S. flava indicated no preference by aphids for any of the switchgrass populations. However, S. graminum displayed a preference for KxS at 24 h after aphid introduction. Feeding behavior studies for S. graminum on switchgrass indicated that aphids had significantly less phloem ingestion on Kanlow than both KxS and Summer, suggesting that resistance factors in Kanlow are associated with the phloem tissue. These studies are the first attempt to analyze the categories of resistance in switchgrass and provide critical information for characterizing the mechanisms of resistance and improving our knowledge of the plant-insect interactions within this system. i ACKNOWLEDGEMENTS I express my sincerest appreciation for my major advisors, Drs. Tiffany Heng-Moss and Jeff Bradshaw, whom provided support, guidance, and mentorship throughout my degree program, and encouraged my growth both personally and professionally. I also express my deepest gratitude to my supervisory committee members, Drs. Z B Mayo and Gautam Sarath for their valuable input, critiques, advice, and support.