Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Sharron S. Quisenberry


This dissertation research was designed with three main objectives: to evaluate the effects of threecornered alfalfa hopper, Spissistilus festinus (Say), feeding on alfalfa, Medicago sativa L., yield, regrowth, root carbohydrate composition, and disease incidence; to screen alfalfa cultivars for resistance to the threecornered alfalfa hopper and develop effective screening techniques; and to determine the mechanism(s) of this resistance. Greenhouse studies determined the effects of threecornered alfalfa hopper infestations on alfalfa growth and their relationship with Fusarium crown-rot. Significant interaction effects between insect population and the presence of Fusarium were found for number of harvestable stems and acid detergent fiber concentration. The main effect of inoculation did not affect any of the plant parameters studied. Insect populations increased plant chlorosis and accelerated plant maturity. Insect populations also reduced plant height, root carbohydrate concentration, stem regrowth, and forage quality. Threecornered alfalfa hoppers also caused an increase in Fusarium crown-rot severity. Choice and no-choice tests were conducted under greenhouse conditions to determine the presence of alfalfa antibiosis or antixenosis to the threecornered alfalfa hopper. Adult threecornered alfalfa hoppers were released into cages containing six alfalfa cultivars. Fewer threecornered alfalfa hoppers and girdles were found on cultivars 'Cimarron VR' and 'Zia'. In the no-choice test, one newly hatched nymph was placed on each plant and development was monitored daily. Nymphal duration was longest when nymphs developed on 'Florida 77' and shortest on 'Zia'. Adult weights were reduced when nymphs developed on 'Dona Ana' and 'Cimarron'. Screening techniques were developed to evaluate alfalfa resistance under field conditions. Resistance was characterized by adult populations, girdle damage, and stand persistence. 'Cimarron VR', 'Dona Ana', and 'GA Plains' exhibited resistance to the threecornered alfalfa hopper by having less girdle damage and greater stand persistence. 'Ladak' showed the highest level of susceptibility. Adult population monitoring was not an effective screening criterion. In addition, multiple girdle counts must be taken to accurately categorize alfalfa cultivars.