The effect of fertilization of yellow starthistle plants on C. succinea was studied under field and laboratory conditions. Uric acid was separated identified and quantified using HPLC analysis. Using these methods, controlled laboratory experiments showed adverse effects of fertilization of plants treated during periods of larval feeding and development. Uric acid levels were significantly increased and the size of pupa was reduced. In outdoor experiments, third instar diapausing larvae were collected from field plots treated with four different rates of nitrogen and nitrogen-phosphate fertilizers. Larval wet weigh was measured and then correlated with the actual field treatments.