In order to find out the relationship between the leaf quality of R. obtusifolius and the ecology of G. viridula, a series of experiments was carried out. The effects of leaf age on rates of growth of instars of G. viridula showed that the leaf age was closely related to the development of the larvae of G. viridula. Also, the results showed quite clearly that the N concentration of R. obtusifolius leaves have significant effects on the growth rates of G. viridula larvae. It is suggested that C : N ratio of food plant plays an important role in the population dynamics of phytophagous insects. A small increase in leaf N concentration in a nutrient poor habitat will substantially increase the G. viridula population.