Abstract: Effects of reproductive organs of cotton, wheat, corn and soybean at low,medium and high rates of nitrogen fertiliser on development and fecundity of the cotton bollowm, Helicoverpa armigera (Hubner) were studied in the environment-simulated chamber (14L: 10D, (27±1)℃ and 60%-80%RH). The causes for those effects were further studied through analyses of C/N ratio in reproductive organs of the four namedplants. As rates of nitrogen fertiliser were increased, the C/N in all four plants was decreased, the order being soybean (0.5, 0.4 and 0.3, repectively), cotton (1.1,0.9 and 0.8, repectively), wheat (4.5, 4.0 and 3.8, repectively) and corn (5.2, 4. 2 and 4.2,repectively). Effects on the bollworm's development and survival took place mainly at young larval stage (1-3 instar), such as its developmental duration getting shorter and survival rate higher in all four plants with increase in rates of fertiliser; and the rates of development and survival were higher in plants with lower C/N's than in those with higher ones. With increase in rate of fertiliser oviposition in all four host plants were increased, and the indexes of population increase enhanced in host plants with higher C/N's, but enhanced first and then decreased in those with lower ones. Plants with medium C/N were more favourable for oviposition and population increase than thosewith C/N's either too high or too low. Effects on the bollworm's development and fecundity were resulted mainly from the difference in contents of carbohydrates and protein as well as their ratio in host plants.

Key words: Helicoverpa armigera (Hubner), C/N, development, fecundity