Abstract: According to the laboratory observation and tests,the 5th instar larva of rice leafroller feeds on the highest amount of rice lcaves, its percentages being 68.42% and 71.6% respectively, as compared with the total fed amount by the 2nd and 3rd generations.The 4th instar larva is on the second place, with 18.30% and 15.78% of total fed amount by the same two generations. The results show that damage can be avoided by controlling the larvae before the 3rd instars.The relationship between different damage levels by larvae and yield losses is also analyzed.The results indicate that the 2nd generation of larvae mainly led to the decrease of 1000-grain-weight of rice and grain numbers per ear,and empty grain ratio is also affected. The damage by the 3rd generation of larvae is the increase of empty grain ratio and the fall of 1000-grain-weight.The leaves fed on by the 2nd generation are mainly the 4th and 5th leaves from the flag leaf. Mild damage by the 2nd generation can slightly increase rice yields. Leaves fed on by the 3rd generation are mainly flag leaf and the 2nd leaf(next to the flag leaf),the yield loss is heavily greater than that by the 2nd generation.The 3rd generation is the key generation to be controlled.Based on the experiments,the dynamic economic threshold model fitting for different generations and various instars has been obtained.

Key words: Cnaphalocrocis medinalis G, Yield loss, Economic threshold model