Economic Injury Levels for the Rice Leaffolder Cnaphalocrocis medinalis (Lepidoptera: Pyralidae) in the Rice Paddy Field

This study was carried out to determine the economic injury level of the rice leaffolder,Cnaphalocrocis medinalis G.. The damage aspects of rice plant (at tilling stage) by leaf folders at different larval density per plant were studied in pot experiment (24 cm in diameter, 18 cm in height). One leaf folder consumed 6-7 leaves during larval stage. The damage by leaf folders was simulated by cutting off 0, 10, 30, 50, 70, and 90% of leaves before and after heading stage July 15th (at panicle initiation stage) and August 15th (at milk stage), respectively. When leaves were cut before the heading of rice, the linear relationships between the leaf cutting rate (X) and each factors of yield (Y) were as following; for grain maturity it was Y = -9.379X + 83.630 (),\;Y = 0.139X + 0.490 () for yield, and Y = -4.880X + 81.116 () for head rice. When leaves were cut after the heading of rice, it was Y = -23.0l4X + 83.589 (), for grain maturity, Y=0.141X + 3.466 () for yield, and Y = -13.795X + 81.964 () for head rice. We found that when leaf cutting after the heading stage caused more damage than before the heading in terms of yield and yield components. Based on theses results the economic threshold level was estimated to be 30% and 7% leaf loss before and after heading stage.