This study was conducted to estimate the control thresholds (CTs) of imported cabbage worm, Artogeia rapae L., injuring Chinese cabbage. The second instar larvae of A. rapae were inoculated with five density levels on each Chinese cabbages transplanted three weeks earlier under greenhouse condition, and checked injury rates after allowing their feeding for one week and two weeks, respectively. The average leaf area consumed by single larvae was 657.7 ㎟ in plots inoculated at three weeks after transplanting (WAT) and 2495.8 ㎟ in plots at 6-WAT, respectively. In the field experiment, different numbers of A. rapae ranged from one to seven larvae were inoculated on 20 plants. The percent yield reduction (Y) of Chinese cabbage infested by different densities of A. rapae (X) for a three-week period was estimated by the following equation, (1) Y=1.764X-0.3049 (R²=0.9901) in plots inoculated at 3-WAT, and (2) Y=1.0305X-0.2976 (R²=0.9398) in plots inoculated at 6-WAT. Based on the relationships between the densities of A. rapae larvae and the yield index of Chinese cabbage, the number of second instar larvae which caused 5% loss of yield (gain threshold proposed by Japan), was estimated as 3.0 per 20 plants for the 3-WAT and 5.1 for the 6-WAT.

This study aimed to estimate control thresholds (CTs) for imported cabbage worm, Artogeia rapae L., injuring chinese cabbage in the field. As a preliminary experiment, five level of densities of second instar larvae of A. rapae were inoculated on plant and checked injury rates under greenhouse condition. Average leaf area consumed for one week by a second larva was 657.7㎟ to the 3-WAT (week after transplanting) and 2495.8㎟ for two weeks to the 6-WAT, respectively. In field experiment, different larvae densities of A. rapae ranged from one to seven were inoculated on 20 plants. The percent yield reduction (Y) of chinese cabbage infested by different densities of A. rapae (X) for a three-week period were estimated by the following equation; (1) Y=1.764X-0.3049 (R2=0.9901) for the 3-WAT; and (2) Y=1.0305X-0.2976 (R2=0.9398) for the 6-WAT. Based on the relationships between the densities of A. rapae larvae and the yield index of chinese cabbage, the number of second instar larvae which caused 5% loss of yield (Gain threshold proposed by Japan), was estimated as 0.15 per plant for the 3-WAT and 0.26 for the 6-WAT.