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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.
