Abstract  [Aim]  To determine the loss inflicted on rice crops by the first generation of Chilo suppressalis in two regions. [Methods]  The relationship between the occurrence of the first generation of C. suppressalis and crop loss was studied by artificially inoculating rice plants with C. suppressalis egg-masses. [Results]  First generation larvae in a second-generation area caused withered sheaths, withered hearts and plant death. As a result, the number of effective panicles per unit area of rice was reduced and yield decreased. The loss rate increased with the number of eggs; the correlation between crop loss and the number of egg-masses per 667 m² can be described by the equation: y = 0.039x﹣2.793. According to the definition of an economic threshold, the control index for the first generation of C. suppressalis is 111 egg-masses per 667 m². By analyzing the sex ratio and oviposition of the overwintering generation, we propose that the control index for adults be set at 20.2 for light trapping, or 8.3 adults for sex-pheromone trapping. [Conclusion]  These results provide important information for the early control of C. suppressalis in rice crops.