he fall webworm (FWW), Hyphantria cunea, defoliates most deciduous trees. After the first detection of FWW in Seoul in 1958, FWW had dispersed to the entire South Korea by 1972. Although currently the FWW density is not high, the defoliation is still reported in the nationwide. In Korea, FWW hibernates as a pupa and emerges between mid May and early June. Generally it is accepted that FWW has a bivoltine life cycle in South Korea: the first generation between mid May and early June and the second generation from late July to late August. However, recently the possibility of trivoltine populations in Korea has been discussed based on the field observations. In this study, we analyzed the field monitoring data and developed a model to predict occurrence patterns of FWW. We discuss the possibility of trivoltine populations in Korea based on the model developed.