The adult activity of M. alternatus caught in a pheromone trap showed a bimodal form consisted of the first peak in mid to late June and the second peak in mid to late September in Jeju area, Korea. The two peaks were separated apparently between mid and late August, showing a valley. The pine trees without oleoresin flow were abundant during early July to early August, and declined thereafter, which did not match with the valley of adult activity curve. Thus, the hypothesis that dying pine trees attract much strongly M. alternatus adults than that of pheromone lures may not fully explain the bimodal pattern, because the first adult activity peak occurred during the peak period of dying pine trees and it declined with decreasing dying pine trees. The accumulated degree-days showed that the emergence of the 2nd generation adults could occur before the second peak when the diapause ecology of M. alternatus was not considered. The voltinism of M. alternatus can affect basically the control strategy of this pest. Consequently, further studies are required to find out clearly the voltinism of M. alternatus in Korea.