Concerning adults of the smaller brown planthopper in its second emergence, the relationship between catching at yellow pan water traps and spacial distribution, and their numerical fluctuation on the early-season transplanted rice plots was studied. The catching showed variability among water traps set on the same rice plot, while the heterogeniety of spacial distribution on a plot was generally low. Even when distributional heterogeniety was high enough for detection, the phenomenon was not reflected on the catching records at the water traps. A general peak in the catching curve tended to appear before the commencement of the intensive arrival of flying adults to the rice plots. From this, the yellow pan water trap can be said to be effective for forecasting the invasion of planthoppers into rice plots.