The highest brown planthopper nilaparvata lugens stal (BHP) damage in the past decade was recorded in western Japan in 1998. The BPH occurrence in 1998 was characterized by early arrivals of immigrants and a moderate BHP immigrant density. Recent studies revealed a new factor that greatly affects BPH population dynamics, i.e., the effect of the whitebacked planthopper Sogatella furcifera Horvath (WBPH) on wing-form determination of BPH population dynamics also comes from a significantly negative correlation between the density of immigrant WBPH and the population growth rate of BPH by analysis of light trap data. The BPH population immigrating into Japan rapidly became virulent to ASD7 carrying the bph2 resistance gene beginning in 1997, and the virulence remained at that high level through 1999. Current forecasting systems of rice planthoppers in Japan are also reviewed, i.e., monitoring of immigration by analysis of weather data, monitoring of immigration by analysis of weather data, monitoring of immigrant density by light and net traps, and field surveys.