Potted experiments of rice (Oryza sativa L.) plants were conducted to produce various scales of brown planthopper [Nilaparvata lugens (Stål), Homoptera:Delphacidae] and leaffolder [Cnaphalocrosis medinalis Guenee (Lep., Pyralidae)] infestations, respectively, for canopy hyperspectral reflectance measurements, and then to identify spectral characteristics (SCs) associated with insect infestations leading to the establishment of spectral models for severity assessment. By linear correlation intensity analysis, correlation coefficients (r) along the spectral domain of 350 to 2400 nm were determined and narrow bands related to infestation severity were selected as SCs. The reflectance at green light (490–560 nm) maximum (RGREEN), red light (640–740 nm) minimum (RRED), and near‐infrared (740–1300 nm) peak (RNIR) were also considered. For canopies infested with brown planthopper, r value at 426 nm was the highest (r = 0.878**). Among the calculated spectral indices using two SCs, the determination coefficient of RNIR/RRED ratio was the highest (R2 = 0.922, P < 0.001). For leaffolder infested canopies, the most negative r value located at 757 nm (r = −0.613*) in active tillering stage but shifted to 445 nm (r = −0.928**) in heading stage. The index RNIR − RRED in the active tillering increased R2 value to 0.422 (P < 0.001), while no increase in R2 was found in the examined SIs in heading stage. Models with more than two SCs yielded from multiple linear regression analysis exhibited a further improvement for discriminating infestation severity.

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Since overwintering populations of brown planthopper (Nilaparvata lugens) and white-backed planthopper (Sogatella furcifera) in Taiwan are very low based on field observations, immigrant planthoppers have become the most important source of serious damage to rice crops (Oryza sativa). Backward trajectory analysis was conducted using trap catch data from 1990 to 2005 to estimate the source of immigrant planthoppers, taking into account the emigration periods and weather conditions, and showed that southern China, Vietnam and the Philippines (Luzon Island) were possible source areas. Southern China was found to be the most important source of emigration. Of all the immigration cases tested, the sources from southern China were estimated to be about 77% in the first rice crop and 65% in the second rice crop. Vietnam came second with about 37% and 56% in the first and second rice crops, respectively. Typhoons were the most important weather factor, inducing mass emigrations from China and Vietnam. Since the population properties are known to differ among the emigration regions and Taiwan, careful monitoring of these insects in the emigration sources and Taiwan is needed in order to establish better pest management practices.

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