Sensitivity of a Ratio Vegetation Index Derived from Hyperspectral Remote Sensing to the Brown Planthopper Stress on Rice Plants

Remote sensing end-products related to vegetation have potential applications in monitoring the health of crops. The sensitivity of a spectral index to crop stress determines its application prospect. Our aim in this study was to explore the sensitivity of a ratio vegetation index (RVI) to identify the damage caused by brown planthoppers (BPHs) on rice plants, and to evaluate the potential application of hyperspectral end-products to monitor population size of BPH. Different numbers of the second-instar nymphs were released onto potted rice at the tillering stage. The plants were exposed to BPH for two, four, six, and eight days, and reflectance from the damaged rice was measured using a hyperspectral spectroradiometer. Measurements were done again two, four, and six days after exposure (recover days), and then the spectral index RVI(746)/(670) was compared among rice plants infested with different numbers of BPH. The relationships between RVI(746/670), the number of BPH and exposure day were simulated by linear and curve models. BPH damage resulted in a decreased spectral index RVI(746)/(670) of rice plants. RVI(746/670) well indicated the damage of rice plants caused by six–eight BPH nymphs per plant in six–eight days, but the index could not identify the damage of these nymphs in two days. The RVI(746/670) showed a two–four-day delay to indicate a slight BPH damage. The spectral index RVI(746)/(670) could indicate the physiologic compensation of plants for the feeding of BPH and the post-effect of BPH damage on rice. The RVI(746)/(670) of rice showed a quadratic curve relation with the number of BPH nymphs and a quadratic or linear relation with the exposure day. The recover day had no significant effects on RVI(746/670). The RVI(746)/(670) (Y) could be simulated by a quadratic surface model based on the number of BPH (N) and exposure day (T): Y = 3.09427 + 0.59111T + 0.44296N − 0.03683T(2) − 0.03035N(2) − 0.08188NT (R(2) = 0.5228, p < 0.01). In summary, the spectral index RVI(746)/(670) of rice is sensitive to damage caused by BPH.