Abstract
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 RVI746/670 was compared among rice plants infested with different numbers of BPH. The relationships between RVI746/670, the number of BPH and exposure day were simulated by linear and curve models. BPH damage resulted in a decreased spectral index RVI746/670 of rice plants. RVI746/670 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 RVI746/670 showed a two–four-day delay to indicate a slight BPH damage. The spectral index RVI746/670 could indicate the physiologic compensation of plants for the feeding of BPH and the post-effect of BPH damage on rice. The RVI746/670 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 RVI746/670. The RVI746/670 (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 − 0.03683T2 − 0.03035N2 − 0.08188NT (R2 = 0.5228, p < 0.01). In summary, the spectral index RVI746/670 of rice is sensitive to damage caused by BPH.
Highlights
A slight change in physical structure or chemical substance of plant leaves would result in changes of their spectral characteristics, and the hyperspectral remote sensing end-products have the potential to reflect stresses imposed on crops [1,2,3,4]
The spectral index RVI746/670 of a rice plant at tillering stage increased when it was damaged by two–eight 2rd-instar nymphs for two days, and the value was significantly higher from the rice plant exposed to 8 brown planthoppers (BPHs) nymphs than from the rice plant without exposure (F4,10 = 4.27, p = 0.028), but there were no significant differences in values from rice plants exposed to two to eight nymphs (Figure 3a)
The different letters above the box plots show significant differences in the RVI746/670 means among different numbers of BPH at p = 0.05 level
Summary
A slight change in physical structure or chemical substance of plant leaves would result in changes of their spectral characteristics, and the hyperspectral remote sensing end-products have the potential to reflect stresses imposed on crops [1,2,3,4]. Herbivore insects feed on crops that alter the leaf area, color, tissue structure, or content of chemical substance, so the feeding damages may be identified by hyperspectral traits of crops [5,6,7,8]. Hyperspectral sensing has a good prospect in monitoring insect pests on crops [9,10,11,12]. Many studies have found that there are constant relationships between spectral index from crops and pest damage levels [13,14,15].
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