Detection of brown planthopper infestation based on SPAD and spectral data from rice under different rates of nitrogen fertilizer

Abstract

Reflectance and SPAD readings were measured in rice grown under different nitrogen fertilizer rates and infested with different numbers of brown planthopper (BPH), Nilaparvata lugens, to assess the relationships among these variables. The results showed that SPAD readings and reflectance from rice were significantly affected by BPH infestations and nitrogen fertilizer rates, whereas there was no interaction between the two factors. SPAD readings and reflectance decreased as BPH infestations increased but they increased as nitrogen fertilizer rates were increased. SPAD readings varied with position of leaf on the same stem of rice. The fourth and fifth leaves were more sensitive to BPH infestations than the first and second regardless of application rates of urea. The ratio indices of SPAD readings of the fourth to first leaf (RSPAD4/1), and fourth to second leaf (RSPAD4/2) were significantly related to BPH infestations, and they were relatively independent of nitrogen fertilizer rates in the single stem rice. The spectral reflectance from rice canopy significantly decreased in the near-infrared wavelength range as BPH infestations increased. The modified chlorophyll absorption ratio index (MCARI710) was found more suitable to relate the numbers of BPH under different nitrogen fertilizer rates and durations of BPH infestation. The main effects of BPH infestations on SPAD reading and reflectance indices were consistent regardless of nitrogen application rates. Therefore, SPAD reading and spectral indices have potential to detect BPH infestations in rice fields.