Leaf Reflectance Spectra of Cereal Aphid‐Damaged Wheat

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Abstract
The efficiency of field monitoring for insect pests would be improved with knowledge of reflected solar radiation from crop canopies during insect outbreaks. The objectives of this greenhouse study were to characterize leaf reflectance spectra of wheat (Triticum aestivum L.) damaged by Russian wheat aphids (Diuraphis noxia Mordvilko) and greenbugs (Schizaphis graminum Rondani) and to determine those leaf reflectance wavelengths that were most responsive to crop stress imposed by these aphid pests. When the ligule was visible on second oldest leaf, wheat plants were infested with four wingless adult Russian wheat aphids, four wingless adult greenbugs, or left uninfested (four replicate plants per treatment). Plants and aphid populations were allowed to grow under greenhouse conditions for 3 wk, after which leaf‐reflected radiation (from the adaxial surface across the 350–1075 nm range), dry weight, area, and chlorophyll concentrations were measured. When compared with the control, greenbug feeding damage caused general necrosis in oldest (first) leaves and dramatically lowered the dry weight, leaf area, and chlorophyll concentration of the second, third, and fourth leaves. Russian wheat aphid feeding resulted in a reduction in leaf dry weight and area in the third and fourth leaves, and a reduction in total chlorophyll concentration in all leaves. Leaf reflectance in the 625‐ to 635‐nm and the 680‐ to 695‐nm ranges, as well as the normalized total pigment to chlorophyll a ratio index (NPCI), were significantly correlated with total chlorophyll concentrations in both greenbug‐ and Russian wheat aphid–damaged plants. Thus, both of these wavelength ranges, as well as this reflectance index, were good indicators of chlorophyll loss and leaf senescence caused by the aphid feeding damage.