Detection of soybean aphid (Hemiptera: Aphididae) feeding using plant spectral reflectance

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important insect pest of soybean throughout the North Central United States. In-field scouting is critical to predict and maintain aphid densities below the economic injury level of ≈670 aphids/plant. Detection of insect feeding by measuring pest-induced changes in the proportion of incident light reflected by plant leaves (i.e., reflectance) holds promise for precise scouting programs. Our objective was to identify potential spectral bandwidths for detecting aphid-induced stress. Field trials were conducted in 2013 and 2014 using caged-plots (1-m width × 1-m length) arranged in a randomized complete block design with 7-8 replications. Three treatments (i.e., early infested, late infested, and aphid-free) were established to create differential soybean aphid populations. Four independent trials were also conducted under greenhouse conditions. Aphid densities were converted to cumulative aphid-days to be regressed against vegetation indices and spectral bandwidths recorded between 350 nm and 2500 nm. Cumulative aphid-days had a strong negative relationship with bandwidths in the near-infrared spectral range (i.e., 770-880 nm). Significant increase on the reflectance of some visible wavelengths (i.e., 400-700 nm) evidenced a reduction in chlorophyll content due to aphid feeding. Furthermore, our results suggested that narrow wavelengths can be satisfactorily combined in vegetation indices to predict aphid densities before economic losses. Decisions for soybean aphid control based on aphid-induced spectral changes may reduce the cost of sampling procedures, and perhaps make scouting more efficient and feasible to farms.