Spectral response of soybean plants to infestation of soybean aphids (Hemiptera: Aphididae)

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), has become the most economically important insect pest of soybeans in the North Central United States and southern Canada. Soybean aphid populations may exceed by up to ten times the economic injury level (EIL) of 670 aphids per plant. For reducing scouting time and improving pest management decision making, we are exploring the use of hyperspectral remote sensing to distinguish among soybean aphid densities in soybean fields. In the first experiment, aphid densities were manipulated using fine-mesh cages infested at different plant growth stages and arranged in a randomized complete block design with eight replications. In the second experiment, disruptive insecticide sprays were used to establish a range of natural aphid populations in experimental open-field plots. For both experiments, spectral reflectance of the plants and number of aphids were recorded at approximately weekly intervals. Cumulative aphid-days was regressed against thirty narrow spectral bands and thirteen vegetation indices (α=0.05). Plants injured by the cumulative effect of aphids over time showed consistent changes in plant reflectance (CV≈5%). In the beginning of aphid infestation, there was a significant relationship between aphid feeding and plant reflectance within narrow bands of NIR range (770-880 nm) or vegetation indices that considered the NIR band (800 nm) in the equation. A more significant relationship (p<0.001) was observed for narrow bands between 450 nm and 880 nm when aphid densities were higher than EIL. In conclusion, our results suggest that spectral reflectance may hold promise for estimating soybean aphid feeding pressure.