Leaf reflectance indices for quantifying Colorado potato beetle damage to potato, Solanum tuberosum (Solanaceae)

BACKGROUND: Rapid response to pests is central to precision agriculture, where measuring crop reflectance may help quantify pest populations. We aimed to determine whether Colorado potato beetle damage changes reflectance of potato plants and, in turn, predict potato yields. Our first greenhouse experiment evaluated changes in canopy reflectance following Colorado potato beetle defoliation and mechanical defoliation on a whole plant. The second experiment was on excised potato leaves. The third experiment tested changes in reflectance due to a commonly used neonicotinoid insecticide imidacloprid. Additionally, reflectance at varied defoliation levels, and how both reflectance and defoliation predict tuber yields were assessed for two growing seasons in the field. The reflectance indices [the normalized difference vegetation index (NDVI) and the normalized difference red edge (NDRE)] were measured with a handheld sensor, defoliation was visually assessed, and tubers were weighed for yield. RESULTS: In the greenhouse, leaf damaged by beetle caused significant reduction in NDVI compared to the mechanically damaged plants and the control plants within 1 day due to changes in canopy structure and leaf color, while changes in NDRE were less pronounced. Imidacloprid treatment did not affect NDVI but NDRE was reduced 20 days after planting. In the field, both NDVI and NDRI were predictive of potato yields, which were strongly associated with mid-season defoliation. CONCLUSION: Defoliation by Colorado potato beetle, but not imidacloprid, changed NVDI and, to a smaller extent, NDRE. Both indices predicted potato tuber yields afield, with the stronger relationship detected for NDVI, therefore these indices are useful predictors of Colorado potato beetle damage to potato crops.