Effects of Popillia japonica (Coleoptera: Scarabaeidae) defoliation on soybean spectral reflectance and yield

Remote sensing has been shown to be a promising technology for the detection and monitoring of plant stresses, including insect feeding. Popillia japonica Newman, is an invasive insect species in the United States, and a pest of concern to soybean, Glycine max (L.) Merr., in the Midwest. To investigate the effects of P. japonica feeding injury (i.e., defoliation) on soybean canopy spectral reflectance and yield, field trials with caged soybean plants were established during the summers of 2020 and 2021. In each year, field-collected P. japonica adults were released into some of the caged plots, creating a gradient of infestation levels and resulting defoliation. Estimates of defoliation caused by P. japonica, ground-based hyperspectral readings, total yield and yield components were obtained from the caged plots. Defoliation from P. japonica was generally most severe in the upper canopy of soybean. Overall mean canopy defoliation (i.e., across lower, middle and upper canopy) ranged from 0.23 to 6.26%, which is representative of defoliation levels observed in soybean fields in the Midwest U.S. Defoliation from P. japonica tended to reduce measures of soybean canopy reflectance in near infra-red wavelengths (~700 to 1000 nm). These results indicate that i) the threat to soybean yield posed by P. japonica alone appears minimal, but this pest adds to the guild of other defoliating insects in soybean whose combined effects could threaten yield; and ii) remote sensing has potential for detection of defoliation from P. japonica and could facilitate scouting for this pest.