Refinement and Validation of Soybean Looper Threshold in Mississippi Soybeans

Previous research has demonstrated that an economic threshold of 20% defoliation during the reproductive growth stage of soybeans can prevent economic yield loss. Studies were initiated in 2017 and 2018 to determine the relationship between soybean looper densities and percent defoliation using both natural infestations and cage studies. Cage studies were infested with varying pairs of soybean looper moths to obtain different levels of infestation. Preliminary results indicate that for each larva found, defoliation increases by 0.64% and for each larvae ½” or larger found, defoliation increases by 0.8%.  Additional studies were performed to compare the drop cloth and sweep net sampling methods to develop and equivalency between the two. This study was conducted in commercial soybean fields at various locations throughout Mississippi using multiple samplers performing 5 sets of 25 sweeps and 5 drops so that results could be paired. Preliminary results indicate that for each larva found with a drop cloth, 3 larvae were found with a sweep net; with larvae 1/2” or larger 1 larvae found with a drop cloth equated to 2.5 larvae found with a sweep net. In addition to the field studies, a lab feeding study was performed to determine the feeding rates of soybean looper larvae based on instar.