Soybean defoliation estimation methods and thresholds in the North Central United States

Leaf-feeding insect pests are often encountered in soybean, Glycine max (L.) Merr., but rarely cause defoliation to exceed established action thresholds in the North Central United States. Pest managers often question these thresholds and apply insecticides when pest densities are well below recommended levels. Furthermore, assessing soybean defoliation in the field is often perceived as too time-consuming. Our objectives were to (i) determine the extent of insect defoliation injury in the North Central United States relative to economic thresholds, (ii) characterize spatial patterns of insect defoliation within soybean fields, and (iii) compare visual estimates of soybean defoliation obtained from the entire soybean canopy with those obtained from individual leaflets. We assessed insect defoliation in 65 soybean fields in 7 states in 2019 and 2020. Only 1 field exceeded an economic threshold computed using updated economic data applied to established relationships between leaf tissue removal and yield loss. The most common pests observed included the Japanese beetle, Popillia japonica Newman; the bean leaf beetle, Cerotoma trifurcata (Förster); the green cloverworm, Hypena scabra (F.); and grasshoppers (Orthoptera: Acrididae). We did not observe consistent "edge effects" in defoliation, likely due to the variety of leaf-feeding insect pest species we encountered. We computed sequential sampling plans that suggested relatively few samples would lead to a decision not to apply an insecticide for defoliating insects in most soybean fields. Defoliating insects rarely cause economic damage in the North Central United States based on current conditions.