Developmental and feeding trials were conducted with Plathypena scabra (F.) under 3 sets of environmental conditions. Durations of the egg, larva, prepupa, pupa, and adult stages were 4.8, 18.7, 1.3, 9.4, and 9.3 days, respectively. Significant differences were found between trials for the egg and total life cycle, but no significant difference in larval stadium was indicated. Soybean leaf consumption of each larval instar was determined with a photometric device. The mean soybean-foliage consumption was 19.89 in2 per larva in trial 1 and differed significantly from the 17.06 in2 and 15.55 in2 obtained in trials 2 and 3. Estimates of total leaf area per soybean plant were made from field measurements. Predictive equations to establish the relationship between soybean defoliation and subsequent yield loss were calculated by using data from previous hail-damage investigations. Theoretical economic-injury levels for P. scabra on soybean in Iowa were calculated by using economists' cost and market data, agronomists' damage-yield data, and feeding data from this study. Calculated levels were 5.9, 18.3, 33.5, 28.7, and 40.5 larvae/row ft for soybean stages 1, 3, 5, 7, and 9, respectively. These values suggested that most recommended chemical treatment levels should be revised upward.