Sequential decision plans based on the binomial distribution were developed for categorizing soybean defoliation caused by the Mexican bean beetle,Epilachna varivestisMulsant. Plants were classified as economically damaged if defoliation was ≥20%. Four plans were tested and compared by simulation on field data consisting of a fixed sample size. The plans showed an overall time saving of 31 to 68% and agreed with the fixed sample outcome in 82 to 91% of the simulations. Procedures that demonstrate the use of sequential decision sampling for soybean defoliation are given, and their application to field situations different from those in the study is discussed.The spatial distribution of within-plant defoliation was characterized and related to soybean growth and the population dynamics of the Mexican bean beetle. The vertical and horizontal distribution of both defoliation and leaf area changed significantly as the soybean plant matured and the overall level of damage increased. The possibility of biased estimates of defoliation, when subsampling plants, is discussed. Variance components at the field, plant, node, trifoliolate, and leaflet levels of sampling also were characterized. Half of the total variance in defoliation per leaflet was attributed to the trifoliolate position on a branch, whereas only 6.4% was due to variation among main stem nodes.