Soybean leaf consumption for the most common semilooper species, Trichoplusia orichalcea (F.) was established in the laboratory using a leaf area meter. The total consumption per larva was 120,85 cm$ sp2$. The yield reduction of our soybean cultivars (Glycine max (L.) Merrill) being Duiker, Gazelle, Roan and SCS1 was measured in relation to different levels of defoliation. The defoliation was induced manually singly or sequentially over three different growth stages. / At soybean maturity, counts of pods per plant, number of seeds per plant, weight per 100 seeds and weight of seeds per plant were obtained. Yields were converted to a percentage of yield and linear regressions fitted to the relationship between percentage yield and defoliation. The percentage yield was influenced by the cultivars, the levels of defoliation, and the timing of defoliation. / The most critical growth stage for defoliation was the seed development stage, but the differences between them were small and not significant. SCS1 was the most resistant cultivar to defoliation, while Duiker was the most susceptible to single defoliation. / The economic thresholds were determined for the soybean cultivars demonstrating significant linear relations between percentage yield and single defoliation. Based on three insecticides and two application methods, the economic thresholds were determined at the flowering stage for Duiker, at the pod development stage for Duiker and Roan and at the seed development stage for Duiker, Gazelle, Roan, and SCS1 cultivars. The numbers of semilooper caterpillars per metre of row required to reach the economic thresholds, are generally higher than the natural levels of infestation.

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