Abstract
Soybean plants were artificially defoliated to simulate the feeding of leaf eaters. The effect of defoliation on the yield was assessed through the change in the recovery pattern of the plant. Based on the data obtained, a method was proposed to determine the economic injury level (EIL) and the control threshold (CT) for leaf-eating insect pests of soybean, e.g. the common cutworm, Spodoptera litura. The early-maturing variety of soybean (Bonminori) was sown on 10th June, 1981. The planting density was 8.33 plants/m2 with an interval of 0.6m between ridges. Artificial defoliation was carried out at 6 different growth stages of soybeans with defoliation rates of 25, 50, 75 and 100%. The pattern of recovery of the plants after defoliation was determined at 10-11 day intervals in terms of leaf area, number of pods, dry weight of leaves, stems, roots and pods. The number of grains per plant and the mean grain weight were measured at harvest. The earlier the stage of growth of soybean at the time of defoliation was, the higher the recovery rate of leaf area. However, no detectable recovery was observed in the plants defoliated at the reproductive stage, especially after R3 (beginning pod stage). Defoliation in the early stages resulted in a yield loss through the reduction in the number of pods, while defoliation in later stages brought about a yield loss associated with the reduction both in the number and mean weight of pods. Based on the relationship between the yield loss and the weight of defoliated leaves at different growth stages of the soybean plant, the extent of defoliation that will cause any definite yield loss can be estimated. The EIL and CT for the common cutworm were calculated on the basis of published data on its feeding activity and survival during the larval stage.
