Mainstem girdling of soybean seedlings by threecornered alfalfa hoppers, Spissistilus festinus (Say), can result in several plant responses: (1) death of the plant, usually within 2 - 3 wks of injury,(2) mainstem breakage at the girdle soon after girdling through maturity, (3) partial breakage at the girdle with continued plant growth, (4) or the injured area (girdle) may heal over and the plant remain upright. Studies were conducted to simulate plant loss occurring during the reproductive growth stages which would result from threecornered alfalfa hopper injury during the early vegetative growth stages and examine the impact of this plant loss on yield of full-season indeterminate, late-planted/double-crop indeterminate, and full-season determinate soybeans, Glycine max (L.) Merr., at specific reproductive growth stages. Yield of full-season indeterminate soybeans was reduced by 6.4 - 15% when plant loss exceeded 20% at the R1–R2 growth stage. At R3 and R5, yields were reduced 7.4 - 30% and 7.2% to 32.1%, respectively, when plant density was reduced by ≥ 10%. For late-planted / double-crop indeterminate soybeans, yield losses of 10.6% to 19.3% occurred when plant loss exceeded 30% at the R1–R2 growth stage. Yields were reduced 6.9% to 20.6% at R3 when plant loss was ≥ 20. At the R5 growth stage, yields were reduced 22.4 - 40% when plant density was reduced by ≥ 30%.