Soybean transpiration, vegetative morphology, and yield components following simulated and actual insect defoliation

Ostlie, Kenneth
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Soybean response to simulated green cloverworm (Plathypena scabra (F.)) (GCW) defoliation was investigated in several experiments from 1980 to 1983. The primary goals of this research were: (1) to assess the fidelity of simulation methods to actual insect defoliation for one basic physiological process, transpiration; (2) to determine if soybean response to defoliation varies between simulation methods; and (3) to elucidate the general effects of defoliation at full bloom on soybean growth, development, yield, and yield components;Water loss from potted soybean after defoliation by simulation methods and actual insects demonstrated significant, but transitory, differences during the first 16 hours. Total water loss over three days did not differ between simulated and actual insect defoliation. Therefore, simulation methods produced acceptable fidelity in soybean water-loss to actual insect defoliation;Simulation methods elicited different responses to defoliation. Each component of a simulation method, technique and temporal pattern, affected remaining leaf area, height, yield, and yield components. Punch-defoliated plants possessed less leaf area, height, and yield than pick-defoliated plants. One-day defoliation resulted in more leaf area, taller plants, and greater yields than either insect-model or equal defoliation. Yield differentials between methods were sufficiently large to significantly affect yield-loss estimates and economic injury levels (EILs);Simulated GCW defoliation at full-bloom (R2) produced linear reductions in leaf area, height, lodging susceptibility, and yield. Defoliation reduced leaf area in nodes 6-11, but reduced yields in nodes 4-9. This downward shift, presumably, involved downward translocation from upper undefoliated leaves. Defoliation primarily reduced yields through decreased pod and seed number and, secondarily, through smaller seed size;The EIL for first generation GCW larvae was established at 298,500 larval equivalents per ha or 23 per m of row (76 cm row width). Comprehensive EILs, reflecting moisture stress on the crop, were also derived. A general theory on incorporating survivorship information into economic thresholds was developed and illustrated using the GCW in Iowa soybean.