Biotic Stress and Yield Loss

egressions. Actually, the title “injury-defined EILs” is slightly misleading, because inprinciple the EIL always defines a level of injury sufficient to cause economic loss. Inpractice, however, pest densities are used as an index of injury, so my terms “injurydefined”and “pest-defined” really just refer to how we express the EIL (in terms of leafloss or insect densities). As background for our calculations, Haile 26 obtained data in1997 and 1998 on simulated insect defoliation to soybean, and defined strong linear relationsbetween the leaf area index (LAI) (the ratio of leaf area to ground area) and interceptedphotosynthetically active radiation (PAR). More specifically, the linearrelationship held for LAIs below the critical LAI of 3.5. (The critical LAI for a crop isthat LAI at which a canopy intercepts approximately 95% of all PAR.) Additionally, heobserved another strong linear relationship between intercepted PAR (immediately postdefoliation)and yield. Haile’s findings are consistent with a growing body of otherresearch on soybean defoliation.The following example is based on calculations with absolute yield, but similarcalculations can be made for proportional yield. In the following equations, a1, b1 anda2, b2 are linear regression parameters. PAR refers to intercepted PAR after defoliation,and LAI refers to the LAI of plants after defoliation. As a first step, we need torelate LAI to yield because we want to define the EIL as leaf tissue lost, and we canconvert lost tissue into an insect density by considering insect consumption rates:YLD a1b1*PAR, andPAR a2b2*LAI, soPAR (YLD-a1)/b1, and[2.2]LAI (PAR-a2)/b2;combining the two previous equations yieldsLAI (((YLD-a1)/b1)-a2)/b2. [2.3]This explains the relationship of remaining LAI to yield, but we need to calculatehow much leaf area would need to be removed to reach an economic level of yieldloss. Without injury, we have what I am calling check yield (CYLD); this occurs ator above the critical LAI (what I have called CRTLAI), soat the EIL (GT gain threshold)CRTLAI ((((CYLD-a1)/b1)-a2)/b2) and [2.4]EILLAI ((((CYLD-GT-a1)/b1)-a2)/b2). [2.5]The gain threshold (defined originally by Stone and Pedigo 19 ) is the amount of yield lossnecessary to justify management; the GT is determined as GT C/V, where C is the costof management and V is the value of production. By subtracting CRTLAI-EILLAI, we