Soybean cultivars and insect defoliation: yield loss and economic injury levels

Understanding the response of soybean [Glycine mas (L.) Merr.] cultivars to insect defoliation is important for identifying differences in insect tolerance and compensation among soybean genotypes, for assessing yield loss differences among cultivars, and for determining if a single economic injury level (EIL) can be used for all cultivars. Therefore, this study was conducted to evaluate the relative tolerance of selected soybean cultivars to insect defoliation. Field experiments were conducted in 1994 and 1995 on three indeterminate soybean cultivars: Dunbar, Corsica, and Clark. Sequential defoliation (46-66%) was imposed manually at the R2 stage (reproductive development of soybean at full flowering), guided by an insect consumption model. Leaf area index, percent light interception, growth, and yield data were determined. Because of ample rainfall during the soybean growing period in 1994, defoliation did not affect soybean yield; all cultivars compensated for all levels of defoliation via delayed leaf senescence and compensatory regrowth, which enhanced the light interception capacity of defoliated soybean canopies. In 1995, however, defoliation caused a significant yield reduction (15-70%) in all cultivars; this yield reduction was greater for Dunbar than the other two cultivars. In both years, yields were directly related to the light interception capacity of soybean canopies after defoliation. There were significant EIL differences among soybean cultivars. Large differences in EILs among cultivars suggest a need to revise existing EILs for defoliating pest species.