Comparison of three methods for estimating defoliation in soybean breeding programs

The proper assessment of defoliation by phytophagous insects in soybean [Glycine max (L.) Merr.] is required when screening germplasm for insect resistance. The objective of this study was to evaluate the effectiveness and relative efficiency of the widely used visual or relative assessment of insect defoliation compared with a more quantitative assessment for use in soybean breeding programs, In 1985 and 1991, visual assessments (whole plot, 1985; whole plant and average of partitioned plant, 1991) were compared with an established quantitative assessment. Twenty leaflets were randomly removed from each of the upper, middle, and lower thirds of the canopy of each plot. These leaflets mere compared with a template of known defoliation in 1985, In 1991, a leaf area meter was used to determine the actual amount of defoliation of each plant, In 1990 and 1991, two types of visual estimates (whole plant and average of partitioned plant) were also compared with each other. Sampling time was estimated for each method. Estimates between visual and quantitative assessments were highly correlated each year. Precision was similar for each method, Sampling time for the visual assessment was at least 30 times faster than the quantitative assessment. The results of these studies show that relative estimates can be used to assess insect defoliation accurately and quickly.