Comparison of three methods for estimating defoliation in soybean breedingprograms

The proper assessment of defoliation by phytophagous insects in soybean [Gl ycine max (L.) Merr.] is required when screening germplasm for insect resis tance. The objective of this study was to evaluate the effectiveness and re lative efficiency of the widely used visual or relative assessment of insec t defoliation compared with a more quantitative assessment for use in soybe an breeding programs, In 1985 and 1991, visual assessments (whole plot, 198 5; whole plant and average of partitioned plant, 1991) were compared with a n established quantitative assessment. Twenty leaflets were randomly remove d from each of the upper, middle, and lower thirds of the canopy of each pl ot. These leaflets mere compared with a template of known defoliation in 19 85, In 1991, a leaf area meter was used to determine the actual amount of d efoliation 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 visua l 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 the se studies show that relative estimates can be used to assess insect defoli ation accurately and quickly.