Evaluation location: South Dakota, United States
Methods: Levels of defoliation from chewing insects were characterized among 11 soybean lines in a thrice-replicated field-plot test at the Eastern South Dakota Soil and Water Research Farm near Brookings, SD, in 2010. Soybean seed was obtained from the USDA-ARS Soybean Germplasm Center, except that lines K1639-1 and KS4202 were obtained from Kansas State University. Soybeans were planted at 4-cm depth on 21 May with a two-row cone seeder at 25 seeds per replicate plot (1.1 m). Row width was 30 inches. Weeds within plots were managed by mechanical field cultivation, herbicides, and hoeing. Soybean plants incurred defoliation mainly during mid- to late season from natural infestations of chewing insects, including green cloverworms, soybean loopers, and grasshoppers, and to a minor extent from bean leaf beetles. Test lines were evaluated using a rating scale that ranged from 1 to 3, in which '1' = 0 to 25 percent defoliation of plants within a replicate plot, mostly from a few, small rounded feeding areas within interveinal area of leaves ('shot holes') and along leaf edges, with leaflets intact and virtually no yellowing of leaves. A rating of '2' = 26 to 74 percent defoliation, with numerous shot holes and large sections of leaf edges eaten, but leaflets mostly intact; and minor yellowing of leaves. A rating of '3' = greater than 75 percent defoliation, with large coalesced feeding areas, often with entire leaves consumed or only veins remaining; moderate yellowing of leaf tissue. Plants were rated on 19 Aug. Two persons produced a consensus rating for each replicate per line on each date. Ratings are reported as averages across replicates. Plants ranged from vegetative V6 (in late maturity checks) to full-seed R6 growth stages (in early MG) when rated because of the wide span in maturity of individual lines. PI 227687, PI 548654 and KS4202 had average ratings of 1, and PI 548374 had an average rating of 1.33. Other lines had ratings of 2 or greater. Ratings of 1.0 to 1.3 were considered resistant (R), 1.7 to 2.0 mostly resistant (MR), and 2.1 and above susceptible (S).