Evaluation location: South Dakota, United States
Methods: Levels of defoliation from chewing insects were characterized among 330 soybean lines in a thrice-replicated field-plot test at the Eastern South Dakota Soil and Water Research Farm near Brookings, SD, in 2010. Soybeans were planted at 4-cm depth on 26 (replicate 1) and 27 May (replicates 2 and 3) with a two-row cone seeder at 25 seeds per replicate plot (1.1 m). Row width was 0.75 m. 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, primarily from 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 remaining leaf tissue. Plants in replicates 1 and 2 were rated on 18 Aug, and those in replicate 3 were rated on 19 Aug. Two persons provided 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. 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). Of the 330 soybean lines, three lines had average ratings of 1 (PI 468906, PI 468909, PI 612753A), 16 lines had average ratings of 1.3, and 67 had average ratings of 1.7. The remaining 244 lines had ratings of 2 or greater, including 13 with average ratings of 3.