Effects of Velvetleaf Competition and Defoliation by Soybean Looper (Lepidoptera: Noctuidae) on Yield of Indeterminate Soybean

Abstract

The individual and combined effects of various levels and timing of insectinduced defoliation and velvetleaf ( Abutilon theophrasti Medicus) competition on soybean yields were measured during four years of study. Larvae of caged adult soybean loopers, Pseudoplusia includens (Walker), were used to attain planned levels and schedules of defoliation in experimental units consisting of two rows of soybeans 1.8m in length, either weed free or infested with two densities of velvetleaf. Estimates of actual levels of defoliation were recorded from leaflet samples in the first 2 yr of the study, and percentage reduction in leaf area index (LAI) compared to nondefoliated controls was measured in the last 2 yr. Soybean yield was significantly reduced by both defoliation and weed competition, and combinations of defoliation and velvetleaf competition had an additive effect on soybean yield reductions in some plots. Soybean yields were generally lowest in treatments consisting of the highest levels of defoliation and densities of velvetleaf. However, these results showed little, if any, interaction between insect defoliation and velvetleaf competition at the levels included in this study. At soybean growth stage R2, percentage defoliation was a better predictor of yield loss than percentage reduction in LAI, but at R4 the reverse was true.