SOYBEAN GROWTH AND YIELD AFTER SIMULATED BEAN LEAF BEETLE INJURY TO SEEDLINGS

Although the bean leaf beetle [Cerotoma trifurcata (Forster)] is a major pest of seedling soybean [Glycine max (L.) Merr.], the effects of beetle injury on soybean growth and yield are poorly understood. This study was conducted to characterize growth and yield responses of soybean to simulated bean leaf beetle injury occurring during seedling stages. Also, single-day and sequential defoliation techniques were compared. The study was conducted on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll) in a soybean ('Elgin 87') field near Mead, NE. Treatments consisted of an undefoliated check and various levels (0-80%) of simulated bean leaf beetle defoliation (sequential and single-day) and/or cotyledon removal. Cotyledons were removed at stage VC, sequential defoliation was begun during stage VI, and single-day defoliation was imposed on the last day of the defoliation period (late V3). Soybean yield significantly decreased 12% as seedling defoliation increased to 68%. Seedlings experiencing sequential defoliation did not reach a critical LAI of 3.5 until well into their reproductive stages, limiting light interception and dry matter accumulation. Cotyledon removal coupled with defoliation also reduced height up to 28% at R2.1 and delayed canopy development up to 11 d. Cotyledon removal reduced leaf area development of the seedling soybean 32%. Plants without cotyledons required up to 34% less defoliation than plants with cotyledons to delay canopy development and reduce yield. Single-day defoliation techniques were less accurate than sequential defoliation techniques and overestimated the amount of leaf tissue a continuously defoliating insect would remove.