SOYBEAN GROWTH AND DEVELOPMENT ALTERATIONS CAUSED BY HELICOVERPA-ZEA (BODDIE) FEEDING

Corn earworm, Helicoverpa zea (Boddie), injury to soybean, Glycine mar (L.) Merr, is characterized Primarily by feeding on leaves, pods, and flowers. Although soybean can withstand considerable damage to each plant part individually or in combination without Yield loss, corn earworm larvae frequently cause yield reductions. To evaluate alterations in plant growth that determine if yield losses occur, the plant growth and yield response of soybean to corn earworm injury was observed at six experimental sites from 1985 to 1987. Insecticides were used to either promote or suppress corn earworm populations, and population levels were monitored during the period of infestation with ground-cloth samples. The effect of corn earworm feeding on leaf area index, number of flowers, number of pods, and pod weight was monitored weekly from the time of infestation until harvest. Leaf area index was reduced at each site. Flower and pod feeding resulted in a delay in pod fill and a reduction in final pod number at several sites. The rate of pod fill was increased in treatments with large corn earworm populations at two sites and was decreased at one site. The number of seeds per pod was also reduced by corn earworm feeding at several sites; the weight per pod was decreased at one site and increased at other sites. At harvest, the seed number, weight per seed, percent oil and protein, and harvestable yield were sampled. Yields were reduced at all but one site, and losses were attributed primarily to reduction in pod and seed number. At one site where infestation occurred during the early flowering stage, yield loss was attributed primarily to reduced weight per seed. Since mechanisms of yield reduction were different, different thresholds may be required for soybean in early reproductive stages and soybeans in later stages.