By Dominic Reisig
Corn earworm, also known as bollworm and tomato fruitworm, is an important pest that causes economic damage to a wide variety of globally important crops, such as corn, cotton, sorghum, vegetables, tobacco, and soybeans. In the Southeastern region of the US, this pest causes major yield loss in cotton and soybeans, but usually not corn. However, the economic threshold for soybeans depends on crop price, cost of control, sampling method, and row width (for more information use the Corn Earworm Threshold Calculator for Soybeans). Usually, the threshold increases when prices are below $10/bushel and at lower levels of earworm feeding, soybean can compensate for potential yield loss. Soybean varieties are classified depending on growth habit.
Soybean varieties with a determinate growth habit terminate vegetative growth on the main stem once reproductive stages begin. In contrast, varieties with an indeterminate growth habit continue vegetative growth once reproductive stages begins. In recent years, many North Carolina soybeans producers are opting to plant indeterminate soybean varieties for the associated yield benefits in both full season and double crop production scenarios.
Indeterminate varieties flower longer than determinate varieties; as a result, there may be more tissue available for a longer time for corn earworm to lay eggs and feed on it. Adult corn earworm prefers to lay eggs during the R1 and R2 flowering stages of soybeans and, once the eggs hatch, the caterpillars will injure flowers, pods, and leaves of soybeans. While it is difficult for caterpillars to eat enough flowers to cause yield loss, once pods form on the plant, the risk of yield loss is much greater. Hence, flowering is not the ideal time to spray, because it will likely not protect yield and might kill beneficial insects, resulting in even more yield loss from flaring other pests.