By Robert Koch, Extension Entomologist
Recent cool and wet conditions may increase the risk of seedcorn maggot infestation in some soybean and corn fields. Seedcorn maggots are small (1/4 inch long), white maggots (fly larvae) that feed on germinating seeds. The maggots can tunnel into seed, which may result in seed death, and can injure the emerging plant tissues, which can affect plant growth or lead to damping off. Such injury can result in stand loss or weakened plants. For example, if the growing point of soybean is killed, "Y-plants" can result when branching develops at the cotyledons. Yield from "Y-plants" may be reduced if competing with neighboring healthy plants. Seedcorn maggot injury can be difficult to distinguish from other problems such as Pythium and other seedling diseases.
Fields at greatest risk are those with decaying organic matter, such as a recently incorporated cover crop or manure. Risk of injury is greater when cool and wet conditions slow germination and emergence, which increases the window of time plants are susceptible to attack. Rescue treatments are not available for this pest. However, preventative tactics can be utilized to protect seed and plants in high-risk situations. Seed-applied and soil insecticides can offer effective protection of germinating plants from seedcorn maggot (be sure to follow instructions on product label). In addition, degree-day models are available to guide decisions about adjusting planting date to avoid periods with high larval abundance (UW seedcorn maggot degree-days).
When significant stand loss occurs, replanting may be required, but this option should be considered carefully. Information is available to guide replant decisions in soybean (U of MN Extension soybean replant guide) and corn (U of MN Extension corn replant guide).