Are you noticing holes in your soybean stems? Holes where the petiole meets the main stem are the entry point where soybean stem borer (also known as Dectes stem borer) larvae tunnel into the main soybean stem. Originally eggs are laid in soybean leaf petioles in the upper canopy. The eggs hatch into larvae which burrow down the petiole then into the main soybean stem. Notice the soybean stem borer infected stem in the middle while the soybean stem to the right has a a non-infested area where the petiole dropped (it is naturally sealed over by the plant). Count how many plants out of 20 have this symptom to get an idea of percent infestation and repeat in several areas of the field. Fields with 50% or more infestation need to be harvested first and perhaps earlier to avoid lodging and yield loss associated with lodging.
Lodged soybeans can be another key for checking for stem borer around harvest time. Notice the stem in the middle of the photo that is lodged (fallen over instead of standing in the row).
Following the stem to the base, the stem easily breaks away from the plant. The stem itself will appear solid. The base of the plant where it breaks is also often sealed off. The stem borer will seal itself inside the base of the stem. In this case, there’s a small portion that hasn’t been sealed off yet.
Gently pulling apart the base of the stem reveals the soybean stem borer larva. The larva will spend the winter and eventually pupate here. Adult beetles will emerge in late June and there’s only one generation per year. For more information specific to life cycle and management, please see the following NebGuide.