By Bob Wright, University of Nebraska—Lincoln Extension entomologist
We are starting to see dead leaves caused by feeding of soybean stem borer larvae in south central Nebraska soybeans. No control measures are appropriate at this time, but monitor your fields and be aware that high populations of soybean stem borers may predispose the field to lodging and make harvest difficult. Fields with higher levels of injury by soybean stem borers should be harvested first to minimize lodging losses.
Soybean stem borers are the immature stages of a long-horned beetle, Dectes texanus. The adults lay eggs in the upper leaf canopy, typically in the leaf petiole. Larvae feed by tunneling within the soybean plant. At this time of year larvae move from the leaf petiole into the main stem of the soybean plant, and at that time the leaf dies. These leaves are easily detached from the stem, and a circular tunnel can be seen where the leaf petiole was attached to the main stem.
As larvae grow larger they continue to tunnel within the main stem, and by the end of the growing season they have tunneled to the base of the plant where they overwinter. In preparation for overwintering, they hollow out a cell at the base of the plant, which weakens the stem and makes it more susceptible to breakage.
For more information see the NebGuide, Soybean Stem Borer in Nebraska.