CENTRAL PLAINS: Central Kansas is accustomed to dealing with Dectes stem borer (often just referred to as soybean stem borer) in soybeans. If you haven’t experienced a Dectes stem borer infestation before, I usually describe them as the southwestern corn borer of soybeans. Unfortunately, I have been seeing higher level of this pest expand their reach outside of the traditional problem areas, which can pose a significant risk for harvest losses in soybeans.
Soon after hatching, stem borer larvae bore into the stem. They will remain in the stem and feed on the inside of the stem until the plants begin to reach maturity. Afterward, the stem borers will migrate down to the base of the stem, which has a girdling effect. With some moderate wind, affected plants can break off right at the soil surface, making them very hard to pick up with the combine.
Dectes stem borers’ choice of fields is fairly unpredictable. While no-till continuous soybean fields face the highest risk, after that, there doesn’t seem to be any real rhyme or reason why soybean stem borers target one field vs. another – or even one variety vs. another. I have seen fields before where, despite being very close in maturity, one variety had a very high infestation level while another was considerably less infested. In other words, it’s hard to say which fields – outside of those with continuous soybeans – are more at risk than others.
Scouting for the presence of stem borer can be done fairly quickly, and it is well worth the trip if they are present. To search for borers, simply grab some plants and snap the stems open between three or four different nodes. If borers are present, the inside of the stems would be dark brown. You also can usually see sawdust, a direct result of the feeding. If you find these symptoms, splitting the stems will usually uncover larvae.
How many is too many? I’m not sure that there is a good answer for this, as having any larvae present can present a problem. The main goal of the assessment is to help you prioritize fields based on risk. Obviously, to save yield potential, we would give harvest priority to any infested or at-risk fields. The longer a compromised field sits unharvested, the more likely that plants will break off at the ground and be missed by the combine.
Reporting from Hutchinson, KS
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