By Adam Varenhorst
Dectes stem borer adults have been collected from soybean in Southeastern South Dakota this week (Figure 1). As a reminder, the larvae of insect pest are capable of reducing soybean yields from 0-17% due to its feeding activity inside of the main stem (Figure 2). Variations in observed loss are potentially due to seasonal conditions including drought. When the soybean reach maturity and begin to dry down prior to harvest, the Dectes stem borer larva moves toward the base of the plant and girdles or cuts the stem. This activity can cause the infested plants to lodge prior to harvest. During the last few years, we have observed fields with infestations that range from 70-100% of the sampled plants in Southeastern South Dakota.
Scouting and Management
The Dectes stem borer adults will be active through August with peak adult activity occurring sometime during mid to late July. Soybean in areas that have had Dectes stem borer infestations during the previous year should be scouted. Recent research from SDSU has demonstrated that an insecticide application during the first to second week of July can protect yields.
Results from the 2024 research indicated that soybean treated with a single application of Hero® insecticide yielded 9.1 bushels per acre more than the untreated control. Two applications of Vantacor® insecticide provided a yield benefit of 7.8 bushels per acre.
We recommend applying a single treatment of Hero® insecticide during the third week of July or if two treatments are needed, we recommend applying Vantacor® insecticide during the first or second week of July followed an application of Hero® insecticide approximately two weeks later. As always, read and follow all insecticide labels for rates and restrictions.
Source : sdstate.edu