Dectes Stem Borer Adults Are Active in Soybean: 2025 Management Recommendations

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.

Dectes stem borer adults have been collected from soybean in Southeastern South Dakota (Figure 1). As a reminder, the larvae of this insect pest are capable of reducing soybean yields by 10-15% due to their tunneling activity in the main stem (Figure 2). When the soybean reach maturity and begin to dry prior to harvest, the Dectes stem borer larva then moves toward the base of the plant and girdles or cuts the stem. This activity can cause 17% of 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.

Adult

Gray beetle with long antennae.
Figure 1. Dectes stem borer adult. (Credit: Adam Varenhorst, SDSU Extension)

Larva

Cream colored larvae on white and green soybean stem on brown dirt.
Figure 2. Dectes stem borer and boring injury through a soybean stem pith. (Credit: Philip Rozeboom, SDSU Extension)

Scouting and Management

The Dectes stem borer adults will be active through August, and 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. From the 2024 research, soybeans 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. Based on the current soybean cash price of $10.15 per bushel, Hero® insecticide applied at 10.3 oz per acre ($1.60 per oz), Vantacor® insecticide applied at 2.5 oz per acre ($11.72 per oz), and an application fee of $9.00 per acre; a single application of Hero® insecticide would provide a benefit of approximately $66.89 per acre and two applications of Vantacor® insecticide would provide a benefit of approximately $2.57 per acre. An application of Hero® insecticide followed by a later treatment of Vantacor® insecticide would provide a benefit of $28.69 per acre.

Our management recommendation that should be considered after Dectes stem borer adults have been confirmed in soybean is to apply either a single application of Hero® insecticide during the second or third week of July or one application of Hero® insecticide during the second week of July followed by an application of Vantacor® insecticide approximately two weeks later. These application timings provided the best yield response in our research. Although Elevest® and Brigade® 2EC insecticides are also labeled for the management of Dectes stem borer in soybean they did not prove to be economical during our 2024 study. As always, read and follow all insecticide labels for rates and restrictions.

Related Topics

Soybean Insects

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