Understanding Stem Borer In Soybeans​

The Dectes stem borer is the larva of the long-horned beetle (Figure 1). While sunflowers are the preferred host crop of this insect, Dectes stem borer can overwinter in soybean stubble. This insect is ty​pically most frequently reported in the Plains states, Mississippi and Ohio river valleys, and the Atlantic coast; however, movement into the Northern Plains and Upper Midwest is occurring due to no-till adoption and warmer winters.

Figure 1. Adult long-horned beetle. Photo courtesy of Ted MacRae.

The insect overwinters as a mature larva within the host plant, pupates and emerges from June through August. Mating occurs approximately five days after emergence. The female beetle then chews a hole in the soybean leaf petiole and lays a single egg. The egg hatches and the larva begins feeding on the inner pith within the stem and burrows or tunnels to the base of the stem (Figure 2). Larvae are cannibalistic, so only one insect will be present in a plant. The mature larva creates a chamber at the base of the stem just above the soil surface in preparation for overwintering.

Figure 2. Dectes stem borer larva tunneling inside a soybean stem. Photo courtesy of Ted MacRae.

Direct yield effects from Dectes stem borer feeding are typically minor under an average infestation. Yield losses from standability issues can occur because of girdled stems. Lodging can often be severe in earlier maturing or an earlier planted soybean crop.

Identification of stem borers can be difficult. Random wilting and dying of leaves at or near the top of the plant is an indicator when surveying a field. A reddish scar around the entrance score indicates that an egg has been deposited. Splitting the stem diagonally can ultimately determine the presence of a Dectes stem borer larva.

Insecticide efficacy on Dectes stem borer is often not economically viable due to their wide window of activity. Several cultural practices are recommended to reduce potential yield losses:

  • Early/timely harvest
  • Fall tillage
  • Weed control of host plants (wild sunflower, giant ragweed, common cocklebur
  • Fuller maturity varieties
  • Crop rotation

For more information, contact your Channel Seedsman or Technical Agronomist.​​​​​​​​​

Sources: ​​

Wright, B. Dectes stem borer emerging in soybeans. CROPWATCH. University of Nebraska. https://cropwatch.unl.edu/dectes-stem-borer-emerging-soybeans.


Wright, R.J. and Hunt, T. E. 2011. Soybean stem borers in Nebraska. NebGuide. G2082. University of Nebraska.

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