How to Identify and Manage Dectes Stem Borer

Categories: GROWING, SOYBEANS
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  • Dectes stem borer populations have increased at alarming rates in many previously unaffected areas.  
  • No-till or minimal tillage fields may be at greater risk for Dectes stem borer.
  • There are currently no insecticides labeled for Dectes stem borer larvae control, making alternative management practices critical for mitigating damage.
Introduction
Instances of Dectes stem borer, also known as soybean stem borer, infestations have increased notably in recent years. This pest has an extensive reach from the Southeastern U.S. and Great Plains, reaching as far north as ND and as far south as TX, with sightings also recorded more frequently in areas throughout the Midwest. The ever-expanding geography of this relatively new pest will be a critical factor to consider in future soybean management.                    

Identification and Life Cycle
The Dectes stem borer beetle has 1 generation per growing season. Larvae overwinter in the base of the stem of the host plant. In the spring, the overwintering larvae enter an 8-10 day long pupal stage after which they emerge as adult beetles. Adults are light gray in color, approximately 3/8-1/2“ in length, with long, slightly curved antennae. Adult stem borers mate soon after they emerge, and females lay eggs in soybean leaf petioles around mid-canopy. When larvae hatch they are a light, creamy white in color, but they darken upon maturing. The bodies of the larvae are “accordion-like” in appearance with reddish-orange or brown colored heads and typically grow to 1/2-5/8“ in length. They feed on the plant by tunneling through the petiole into the stem. Usually, only 1 mature larva will be found per plant because they are cannibalistic.

Injury to Soybeans
The greatest contributing factor to yield loss because of Dectes stem borer is soybean lodging, ultimately due to girdling of the lower stem brought on by larva feeding on the plant pith and frass build-up. An outside force, such as high winds or heavy rain, is usually needed for lodging to occur. Excessive pith and vascular tissue feeding weakens the plant structure and restricts moisture and nutrient flow, effectively reducing pod fill and yield in heavily impacted fields. An indicator of an infestation can be the wilting or senescence of the upper canopy leaves due to the larval entrance points and feeding.

Management and Additional Considerations​​​​​​​
The most conducive conditions for Dectes stem borer are thought to be conservation or no –till systems, where a buildup of minimally disturbed residue provides hosts for larvae to overwinter. Several weeds are also hosts to the Dectes stem borer, such as cocklebur and giant ragweed. Fields with high presence of these weeds are more likely to harbor the pest and risk infestation. Soybean field edges are also typically more susceptible and are oftentimes more heavily impacted than the center of a field.

Treatment options are limited at this time as there are currently no insecticides labeled for burrowed larvae, and resistant soybean varieties have not yet been developed. There are several cultural practices, however, that can be used to mitigate impacts of the pest:
  • Harvest soybeans as soon as possible after maturing to limit opportunity for lodging to occur.
  • Utilize effective weed management techniques to control hosts such as cocklebur and giant ragweed in and around the field.
  • Consider fall tillage to reduce survival rates of overwintering larvae.
  • Consider increased crop rotation and avoid planting soybeans near recently infested fields.
For more management advice and insights into Dectes stem borer, contact your local Golden Harvest Seed Advisor.

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