Close up adult dectes stem borer; Image Courtesy: Adam Varenhorst

Dectes Stem Borer in Soybeans

December 18, 2023

As South Dakota producers prepare for the 2024 planting season, there’s a little-talked-about-pest for consideration. The Dectes stem borer is a small, long-horned beetle whose larvae feed within soybean stems and can wreak havoc on stalk integrity.

The Dectes stem borer larvae cause 10-15% yield losses just by feeding within the stem and causing lodging which - without wind - can cause an additional 19% yield loss before harvest. If strong winds or storms occur before the harvest of infested fields, much greater yield loss can occur.

In-Season Management

For soybeans, there are only a few approved insecticides that are labeled for Dectes stem borer management in soybeans. However, these are generally not recommended due to limited yield improvements and the need for correctly timed applications. However, research being conducted by Dr. Adam Varenhorst at South Dakota State University - and funded by South Dakota’s soybean checkoff - is evaluating application timings and foliar insecticides to develop a management plan for the near future.

Pre-Season Management

There are also pre-planting steps available to reduce the impact of Dectes stem borer. According to research conducted by Varenhorst, there are three approaches for managing Dectes stem borers.

  • Plant an alternative host along the edge of your soybean crop that the beetles prefer for egg-laying like wild sunflowers

  • Manage weeds like wild sunflower, cocklebur and giant ragweed which are preferred hosts.

  • Consider tillage to bury soybean stubble at least 2 to 3 inches deep. This strategy is not recommended for areas where no-till practices are adopted.

  • Rotate crops.

How to Identify the Dectes stem borer

Image Courtesy: Adam Varenhorst

Dectes stem borer adults are gray and slender. They are approximately three-eighths of an inch in length with antennae that are as long as the body. The antennae have a distinctive black and gray alternating pattern.

Image Courtesy: Adam Varenhorst

The larvae of the Dectes stem borer are present within soybean stems. They are white to cream-colored with an orange to brown head capsule. The larvae have an accordion-like appearance due to constrictions between each body segment. They are legless and can be approximately one-half to five-eighths of an inch in length.

Scouting for Adults and Larvae

Producers should scout for beetles beginning in June, but adults can be present through August. During the summer, producers should not only watch for the beetles themselves but for the loss of trifoliate and broken plants hanging in the canopy as newly hatched larvae tunnel through the petiole toward the main stem.

Local agronomists have noted that “a dead trifoliate surrounded by healthy leaves is a telltale sign that Dectes stem borer is present.” It’s important to remember that once the larvae are in the plant, a foliar insecticide will not be capable of reaching them.

Harvest Considerations

The Dectes stem borer will move to the base of the plant and girdle the stem sometime in late September to early October depending on moisture and soybean growth stage. According to Varenhorst, “drought conditions can force the larvae to prepare for overwintering earlier in the season.” He recommends that producers consider harvesting infested soybeans as early as possible to reduce the impact of lodging.