Dectes Stem Borer may be more of an issue during 2021

Dectes stem borers are an annual sunflower pest in South Dakota.

During most years, Dectes do not cause major issues in sunflower, which can be attributed to stalk circumferences that prevent the larvae from effectively girdling the stalk and causing lodging.

However, we have observed increased issues with Dectes stem borer during dry years. This is because dry conditions result in smaller stalks, which the larvae can girdle much more easily.

The dry conditions also will cause sunflower stalk conditions to deteriorate earlier in the season, which forces the larvae to prepare for overwintering earlier in the season. As a result, lodging occurs before the sunflowers can be harvested, which results in increased yield loss.

Based on the current drought conditions in our sunflower production areas, it is going to be likely that Dectes stem borer will cause lodging earlier than normal in the 2021 season.

Identification

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.

The larvae of the Dectes stem borer are present within sunflower 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 and management

Dectes stem borer adults emerge from June to July and will seek out host plants. Although the adults may be observed, management is not successful or recommended due to the long emergence period. Females lay single eggs in each stem of the host plant.

In sunflower, the Dectes stem borer larvae tunnel through the stem and feed on the pith tissue. Only one Dectes stem borer larva is observed in a stalk due to their cannibalistic nature.

Although the larvae will feed on tissue within the stalk that results in a reduction of water and nutrient movement within the plant, no yield reductions have been observed from this activity in sunflower. In the late summer, the successful larva moves towards the base of the plant and will girdle the stem approximately two inches above the surface of the soil.

The larva creates a cell below the girdle line for overwintering. In sunflower, the Dectes stem borer can only girdle approximately one-half of an inch outward from the center of the stalk. Therefore, larger-diameter stalks can prevent the girdling from being successful and reduce the risk of lodging.

Scouting for this pest should consist of monitoring fields during June and July for adult beetles.

Insecticides are not an effective management strategy for the Dectes stem borer. If adults are observed, monitor the field throughout the summer to determine the level of infestation. In the fall, scout lodged plants by splitting the stem and examining for evidence of feeding or the presence of the larva.

Insecticides are not effective at managing this pest due to the difficulty of timing an insecticide application to eliminate adult beetles. However, there are several other practices that can be implemented to reduce lodging and yield losses. These include:

• Reduced planting populations to increase the diameter of the stalks.
• Management of weeds within and around fields. Alternative hosts that are preferred for egg laying by the Dectes stem borer include cocklebur and giant ragweed.
• Harvest as early as possible to reduce the impact of lodging. This strategy may result in the increased need to dry the sunflowers for storage.
• Tillage to bury sunflower at least two-to-three inches deep. This strategy is not recommended for areas where no-till practices are employed.