Every year, there is a risk that sunflower in South Dakota will be infested by Dectes stem borer larvae. At this point in the season, adults are active, and females have likely been laying eggs in sunflower plants.
During drier years, the impact of Dectes stem borer can be much more significant. However, with the wetter conditions of 2019 the number of Dectes infested plants seemed to be lower. So, what about Dectes populations in 2020?
According to the U.S. Drought Monitor (Figure 1), there are still some sunflower production areas in South Dakota that are experiencing abnormally dry (yellow), moderate drought (light orange) and severe drought (orange). In the drier areas, Dectes stem borers could become an issue and cause stem lodging later in the season.
Dectes stem borer adults are slender, gray beetles. They are approximately 3/8 of an inch in length with antennae that are as long as the body and have segments that alternate between black and gray (Figure 2).
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 ½ to 5/8 of an inch in length (Figure 3).
Scouting and Management
Dectes stem borer adults emerge in June-July and will seek out host plants. Although the adults may be observed, management of adults 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 larva tunnels through the stem and feeds on the pith tissue. Although this results in a reduction of water and nutrient movement within the plant, no yield reductions are observed from this activity in sunflower. In late summer, the 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 girdled area for overwintering. In sunflower, the Dectes stem borer can only girdle approximately ½ of an inch outward from the center of the stalk.
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 due to the difficulty of timing an insecticide application to eliminate adult beetles. 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. However, there are several other practices that can be implemented to reduce lodging and yield losses. These include:
- 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.
- Early harvest to reduce the impact of lodging.
- Reduced planting populations to increase the diameter of the stalks.