By Adam Varenhorst, Extension Field Crop Entomologist
In South Dakota, one of the insects that is commonly observed in sunflower is the Dectes stem borer larva. At this point in the season, adults are beginning to become active with females 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, there is the possibility that the Dectes stem borer might not cause nearly as much of an issue for sunflower farmers.
Dectes stem borer adults are slender, gray beetles. They are approximately 3/8” in length with antennae that are as long as the body and have segments that alternate between black and gray (Figure 1). 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” in length (Figure 2).
Lifecycle and Behavior
Dectes stem borer adults emerge in June-July and will seek out host plants. Although the adults may be observed, management of that life stage 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 feed 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 the 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” outwards 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.
- Tillage to bury sunflower at least 2-3 inches deep.
- Reduced planting populations to increase the diameter of the stalks.