Figure 1. Dectes stem borer adult. Credit: A. Varenhorst
Dectes Stem Borer
For the last two years, we have been observing large populations of Dectes stem borer in South Dakota sunflowers. This spring, we noticed large numbers of the adults on volunteer sunflowers in field edges. This observation is consistent with ones we made in 2016, which ended up being a bad year in regards to Dectes stem borer. One of the concerns we have regarding this pest is that last year, it seemed to cause more lodging in areas that were drier. This year, much of Central South Dakota is under significant drought stress (Figure 2). An explanation for this increased lodging is that the Dectes stem borers began girdling drought stressed plants earlier in the season. Based on observations from last year, it is possible that the Dectes stem borer may be a more significant pest of sunflower in 2017. The dryland oilseed sunflower fields are at an increased risk due to higher planting populations.
Dectes stem borer adults are gray and slender. 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 plant 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 3).
Figure 3. Dectes stem borer larva. Credit: A. Varenhorst
Dectes stem borer adults emerge through June and July and will seek out host plants. Although the adults may be observed, management of this life stage is not successful 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 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 girdling 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. 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:
- 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 residue at least 2-3 inches deep.
- Reduced planting populations to increase the diameter of the stalks.