The Dectes stem borer, Dectes texanus LeConte (Coleoptera: Cerambycidae), is an insect pest native to North America. It was first reported infesting soybeans in Missouri in the 1960s. We observed the first emergence of Dectes stem borer adults (Figure 1) during the first week of July in a couple of locations in Missouri. Dectes stem borer has a single generation per year. It overwinters as mature larvae within the stems of its host plants, and the adult beetles begin emerging sometime in June or early July.
Figure 1 Dectes stem borer adult on soybeans.
After emerging, females take approximately 7 to 14 days before beginning oviposition. During oviposition, the female chews the petiole, leading to wilting of the petiole and leaf abscission. Larvae tunneling and girdling into stems weaken them, making the plant very susceptible to lodging, a great potential for yield losses. My lab is currently part of a multistate project funded by the North Central Soybean Research Program to evaluate Dectes stem borer activity, potential yield losses, and management strategies using insecticides.
Management considerations
There are currently no economic thresholds for Dectes stem borer. A few insecticides are registered for Dectes stem borer control. While these insecticides can kill the adults, applications are generally not recommended because it is extremely difficult to time the sprays properly.
Several cultural practices can be effectively employed to help reduce yield losses from Dectes stem borer:
- Effective control of weed host plants (e.g., cocklebur, giant ragweed) within and around fields is necessary to reduce the number of preferred egg-laying sites for the female beetles.
- Scout soybeans at several locations throughout the field before maturity for tunneling and live larvae. Fields with high percentages of infested stems should be harvested as soon as possible to help reduce losses associated with lodged plants.
- Avoid planting soybeans in fields adjacent to fields that were heavily infested the previous year. Rotate crops to reduce insect pressure.
- Fall tillage can increase overwintering mortality of Dectes stem borer larvae. Plant stubble should be buried at least 2 to 3 inches.
- Certain soybean varieties have shown reduced damage, but these soybean varieties are not commercially available.