The Dectes stem borer (Dectes texanus texanus) is a small, long-horned beetle whose larvae attack soybean. Also known as the soybean stem borer, it is a native insect species in North America east of the Rocky Mountains. Although Dectes stem borer damage to soybean has been reported since the 1970s, it has not generally been considered a major pest. In recent years however, reports of damage have increased. Dectes stem borer damage to soybean has been reported most frequently in areas across Texas, Kansas and Nebraska, along the Mississippi and Ohio Rivers, and along the Atlantic coast (Figure 1).
Figure 1. Areas of frequent Dectes stem borer infestation in soybean. (Adapted from Buschman and Sloderbeck, 2007.)
Appearance and Life Cycle
Dectes stem borer larvae are legless with small, brown heads. Their bodies are deeply segmented with an accordion-like appearance, and enlarged near the head with the body gradually tapering toward the rear end (Figure 2). Newly hatched larvae are creamy white and less than 1/16 inch long, but darken as they mature to a length of 1/2 to 5/8 inch long. Adult beetles are about 3/8 inch long and pale gray, with long, black and gray banded antennae (Figure 3). Eggs are yellowish, shiny, and elongated, and darken to an amber color prior to hatching.
Figure 2. A Dectes stem borer larva that has tunneled within the stem of a soybean plant.
Dectes stem borer goes through one generation per year. The insects overwinter as mature larvae within the stem of their host plants. Larvae pupate early in the summer. The pupal stage lasts eight to 10 days and adult beetles begin emerging in late June or early July.
Mating takes place approximately five days after adults emerge from the pupal stage. The female beetle chews a hole in the soybean leaf petiole or stem, where it lays a single egg. After hatching, the larva feeds on the outer stem and then bores into the main stem.
Figure 3. An adult Dectes stem borer beetle. (Photo: Michael L. Boyd, University Of Missouri–Columbia)
Larvae are cannibalistic, with only one survivor per plant. But by the time a soybean plant reaches maturity, this larva will have tunneled down to the base of the plant, where it will overwinter as a mature larva. To create a protective cell for overwintering, the larva girdles the interior of the stem at a point near or just above the soil line and plugs the stem with its frass (Figure 4).
The larva feeds on the pith tissue as it tunnels within the main stem, causing damage that can reduce the plant’s ability to translocate water and soil nutrients to developing pods and seeds. Yield losses of 7 to 12% caused by larval tunneling have been reported (Johnson, 2006). Soybean yield losses can also result from lodging caused by the girdling of stems prior to harvest. Instances of 30 to 40% lodging due to Dectes stem borer damage have been reported (Patrick et al.). Significant damage resulting in soybean yield loss has traditionally been sporadic; however, the pest status of Dectes stem borer appears to have increased in recent years (Buschman and Sloderbeck, 2007).
Figure 4. Soybean stem girdling caused by Dectes stem borer. (Photo: Scott Stewart, University of Tennessee)
Although the specific reason for this increase is unknown, it may be due to increased adoption of no-till, which leaves the habitat of overwintering larvae undisturbed. Another possible explanation is the recent trend of warmer winter temperatures in some areas, which may allow greater numbers of larvae to survive the winter. Increased reports of Dectes stem borer activity may also indicate more people actively looking for the pest. Since the majority of damage occurs after the plants have matured and not all injured plants will lodge, occurrences of Dectes stem borer damage may have gone largely overlooked in years past.
Resistant soybean varieties and chemical controls for Dectes stem borer are not currently available; therefore, cultural control methods are the only means of reducing damage and yield loss. Dectes stem borer beetles are not strong flyers, and are limited in their ability to move among fields. For this reason, crop rotation can reduce the potential for damage in areas with limited soybean acreage. The potential benefits of rotation are reduced in areas with extensive soybean production or with sunflower production, as this crop also serves as a host for Dectes stem borer.
Fall tillage can help manage Dectes stem borer populations by reducing winter survival. Burying stubble two to three inches deep in the fall by discing or bedding can reduce winter survival by as much as 60 to 70 percent. Even in no-till, a light discing to tear the plant crowns from the soil may provide some benefit. Solid contact with soil provides moderation of winter temperatures, which favors winter survival. Removal of the crown from this environment may increase larval death.
Effective weed management can also help to control Dectes stem borer. Weed species such as cocklebur, giant ragweed, and common sunflower can serve as alternate hosts. Control of these species in and around soybean fields will reduce the number of favorable egg-laying sites available.
Fields should be sampled before maturity for the presence of stem borer tunneling and live larvae by splitting stems at several locations throughout the field. Fields with high percentages of infested stems should be harvested as soon as possible to avoid yield loss due to girdling and lodging.