Dectes stem borer is making its presence known
click image to zoom We have been receiving numerous phone calls about Dectes stem borer in soybean this year. We began seeing large numbers of adults in soybean fields as early as mid-June this year and populations appeared to peak in early- to mid-July. Now we are beginning to receive reports of severe infestations in maturing Group IV and early V soybean fields. For those unfamiliar with Dectes texanus texanus, it is a native species and there is one to two generations of this pest for year. The adult is a buff gray colored beetle (Fig. 1) of the cerambycid family of beetles which are commonly referred to as longhorned borers because their antennae are often longer than the body as in the case of Dectes. The larva, which is the stage that damages soybeans, is an off-white color and is usually found in petioles or the main stem of the soybean plant usually in the months of July through September (Fig 2 and 3).
Dectes is found all through the Mid-South, the Eastern Seaboard and the Midwest. It’s host range includes cocklebur, giant ragweed, and sunflower (preferred host). Most of the literature on this pest indicates that it is not a major pest and that damage is usually isolated to field edges and ditch banks where wild hosts are found. It states in the ESA Soybean Insect Handbook that the importance of the pest may increase with increased no-till acreages. Certainly in recent years with the advent of Roundup Ready soybeans and increasing reduced and no-till acreage, it appears that the occurrence of this pest has increased significantly. In recent years we have observed an extremely high larval population, where parts of the field were exhibiting symptomology associated with high infestation levels. Upon closer examination of these lighter colored areas, we observed leaf petioles dying as a result of larval tunneling. Many of the leaf petioles were dying from infestation and upon removing them we could see where the larva had entered the main stem (Fig. 4) and had begun to bore in the main stem (Fig. 5).
click image to zoom According to the literature, adults lay their eggs in the upper petioles of the plant in July and August—this year I think it may have begun in mid- to late-June. The larvae then begin to feed on the petiole and move quickly into the main stem. The damage shown in Figs. 4 and 5 shows the damage to the leaf and petiole resulting in the petioles and leaves dying. The trifoliate and damaged petiole where the small larva is tunneling in the petiole will cause the leaf to wilt, turn chlorotic and it will eventually cause the petiole to drop from the plant. Obviously this could easily be misdiagnosed as a disease or nutrient imbalance. Once the larva hits the main stem, it will tunnel up or down the main stem. As the larva develops it will eventually move down the main stem and girdle the inside of the plant 2-3 inches above the soil. This can result in lodging (Fig. 6) during high winds or heavy rain. Examination of lodged plants shows the larvae in the stubble of broken and lodged plants (Fig. 7).