Dectes Stem Borer in Soybean

Crop Injury: Dectes stem borer (Dectes texanus texanus) can occasionally pose a problem from larval feeding in soybean stems, especially near ditches and field edges. Common weedy hosts are cocklebur and giant ragweed. Larval feeding has not been shown to cause enough physiological stress to reduce soybean yields. But late-season feeding can cause plant stems to become vulnerable to breaking or lodging, especially during high winds or heavy rains. Lodged plants can occasionally cause severe harvest losses.

Pest Description: Larvae have an orangish-red head and an accordion-like body extending up to ½ inch long. Adult beetles are long and gray and about ⅜ inch in length with long, dark antennae.

Life Cycle: Dectes stem borer larvae overwinter in the base of plant stems, emerging as adults throughout June until September. Following a 10 to14 day pre-ovipositional period, females can insert eggs into soybean petioles near the top of the plant over a period of several weeks. The larvae bore in the petiole toward the main stem and then feed up and down the main stem. In the fall, the larvae borer to the base of stems where they girdle the inside of the stem just above the soil line. They pack the tunnel with a frass plug. The larvae overwinter below the plug and then pupate in the spring to early summer. One generation occurs each year.

Scouting: Dectes stem borer scouting is best done from July through August when adults emerge. Use a sweep net to collect them and inspect for tunneling larvae in plant stems, especially in border rows. However, there are no thresholds developed for this insect.

Management: While foliar insecticides will kill adults, no insecticides are labeled for this insect and the extended period of adult emergence and the short residual of insecticides makes this option uneconomical. There are no chemicals available to control the larvae. Burial of infested stubble by plowing or disking has been shown to reduce larval survival. Dectes stem borer damage is most common along border rows near weedy hosts, so removal of weedy hosts may reduce populations. Since lodging is the primary problem from this pest, timely harvesting of infested fields should be a priority to reduce harvest losses.

Developed and reviewed by Fred Musser, Mississippi State University; Erin Hodgson, Iowa State University; and the Iowa State University Integrated Pest Management Program.