There’s a new(er) soybean pest in town that’s receiving some attention from southern Illinois soybean growers. Feeding caused by the Dectes stem borer contributed to stalk breakage and lodging found in area soybean fields around harvest, according to Kelly Estes, Illinois Natural History Survey agricultural pest survey coordinator.

Grayish-blue in color, the Dectes stem borer is a small, longhorned beetle, approximately ½ inch in length with antennae that are usually as long, or longer than the length of the beetle. Adults are present late June through August. During this time, adult females lay eggs in the leaf petiole of soybean plants.

After hatching, grubs feed and tunnel within the length of the leaf petiole, making their way to the main stem. Once reaching the main stem, they feed on the pith, and will settle just below the soil line where it girdles the stem and overwinters. Larvae will complete pupation in the spring.

The Dectes stem borer has one generation each year in Illinois, noted Nick Seiter, University of Illinois research assistant professor. The first sign of infestation is usually the appearance of yellowed, wilting leaves that are being shed from the plant. These dying petioles do not lead to meaningful levels of defoliation but are a sign that an infestation has begun.

Splitting the stems of infested plants will often reveal the larvae burrowing through the stem. Surprisingly, this burrowing usually causes little to no yield loss. Girdling of the stem and subsequent lodging is the more serious concern, as stems often break off at the site of the girdle. Lodging generally occurs when soybeans remain in the field long after reaching maturity. When harvest is timely and lodging does not occur, even heavily infested fields typically do not exhibit obvious symptoms or economic damage.

While the Dectes stem borer has been present in Illinois for many years, it is only recently that injury in soybeans has garnered the attention of soybean growers. Delayed harvests in much of Southern Illinois in 2018 resulted in lodging in many Dectes-infested fields, putting this pest squarely on the radar of many farmers.

The most effective management tactic is to harvest infested fields before lodging occurs. Prioritizing known infested fields will help reduce the potential for lodging caused by stem girdling.

For more information on Dectes stem borer biology and management, view a new fact sheet.