Protect Soybeans from Dectes Stem Borer

By Scout Nelson

Dectes stem borer adults have been detected in soybean fields across southeastern South Dakota last week. The insect is an important soybean pest because its larvae feed inside the main stem, reducing plant health and lowering yields. Research shows that feeding damage can reduce soybean yields by as much as 17%, while severe infestations may also increase the risk of plants falling over before harvest.

Adam Varenhorst, Associate Professor & SDSU Extension Field Crop Entomologist, along with Philip Rozeboom, Patrick Wagner, and Brad McManus, reported that Dectes stem borer adults are now active in soybean fields. They noted that infestations ranging from 70% to 100% of sampled plants have been observed in some southeastern South Dakota fields during recent years.

Adult Dectes stem borers are gray beetles with long antennae. Their cream-colored larvae tunnel through the inside of soybean stems, feeding on the plant tissue throughout the growing season. As soybean plants mature and begin drying before harvest, the larvae move toward the base of the stem and cut or girdle it. This damage weakens the plant and often causes lodging, making harvest more difficult and increasing crop losses.

Adult beetles remain active through August, with the highest activity usually occurring during the middle to late part of July. Soybean fields that experienced infestations during previous growing seasons should be scouted carefully during this period to identify new infestations early.

Recent SDSU research has shown that properly timed insecticide applications can protect soybean yields. Results from 2024 trials showed that a single application of Hero insecticide increased yields by 9.1 bushels per acre compared to untreated soybeans. Two applications of Vantacor insecticide provided a yield increase of 7.8 bushels per acre.

Current recommendations suggest applying one Hero treatment during the third week of July. When two applications are needed, Vantacor should be applied during the first or second week of July, followed by Hero about two weeks later. Growers should always read and follow insecticide labels for approved rates, timing, and application restrictions.

Photo Credit: pexels-erik-karits