We observed the emergence of Dectes stem borer adults (Figure 1) on June 10 in Cass County, with adult emergence at all monitoring sites in eastern Nebraska by June 19. Adults were first observed in North Platte on June 17 and reports from Grant, Nebraska were received on June 28. While Dectes soybean stem borer populations have previously been reported mostly in the eastern and south-central parts of the state, their presence in west-central and southwestern Nebraska seems to be increasing within the last several years.

With adult emergence comes calls regarding recommendations for treating to prevent larval infestations that could lead to stem lodging at harvest. Dr. Robert Wright, a recently retired faculty member in the UNL Department of Entomology, conducted research on Dectes stem borer for the better part of a decade. Research studies at the South Central Ag Lab found that peak stem borer adult activity was in early July in soybean fields. Although we are not tracking this information this year, we have noticed significant adult activity in soybean fields over the last seven to 10 days. At the Eastern Nebraska Research, Extension and Education Center, careful inspection of the upper four nodes of early planted soybean on June 20 showed scarring on leaf petiole associated with egg laying and small larvae present within those petioles (Figure 2).

Management of Dectes stem borer is challenging because of the inability to detect consisten yield losses from larval feeding in the stem without lodging. Studies conducted in multiple states from 2007 to 2011 found that with an 84% reduction in infested plants, yields only differed by 0.7 bu/acre from larval feeding within the stem. Significant yield losses can occur when the larvae prepare their overwintering chamber by girdling the base of the stem towards the end of the growing season. This girdling behavior doesn’t completely cut the plant but weakens it to a point where strong winds or storms can cause the plants to fall over.

Scouting in August for signs of Dectes stem borer can help prioritize fields that are at risk for an early harvest. Scouting at that point in the season is easier as you can watch for signs of a dead or wilting trifoliate on an otherwise healthy plant. Splitting the main stem can confirm the presence of absence of larvae. Fields with 50% or greater stems infested should be prioritized for early harvest.

The risk for lodging is greater on soybean that are planted early in the season to a shorter maturity group. In addition, 30-inch row spacings are a greater risk of lodging compared to narrower rows.

To date, there are no economic thresholds for Dectes stem borer. A great deal of research on Dectes has been conducted by entomologists at Kansas State University. (See the KSU Department of Entomology site.)

People planning on using an insecticide to control Dectes beetles should be aware that adult control is not highly correlated with levels of larval infestation at the end of the season, possibly due to the long period of activity and egg-laying of Dectes beetles. Multiple insecticide applications to control Dectes may not improve grower profits and may lead to late-season outbreaks of other pests such as spider mites or soybean aphids, by eliminating natural enemies that help suppress these arthropod pests.

See NebGuide G2082, Soybean Stem Borers in Nebraska for additional information on soybean stem borer.