Multi-Site Extension Study Provides New Data on Soybean Pests

September 5, 2025

Multi-Site Extension Study Provides New Data on Soybean Pests

Late Season Update from Nebraska Soybean Board-Funded Project on Soybean Gall Midge, Dectes Stem Borer and Plant Disease Impacts

By Thales Rodrigues da Silva - MSc Graduate Student, Justin McMechan - Crop Protection and Cropping Systems Specialist, Dylan Mangel - Extension Plant Pathologist, Wayne Ohnesorg - Extension Educator, Travis J. Prochaska - Extension Educator, Ron Seymour - Extension Educator, Aaron Nygren - Extension Educator, Matheus Ribeiro - Assistant Extension Educator, Ritika Lamichhane - Extension Educator, John Nelson - Extension Educator

Split image showing a Dectes stem borer adult on a soybean leaf and soybean stem damage from gall midge larvae near a field edge, highlighting pest pressure before plants mature.

Late-season scouting across nine Nebraska sites reveals shifting pressure from soybean gall midge, Dectes stem borer and key plant diseases. See the latest findings and what they mean for fields headed toward harvest.

Soybean gall midge, Dectes stem borer and plant diseases are key threats to soybean production in Nebraska. In early July and August, we provided updates across nine sites in the state regarding soybean gall midge larval abundance, Dectes stem borer adult activity, and plant diseases. 

In this article, we provided an update on data and observations of these pests collected Aug. 11-25 at nine sites in Nebraska.

Highlights

  • Soybean gall midge larvae counts declined at all sites, which is expected at this point in the season.
  • Wilted and dead plants from soybean gall midge increased at most sites.
  • Dectes stem borer punctures declined at all sites except Crete; larval counts increased at five of the nine sites.
  • Phytophthora was found only at two sites and in lower rates. 
  • Co-occurrences of these pests within the same plant occurred at four sites, even on sites with lower pest and disease pressure. 
Map of Nebraska with nine survey sites showing soybean gall midge larval counts per plant and percentages of wilted or dead soybean plants. Highest larval counts are in southeast Nebraska, with some fields showing over 60% plant loss.
Figure 1. Soybean gall midge larval counts per plant (grey circle) and the percentage of wilting and dead plants (yellow square) collected in mid- to late August for each of the nine sites.

Soybean Gall Midge 

  • Larvae were found in all sites except for Adams County, like the last report (Figure 1).
  • Southeast Nebraska sites had the highest larvae/plant counts, although they are very low when compared to July’s numbers.
  • At least 20% of the plants were wilted or dead in central and southeastern Nebraska (Figure 1).
  • Larval counts are expected to drop later in the season as harvest approaches.
Map of Nebraska with Dectes stem borer punctures per plant shown in yellow boxes and larval counts in gray circles for nine survey sites. Highest puncture rates are in south-central Nebraska, while larval numbers are elevated in several southeastern counties.
Figure 2. Dectes stem borer larval counts per plant (grey circle) and number of punctures from adult females (yellow square) collected in mid- to late-August for each of the nine sites.

Dectes Stem Borer

  • Punctures are small grooves made by the female Dectes beetle on the petiole for both feeding and egg-laying purposes.
  • Not all punctures result in eggs.
  • Adult activity was detected in all sites (Figure 2), following the pattern observed in the last report.
  • Southeast and south-central portions of the state had higher incidence of larvae averaging 50%, while north sites averaged 18%.
  • Most of the larvae moved from the petiole to the main stem. 
Map of Nebraska showing survey sites for Phytophthora in soybeans, with percentages of symptomatic plants marked in gray circles. Most sites show 0% incidence; two sites in central and southeastern Nebraska report 39% and 6%. An inset photo shows a soybean stem with dark lesions typical of Phytophthora infection.
Figure 3. Incidence of Phytophthora from observations in mid- to late August.

Plant Diseases

  • Phytophthora (Figure 3) was present at two sites.
  • Soybean sudden death syndrome, downy mildew, frogeye leaf spot and white mold were also found but in low frequencies.
  • The frequency of plants with Phytophthora did not increase at any of the sites.
Close-up of a split soybean stem showing orange soybean gall midge larvae and feeding damage within darkened stem galleries.
Figure 4. Soybean branch infested with soybean gall midge larvae, likely as a result of the leaf drop caused by Dectes stem borer larvae activity.

Co-Occurrence of Pests

  • In this collection, fewer soybean gall midge larvae were found associated with Phytophthora symptomatic plants.
  • Dectes and soybean gall midge were found in the same petiole, and in some cases, in the same stem gallery (Figure 4) on all sites except Adams and Colfax counties.
  • More soybean gall midge larvae were found in the Dectes galleries than July’s observations.

The next article will be on soybean yields and Dectes girdling, so watch for an article in October. 

For more information on soybean gall midge, visit the Soybean Gall Midge Alert Network. 

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