Figure 1. Adult soybean stem borer, Dectes texanus texanus

Figure 2. Larval soybean stem borer, Dectes taxanus texanus

We have been seeing emergence of Dectes stem borers (Figure 1) in south central Nebraska for the last two weeks. I have been getting calls about recommendations for treating adults to prevent larval infestations, which can lead to stem lodging at harvest (Figure 2).

Previous research has not established an economic threshold for this insect. A great deal of research on Dectes has been conducted by entomologists at Kansas State University. (See http://entomology.k-state.edu/extension/insect-information/crop-pests/soybeans/sbsb/.) 

An interesting study by Phil Sloderbeck and Larry Buschmann at KSU's Southwest Research and Extension Center, Garden City, Kan. was published in the Journal of Insect Science. They evaluated application of lambda-cyhalothrin (the active ingredient of pyrethroid insecticides such as Warrior) in nine replicated on-farm studies over a three-year period.  Applications were timed to correspond with the peak density of Dectes in soybeans and a second treatment was made 14 days later. They monitored adult densities and end of season percent of plants with larval tunneling.

The results varied from year to year. (See Figures 3-5.) For example, in 2001 the first application resulted in 70% reduction in adult density and the second application had 0% reduction; two applications had 74% reduction in adult density. However, adult control levels did not impact final end of season stalk tunneling, and there was no significant difference between the untreated control and the insecticide treatments. In 2002, two insecticide applications resulted in 89% reduction in adult populations but only 59% reduction in larval infestations at the end of the season. In 2003, the second application by itself provided 89% adult control and 56% reduction in larval infestations, whereas two applications provided 98% adult control and 75% reduction in larval infestations.

I have been doing research on soybean stem borer in Nebraska since 2013 with support from the Nebraska Soybean Board. Seasonal occurrence of Dectes in Nebraska soybeans occurs later than in Kansas, so proper timing would be different, and would vary from year to year. Last year in studies at UNL's South Central Ag Lab near Clay Center, peak Dectes emergence occurred in early July with emergence continuing through July. Studies in commercial soybean fields in south central Nebraska showed peak numbers (more than three beetles per 20 sweeps) around July 8-15, with adults detectable through early August. Seasonal occurrence appears to be somewhat later this year than last in south central Nebraska.

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 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 which help suppress these arthropod pests.

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

Bob Wright
Extension Entomologist

 

Figure 3 (left). Results for 2001. B. Season-long total beetle numbers in the four treatments and percentage reduction relative to the control. C. Total percentage of plants tunneled in the four treatments and percentage reduction relative to the control. (Source: Kansas State University)

Figure 4. (bottom). Results for 2002. B. Season-long beetle numbers in the four treatments and the percentage reduction relative to the control. C. Total percentage of plants tunneled in the four treatments and percentage reduction relative to the control. (Source: Kansas State University)

Figure 5. Results for 2003. B. Season-long beetle numbers in the four treatments and the percentage reduction relative to the control. C. Total percentage of plants tunneled in the four treatments and percentage reduction relative to the control.

Journal of Insect Science: 2011. Vol. 11 | Article 49. Aerial insecticide treatments for management of Dectes stem borer, Dectes texanus, in soybean http://www.insectscience.org/11.49/ P. E. Sloderbeck and L.L. Buschman, Kansas State University, Southwest Research-Extension Center and Department of Entomology, Garden City, KS. 67846.