It is expanding its range in Nebraska: the Dectes stem borer (DSB), a long-horned beetle native to the central part of the U.S., where its larvae attack soybeans. Long considered only a minor pest in soybeans, DSB has been slowly extending its range on soybeans into Nebraska from north-central Kansas.
First documented as a potential soybean pest in a field in Nuckolls County in 2000, DSB has been reported across soybean-producing counties in eastern and south-central Nebraska. But the past few years have shown an extension of that range into southwest Nebraska.
Last summer, DSB adults emerged on June 10 in Cass County and at all monitoring sites in the eastern part of the state by June 19. Adults were observed in North Platte on June 17 and at Grant by June 28.
Here’s the problem
But that impact on yield from DSB during this period is debatable. The more worrisome effects most likely come when the larvae prepare an overwintering “chamber” in the base of the stem at the end of the growing season. This weakening of the stem causes lodging before harvest.
At a recent University of Nebraska Extension Crop Production Clinic in Norfolk, crop protection and cropping systems specialist Justin McMechan told producers that in 2024, south-central Nebraska in Clay County, for instance, had some field borders where 100% of soybean plants were infested. There were plants where there were multiple borers in a single plant.
“The only good thing about that is that they are highly cannibalistic,” McMechan said. “And the biggest one wins.”
There were regions of the state where more DSB were found in fields in 2024. “Infestations were pretty bad in south-central Nebraska from my tour of the state,” he said, “and I know there was an increase in presence of DSB in northeast Nebraska, compared to previous years.”
Heavy infestations, even if they are confined to field borders, slow harvest considerably and lower yields because it is so difficult to pick up lodged stems with a combine header. Early-planted, early-maturity soybeans are the worst-case scenario for DSB infestations, McMechan said. “These are most likely to see a lodging situation,” he explained.
Scouting in early August for common signs of DSB, like a dead or wilting trifoliate on an otherwise healthy plant, helps farmers monitor where infestations are the worst. Splitting the main stem of a plant will confirm the presence of larvae, he said. Fields where about 50% or greater of the stems are infested definitely should be harvested first, McMechan added.
Where there are DSB in the field, slow harvest speed down and try to pick up the plants that are there and salvage as many plants as possible, he advised, perhaps employing metal lift guards on the combine header to assist in the process.
Trap crop
Studies in Kansas have shown that planting a trap crop such as sunflowers along field borders can drastically reduce DSB numbers in the adjacent soybean fields. “They prefer sunflowers over soybeans,” he said. “But the question for farmers is what to do with the sunflowers at the end of the season. Destroying the sunflowers would have to be part of the equation,” otherwise there would be concerns about the sunflower stubble harboring an increased overwintering population.
Even a single row of sunflowers may help, said McMechan, citing a Delaware study that found more than 30 DSB larvae per plant over a nine-day period in July.
Rotating crops is one way to avoid DSB, but McMechan explained that ragweed and cocklebur can serve as hosts if they remain in the field, and the borers will move from a field without a host crop to the nearest field with a host.
Foliar insecticide treatments to kill the adults are difficult to time because spraying may be needed multiple times during the season. It isn’t economically feasible for many producers.
Learn more about DSB in Nebraska at cropwatch.unl.edu.