Research Highlights

To Spray or Not to Spray for Dectes Stem Borer?

By Laura Temple

As an adult beetle, the Dectes stem borer causes no notable damage in soybeans. However, its larvae hatch inside soybean leaf petioles and eat tissue inside the stems, causing significant damage. 

A sporadic pest, Dectes stem borer can be found in soybean fields from the eastern shore of Maryland to the panhandle of Texas, says Alan Leslie, Ph.D., an agriculture and food systems educator with University of Maryland Extension. With checkoff support from the Maryland Soybean Board, he led an in-depth, three-year study of the pest to develop recommendations to better monitor and manage it.

“Soybean producers on the eastern shore of Maryland reported lodging due to Dectes stem borer,” Leslie explains. “They wanted to know if and when they should spray. However, very little data existed to monitor populations or establish an economic threshold for control.”

Based on farmer input, his team took a detailed look at the biology and life cycle of the pest, the yield loss it causes and a wide range of potential control options. They observed characteristics that make Dectes stem borer a frustrating, challenging pest to manage, and the team summarized their findings in a fact sheet for farmers. 

“Our research shows that insecticides are not the best answer for this pest,” he says. “Cultural control, especially crop rotation, tends to be more effective and economical.” 

Life Cycle

The Dectes stem borer produces just one generation per year. In addition to soybeans, host plants include sunflower, cocklebur, ragweed species and others. In Maryland, adult beetles emerge over the span of a month, starting in late June or early July and finishing in early August.

“Emergence peaks in mid-July, but based on our research, that peak isn’t significant enough to warrant an insecticide treatment,” Leslie says. “Contact insecticides control emerged beetles, but after they emerge, they lay eggs in soybean petioles. Inside the plant, they are not vulnerable to insecticide.”

Those eggs hatch within five to seven days. Then the larvae feed on the petioles and work their way toward the main stem. Leslie notes that systemic insecticides don’t control the larvae because they don’t feed on vascular tissue.

“As the soybean plant matures and dries down, the larvae move toward the base of the stem, girdling them,” he continues. “Those weak stems can easily lodge just before harvest, causing direct yield loss.”

The larvae overwinter in soybean crop stubble, emerging the following summer.

Intensive measurement of individual soybean plants infested with Dectes stem borer found that on average, it causes 4 to 5% yield loss per infested plant. 

“We counted and measured pods and beans by hand to quantify damage and found that the only impact of Dectes stem borer is that the seed size is a bit smaller,” Leslie explains.

He adds that farmers may see as much as 20% yield loss from lodging under severe infestations. However, infestation rates average only about 25% of the plants in a field. If farmers can harvest an infested field before the plants lodge, yield loss will be significantly limited.

Management Options

This life cycle limits the effectiveness of traditional insecticides for Dectes stem borer control. Leslie says multiple insecticide applications would be required while adults emerge, and that isn’t economical.

He adds that monitoring populations is difficult. The team found that sweep net counts were inconsistent and hard to correlate with actual populations because of the beetle’s extended emergence period. Plus, the pest is hard to scout once it moves inside soybean plants. These factors make setting and using an economic threshold for treatment impractical.

“Our project included testing pheromone traps as a way to monitor populations,” Leslie says. “While this approach works for moths and other pests, it didn’t prove to be very effective for this beetle.”

His research explored a wide variety of chemical and cultural control options, besides harvesting before plants lodge.

“Crop rotation is key to reducing Dectes stem borer populations,” Leslie reports. “Many of the fields we monitored during this study were continuous soybeans. Rotating to a non-host crop forces adult beetles to move to other hosts or starve when they emerge.”

Tillage buries the soybean stubble where larvae overwinter, reducing beetle emergence the following year. However, Leslie says this practice doesn’t fit well in Maryland production systems, which are primarily no-till. 

The team also studied whether parasitic wasps or flies have potential to control the larvae. Unfortunately, the team didn’t find evidence of larvae mortality due to parasitic predators. 

“Anecdotal evidence suggests that maturity group and drought may limit feeding, but these factors have not yet been studied,” he adds. 

Additional Resources:

Soybean Stem Borer: https://soybeanresearchinfo.com/soybean-pest/soybean-stem-borer/

Dectes Stem Borer Management in Soybeans: https://extension.umd.edu/resource/dectes-stem-borer-management-soybeans-fs-1196

Other articles on SRIN: https://soybeanresearchinfo.com/?s=Dectes+stem+borer

Published: Oct 31, 2022