Published on Monday, October 21, 2019
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A DEVELOPING PEST IN SOYBEANS
In recent years, the Dectes Stem Borer (Dectes texanus texanus) has become a regular pest in soybean fields throughout the lower Midwest. The Dectes Stem Borer (DSB) adult is a gray beetle that is approximately 3/8 in. in length and has long black antennae on its head. The DSB larvae, which is the stage in which it inflicts crop damage, is creamy white to yellowish in color with a reddish-brown head. The larvae are shaped like an accordion and are 1/16 in. long at hatching. They eventually grow to a length between 5/8 in. to 1 in.
Adult DSB typically emerge between July and August and begin to mate approximately five days after plant emergence. In soybeans, the female chews a small hole in the stem; typically, at the base of a leaf petiole. She then lays an egg in that hole. Unfortunately, the females can live up to 8 weeks and lay upwards of 50 eggs. After the larvae hatch, they feed on the outer stem for several days before they begin to bore into the main stem. The petiole at the entry site typically wilts and falls off. This wilted petiole has proved to be an effective tool for identifying the presence of DSB within a field and understanding the severity of the infestation. Once in the main stem, the DSB tends to tunnel up and down within the plant, feeding on the pith and reducing the plant’s ability to translocate water and nutrients. The DSB only produces one generation per year.
In late summer, the DSB travels to the base of the plant where it hollows out an area of the stem approximately 2 in. above the soil surface. As the DSB prepares to overwinter in the hollowed-out stem, it plugs the stem with frass to protect itself from the winter elements. At this point, the soybean stem has been girdled by the DSB, and the soybean plant is vulnerable to lodging.
There are no insecticides specifically labeled for DSB, and insecticide applications are not generally recommended because application timing is incredibly difficult. DSB adult emergence occurs over several weeks, which would likely require several applications in order to be effective. Research from the University of Nebraska has found that while insecticides could decrease adult populations by up to 70%, there did not appear to be any impact on final stalk tunneling and the yield differences between the treated and untreated controls were negligible.
Some cultural practices can mitigate DSB’s impact; however, there is no totally effective treatment.
Author: Sean Nettleton
Categories: Agronomy, Agronomy Talk
Tags: soybean, Agronomy, Agronomy Talk, dectes stem borer, soybean pests