It seems like every year there is a new or emerging insect pest in northeast Nebraska. This year that insect is the soybean stem borer, sometimes called the Dectes stem borer. In the past this insect has not been of concern for northeast Nebraska farmers, but during the last few years it has reached economically damaging levels in a few fields as far north as Knox County.
The soybean stem borer is a native species that is widely distributed across North America east of the Rocky Mountains. Adults are elongate, gray beetles with antennae longer than the body, and about 1/4 to 1/2 inches long. The larvae are found in soybean stems and are legless, creamy white with a strongly segmented body, enlarged near the head and gradually tapering toward the rear, with brown heads, and about 1/2 to 5/8 inches long when mature. Its main hosts are cultivated sunflower, soybean, giant ragweed, and cocklebur.
In mid-summer the females lay eggs in the upper soybean leaf petioles and the larvae gradually borer into and up and down the stem. It is not the “boring” that is really the problem, but the girdling of the stem base late in the season. If enough of the plants are infested, a strong wind causes the stems to break about 2 to 4 inches from the ground, so plants fall over (lodge) and are not harvestable.
In the past this insect only reached pest levels in soybean south of Nebraska, but over the last 20 years it has gradually reached damaging levels in the north central U.S. This could be because of climate change, changes in agronomic practices, or some other combination of factors, but whatever the reason, we now must watch for it in northeast Nebraska.
The first indication you may have soybean stem borer in your soybean field is leaf flagging, which is a single wilting or brown leaf on an otherwise healthy plant. This is caused by the young larvae boring through the leaf petiole and into the main stem.
Unfortunately, there are few consistently effective management options for soybean stem borer. Adults emerge and lay eggs over a long period, so it is hard to target a good time to treat them with insecticides. Larvae are in the stem, so it is hard to reach them with insecticides. However, there are several cultural practices can help reduce losses.
Weed control to reduce alternate hosts, such as ragweed and cocklebur, can help reduce soybean stem borer. Burying borer-infested stubble 2 to 3 inches after harvest can reduce soybean stem borer the following year. Adult soybean stem borers are not strong fliers, so crop rotation may reduce damage in areas where soybean acreage is limited. Early planted, short-season soybean varieties are more likely to suffer harvest loss from lodging, so longer season varieties that mature later may allow more time to harvest before lodging occurs. If one has a history of injury, or observes injury symptoms this year, they should carefully monitor these fields August through September. If extensive stalk tunneling occurs, the fields are at risk for lodging and should be targeted to harvest first to minimize harvest loss due to lodging.
The soybean stem borer is still at low levels in most of northeast Nebraska, but bears watching. Additional information on scouting and management for soybean stem borer can be found in the University of Nebraska-Lincoln Extension CropWatch Newsletter at https://cropwatch.unl.edu/ .
-Tom Hunt, UNL Professor of Entomology stationed at the Haskell Ag Lab near Concord, NE