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Found in some soybeans: Stem borer can be pest in Louisiana

I've been getting phone calls in recent weeks about suspicious-looking soybean plants — wilting or pale green off-color tri-foliates interspersed in fields. The trifoliate looks as if it is diseased but is not characteristic of anything we normally deal with.

After we split leaf petioles and stems, we found small larvae that resemble the sugarcane borer except that they have no spots. They are not borers but beetle larvae called soybean stem borers.

Unlike the Lepidoptera species that includes the sugarcane, southwestern and European borers, this beetle is officially taxed as Dectes texanus.

So far, Rapides and Avoyelles are the only parishes where I have found them.

The beetles are about three-eighths inch long as adults and lay their eggs on the petioles on the leaves of soybean plants.

Once hatched, the larvae bore into the petioles and eventually get into the main stem of the soybean plant. As they bore into the petiole headed to the main stem, a reddish residue is left behind.

Once in the main stem, they will bore down to the soil line and cause the soybean plant to lodge, especially if the soybean has good yield potential.

As to chemical control, producers have few options for this borer.

Scouting is difficult because of a number of factors. The adult phase is short, and the flight pattern comes in early August when the crop is growing rapidly.

The key symptom for detecting this borer is a pale or off-type single trifoliate. This is an external symptom indicative of what the borer is doing inside the petiole.

Matt Baur and Jack Baldwin, both LSU AgCenter entomologists, have seen these borers before. They usually are not really detrimental to overall yield. Baur and Baldwin emphasized that the borer is an occasional pest of soybeans. You should not expect to see them every year, especially if you practice crop rotation.

Baldwin said, “If populations are severe enough, there could be yield loss, but the numbers usually do not get high enough. From a scouting standpoint, the off-color trifoliate is the best bet to find the larvae, but they cannot be controlled at that stage.”

Baur mentioned that the distribution of the borers is widespread but definitely more common in the mid-part of the state. “Economic impact depends on when the insect attacks. If later in the season, there should be little to no damage; earlier heavier infestations could cause some slight loss,” he said.

It seems as if good cultural practices, including disking fields after harvest, then allowing them to sit over the fall and winter, will be the most effective methods to combat the insects. Crop rotation and good weed control during the fall and early spring will help. Weed control is important because cocklebur and ragweed can serve as alternate hosts.

David Y. Lanclos is the soybean, corn and grain sorghum specialist at LSU AgCenter. [email protected].

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