After splitting leaf petioles and stems, small larvae were found that resemble the sugarcane borer except that they have no spots. They turned out to be 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 them.
The beetles are about three-eights-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 controlling this borer. Scouting is difficult because of a number of factors. The adult phase is short, and the flight pattern coincides with early August when the crop is growing rapidly. The key symptomology for the detection of 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.
Dr. Matt Baur and Dr. Jack Baldwin, both LSU AgCenter entomologists, both told me they have seen these borers before and that they are usually not really detrimental to overall yield. They emphasized that this borer is an occasional pest of soybeans, and you really should not expect to see them every year especially if you practice crop rotation.
Dr. Baldwin mentioned that “if populations are severe enough, there could be yield loss but usually the numbers really 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.” Dr. Baur mentioned that “the distribution of the borers is widespread but definitely more common in the mid-part of the state. Also, 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.”
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 these insects. Crop rotation in addition to practicing good weed control during the fall and early spring will help to combat this insect. Weed control is important because cocklebur and ragweed can serve as alternate hosts.
David Lanclos is Extension corn and soybean specialist with the LSU AgCenter.