Lesser cornstalk borer found in North Carolina soybeans

When veteran North Carolina Crop Consultant Billy McLawhorn first scouted the soybeans on the Stilly Farms near Trenton, N.C., he thought the late-planted beans might make 30 bushels per acre.

Though drought and intense heat took some toll on the beans by the time he agreed to scout the beans for late season insects, chances of a good yield and good profit for the grower looked good.

Within a few weeks optimism turned to disaster, and despite 25 years of experience and a reputation for being one of the most knowledgeable consultants in the state, McLawhorn was not sure what the cause was for such sudden and dramatic damage to the crop.

“I had seen what I thought was charcoal rot, and initially it looked like the stand loss might create five percent or so yield loss. Within a few days, it was clear those same plants would lose 60 percent or more of their yield potential. Because of the timing of finding the initial damage, it took us a few days to get samples in to North Carolina State University for testing. When they told me the soybean plants were being killed by lesser cornstalk borers, I couldn’t believe it,” the veteran crop consultant recalls.

“I’ve worked in these fields and on farms in this area for a long, long time, and I’ve seen a few lessers from time to time, but never anything remotely close to this level — and not in soybeans,” he adds. Finding lesser cornstalk borer on cotton or peanut plants was more a novelty than frequent occurrence, according to McLawhorn.

Trying to make some sense of this unusual infestation, McLawhorn called on long-time North Carolina State Entomologist Jack Bacheler. After taking a thorough look at the 40 acre field of soybeans in late August, Bacheler called it one of his career-defining fields.

Tobacco was planted in the field in the spring, but an early June hailstorm wiped out most of the 40 acre field and the grower decided to plant beans. By the time insurance adjusters had given the okay to plant, it was July 12.

It was too late to re-plant tobacco, so the grower decided to plant soybeans — a logical choice considering the price of beans and the time of year. He planted Group VI beans in seven-inch rows. Up until mid-August the soybeans looked really good and the calf high beans looked fine, except for very sporadic stand loss.

Once McLawhorn got the lab results that showed cornstalk borer damage, plants in more and more areas of the field had died. After consultation with Bacheler, McLawhorn advised the grower to spray with a high rate of Lorsban, which appeared to help some, but proved only a temporary reprieve.

Within a few weeks about half the field was totally destroyed, with only sporadic low spots devoid of damage from the insects. Lessers burrowed into the root system of the soybean plants just below the soil line, robbing the plants of what little moisture was available. The damage was so severe the grower abandoned it, leaving hearty, fast growing Palmer pigweed to take over the field.

Lesser cornstalk borer over-winters as a larva or pupa. The moth emerges early in the spring and lay its eggs on the leaves or stems of plants. The small larva begins to feed on roots and leaves. Later the larva constructs underground silken tubes at the soil surface from which they feed on plants. It becomes fully grown in 2 to 3 weeks after the egg hatch, and pupates in silken cocoons under trash on the surface of the ground.

The moth emerges from these cocoons in 2 to 3 weeks.

Subsequent generations typically are not economically important, but in the North Carolina fields damage appears to have occurred from more than one generation of the insect.

Serious damage by this insect is usually limited to late-planted crops, primarily corn in the Southeast. Rainfall and irrigation will kill many larvae. Soil applied insecticides used at planting for corn rootworms or cutworms may control this pest. Where this insect has been a problem, careful inspection during the seedling stage is important, particularly in drought years.

Bacheler says under some unique conditions lesser cornstalk borer will attack and kill soybeans. On the Stilly farm, the soybeans were planted late on extremely sandy soil and the damage occurred during a 10-12 day stretch of drought in which daytime temperatures stayed at or above 100 degrees for extended periods of time.

For lesser cornstalk borer and soybeans, these conditions provided the perfect ingredients for an unusual perfect storm.

Though the Stilly farm was by far the most severely affected by lesser cornstalk borer, McLawhorn says they also found some scattered and less severe damage from these insects on double-crop beans planted in mid-June.

Lesser cornstalk borer are typically not a major pest, even in corn, in eastern North Carolina though McLawhorn says it’s not unusual to find a few from time to time.

Whether full season beans would be affected by lesser is not certain. With soybean acreage in the upper Southeast expected to increase in 2008, managing lesser cornstalk borer on late-planted beans on sandy soils may be an issue, especially if heat and drought conditions return for an encore performance next year.

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