The Dectes stem borer is a small, long-horned beetle that attacks soybeans.
It has been a common occurrence in soybeans over the past several years and this year is no exception. The insect has been easy to find in fields throughout the county and they seem to prefer an occasional giant ragweed poking through the canopy.
The adult beetle is pale gray and about 3/8 inch long. It has unusually long, black, and gray banded antennae that are about twice as long as its body. Single eggs are deposited in cavities that female beetles chew into leaf petioles or stems.
If eggs are laid into the petiole, you may see random dry leaves that are often confused with sudden death syndrome. It is worth the time to pull a few of those plants to observe tunneling in the upper portion of the main stem.
The larvae hatching from the eggs deposited are legless with small, brown heads. Their bodies are deeply segmented in an accordion-like fashion.
The soybean stem borer overwinters as a larva in the base of hollow, girdled stems. University of Kentucky research indicates that larvae begin to pupate around the beginning of May.
Adults begin to emerge in June. Adults lay eggs during July and August, and some live into September. Larvae tunnel up and down the stems until late August or September. By the time plants mature, larvae will have tunneled down to the base near the soil line.
In preparation for overwintering, many larvae girdle the interior stem surface at a point near or just above the soil line. Plants break off easily where girdling occurred, allowing them to lodge.
Girdling generally occurs when plants are mature and ready for harvest. The yield losses caused by larval tunneling appear to be in the range of 7 to 12% but significant losses can occur when plants are girdled and lodging occurs before harvest. A timely harvest can be effective in reducing yield losses.
This pest is difficult to control because it spends most of its life inside plant stems, protected from standard chemical controls. Insecticides at the time of planting have been ineffective because of the length of time between planting and the emergence of beetles.
The only exposed stage of this pest is the adult, but adults are active for several weeks in the summer, making insecticide timing difficult.
Field trials have shown that insecticides applied when adults are present will temporarily reduce beetle numbers, but not significantly reduce stem borer. Trials of both foliar applications of non-systemic insecticides and systemic insecticides applied at planting or as foliar applications have not shown consistent ability to protect against yield loss.
Since resistant varieties and chemical controls for this insect are not available, cultural controls are the only means of reducing losses from the soybean stem borer.
Crop rotation and timely harvesting have been suggested as beneficial management strategies. These beetles are not strong flyers and when soybean production in an area is limited, crop rotation can limit the potential for damage.
Fields should be sampled before maturity for the presence of stem borer tunneling and live larvae by carefully splitting stems at several locations throughout the field. Fields with high percentages of infested stems should be harvested as soon as possible to avoid girdling and lodging.
In addition, researchers found that soil moisture and type can be a factor in reducing soybean stem borer infestations. Wet soils seemed to harbor fewer larvae than well-drained soils. Stubble in low, wet areas led to 50 to 70% larvae mortality, while well-drained areas experienced only 11 to 38% mortality.
Weed control may be another way to reduce stem borer problems. Stem borers are known to use wild sunflower, giant ragweed, and cocklebur as alternate hosts.
If you do not see soybean lodging at harvest, random plants retaining green leaves may be an indication of tunneling. The tunneling effect disrupts the natural plant physiology causing a disruption in natural plant senescence. If you do see this, examine a few of the plants to determine if dectes stem tunneling is the cause of the retained leaves.
Clint Hardy is the agricultural extension agent for the Daviess County Extension Office. He can be reached at 270-685-8480.