What is in this article?:
- Rare insect problem cut 2012 yield for top South Carolina soybean grower
- 15-20 percent yield loss
- Limited insecticide effectiveness
• Irrigated beans were particularly hard hit because they were up and growing a little faster than the dryland beans.
• The big problem caused by the Dectes stem borer was more from lodging, caused by the weakened stems, than from actual damage to the beans, though that was bad enough.
A RARE PEST, Dectes stem borer, cut yields for award winning South Carolina soybean grower Jason Carter.
Limited insecticide effectiveness
Insecticide applications can provide only limited success in reducing the damage caused by soybean stem borers, due to the lifecycle and feeding habits of the insect.
An extended period of adult emergence makes timing of applications difficult, and once the larvae enter the plant, they are protected from insecticide treatments.
There is no evidence of soybean varieties having any resistance to soybean stem borers. Therefore, cultural control practices are the only effective means of reducing losses from the soybean stem borer.
Damage from Dectes stem borers, commonly called soybean stem borers, is frequently confused with nematode damage. Without testing for nematodes, growers could easily treat for a problem they don’t have with expensive soil fumigants.
Once the problem is identified in a field, the chance of it over-wintering and causing problems in future years has been high in Kentucky and other states farther west.
The best solution appears to be crop rotation. For Carter this is an easy solution, because he routinely rotates his soybeans with two years of corn.
However, in areas where soybeans are commonly grown and beetle populations are high, the value of crop rotation may be limited.
In most areas of the Southeast, soybean production is generally spread out over a wide geographic area, so crop rotation may be of more benefit than in the top soybean producing states.
Though a high percentage of grain crops in the Southeast are grown under no-till or strip-tillage systems, one of the recommended practices to manage Dectes stem borers is fall tillage.
Tillage that buries stubble 2-3 inches is most effective, but even light disking that tears up plant crowns and moves these away from the soil can be helpful.
Giant ragweed, which is on the increase in the Upper Southeast, common cocklebur and wild sunflowers are known to be alternative hosts of Dectes stem borers.
Clearly, maintaining a good weed control program and extending it to the perimeter of fields will be beneficial in managing these pests.
Carter switched last year from Maturity Group VII to Maturity Group V soybeans and that may have helped lessen the damage from this rare pest.The change from Group VII to Group V was to have an earlier pod fill and avoid pod fill in late September and early October, which is usually dry. “This also allows me to get my cover crop planted on time,” he says.
Compared to Maturity Group VII beans, he says Group V beans can typically be harvested the first two weeks in October, versus the first two weeks in November.
Typically, early planted and early harvested beans have been less susceptible to lodging problems caused by Dectes damage.
Another possible way to reduce the risk from these pests, should they become a bigger and more widespread problem in the Southeast, is to plant a trap crop along the border of the field.
Sunflowers in particular are a more desirable host for Dectes stem borers than soybeans.
If the insect shows up in soybeans this year, one of the keys to minimizing the damage, says Doug Johnson is to scout and watch soybeans in August and September.
Fields with extensive stalk tunneling of more than 50 percent should be harvested first to reduce risk of lodging.