Winning soybean yield contests is a common occurrence in recent years for Eastover, S.C., grower Jason Carter.

But winning last year’s contest with a rare and new-to-South Carolina insect handicap was an adventure he doesn’t want to try again.

Carter says he found the rare Dectes stem borer in his soybeans and finding out what was killing his beans proved to be about as frustrating as dealing with the problem.

“Our soybean crop looked outstanding from the start last year, and I was expecting to harvest 70-80 bushels per acre under irrigation and 60-70 dryland,” Carter says.

“Based on the season we were having and past production levels, those kinds of yields seemed reasonable,” he adds.

Carter has won the state soybean yield championship several times in his 17-year farming history, but says winning the yield contest in the irrigated category, with the problems he had with Dectes stem borers, was unexpected.

“Last year our dryland beans out-yielded our irrigated beans, and most of that was because of the problem we had with Dectes borers,” Carter says.

“The soybean crop looked so good last year, then I started seeing spots in the field that didn’t look right. At first I thought it was a nematode problem, but soil samples came back clean. Once we identified the problem, it was too late to do anything about it.”

Literally, no one knew the source of his problems. “I had my insecticide dealer come look at my beans, and Clemson Extension personnel, and finally Jess Easterling, a Monsanto sales representative from South Carolina, brought Mike Baker, a Monsanto Agronomist from North Carolina out to my farm, and they identified the stem borers,” Carter says.

15-20 percent yield loss

“I cut open the soybean stems in the trouble spots in the field and the whole inside of the stem was gone. It was amazing to me that the soybeans even survived, much less produced any beans. I have no doubt my yields were cut by 15-20 percent — maybe more in some spots,” he adds.

Irrigated beans were particularly hard hit because they were up and growing a little faster than the dryland beans. The big problem was more from lodging, caused by the weakened stems, than from actual damage to the beans, though that was bad enough, Carter adds.

University of Kentucky Entomologist Doug Johnson has likely had as much experience with Dectes stem borers as anyone in the South.

He says the insect showed up sporadically a decade or more ago in some parts of Kentucky, but has become more troublesome the past few years in traditionally high soybean producing areas.

How widespread the problem is in the Southeast is unclear, primarily because of the rare occurrence of the pest and the likelihood that its damage is attributed to other causes.

No one, including Johnson, is predicting the pest will become a major threat to soybeans in the Southeast, but it is one growers need to know about and scout for in their fields.

Johnson says the Dectes (or soybean) stem borer is a small (3/8-inch long beetle. The adult beetle is pale gray and has prominent black and gray banded antennae that are as long as or longer than the body.

Single eggs are deposited in cavities that female beetles chew into leaf petioles or stems. If eggs are laid in leaf petioles, larvae will feed in the petiole for several days before tunneling into the stem.

The trifoliate leaf and the petiole then wilts, dries up and drops from the plant.

The University of Kentucky entomologist says dead leaves can be observed in the canopy for a number of days. For Jason Carter this stage of development was the first time he noticed damage in his beans, though most of the damage was already done by the point.

Johnson says Dectes larvae are legless with small, brown heads. Their bodies are deeply segmented in an accordion-like fashion and conspicuously enlarged near the head with the body gradually tapering toward the rear end. Fully-grown larvae are creamy white and one half to five eighths inch long.

Research in several states indicates soybean stem borer over-winters as a larva in the base of hollow, girdled stems.

Carter says this concurs with his production practice. He has strip-tilled all his land for the past 15 years. In addition, the winter of 2011-2012 was unusually warm, which likely contributed to the problem.

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.

rroberson@farmpress.com