Tunneling from Kansas to the Carolinas, the soybean stem borer is really bugging growers and crop scientists who have yet to find an adequate method of controlling the pest.
The insect continues to bore through bean stems while researchers and extension specialists search for a chemical, biological or cultural practice that will end borer damage without throwing a wrench in production practices.
“This is an insect that's really puzzling for us,” says Calvin Trostle, a Texas A&M University extension agronomist who monitors stem borer populations and damage in soybeans and sunflower. “It's frustrating for growers and for those of us working to find more efficient crop-production systems.”
Many counties in the western two-thirds of Kansas continue to see stem borer infestations. The insects are also seen in Arkansas, Louisiana, Alabama, Missouri, Tennessee and North Carolina. They overwinter as mature larvae within tunnels in the stubble of soybeans, sunflower, ragweed, cocklebur and other weeds. Pupation occurs in late spring, and adult beetles emerge over several weeks in early summer.
“The adult beetle is pale gray and about ⅜” long with antennae that are longer than the body,” says Phillip Sloderbeck. A Kansas State University extension entomologist, Sloderbeck has received Kansas soybean checkoff funding to study the pest. “Their eggs are deposited singly in cavities that female beetles chew into leaf petioles and stems.”
Problems for plants begin when eggs hatch. Larvae, which are ½-⅝” long, tunnel into the stalk and feed on the pith. “If eggs are laid in leaf petioles, the larvae will feed on the petioles for several days and then tunnel into the main stem,” says Sloderbeck.
Lodging can then cause severe damage and likely hamper or kill a good harvest. “We are getting some pretty significant levels of infestation,” he says. “We're seeing 80-90% of plants in some fields infested, with 30-40% or more lodging in some of those fields.”
Stem borer pressure hit parts of Kentucky in 2002. Douglas Johnson, University of Kentucky extension entomologist, says the invasion was not new for the Bluegrass State, but the wide distribution of high levels of infestation was unusual. The borer was found in several more northern counties in western Kentucky.
“When lodging occurs, plants will often be broken off smoothly near the soil line,” says Johnson. “The two opposing surfaces of the break (upper face of root end and lower face of stem end) will be smooth and often closed over so you can't see a tunnel. However, if you split the stalk you'll find the borer in the root end, and a tunnel packed with frass (insect excrement) in the upper end.”
Is there control for stem borer? So far, pyrethroid insecticides show success in killing adult beetles. But they haven't stopped larval damage.
“Apparently, the best method of controlling them is through tillage,” says Trostle. “But that goes against many of the more current production practices of minimum- and no-till farming.”
Our cropping practices are going the other way, adds Sloderbeck. “We're seeing more reduced-tillage and no-till farming, and tillage is what is needed to control overwintering larvae.”
According to research by Gary L. Lentz, University of Tennessee entomologist, some varieties are more susceptible to stem borer damage than others. “In a (Tennessee Soybean Promotion Board) checkoff-funded project, cultivars and planting dates were evaluated for their susceptibility to Dectes texanus texanus (soybean stem borer) following a spring disking,” says Lentz. Of 28 cultivars evaluated from Groups IV and V, he says the least damaged varieties were Terral TV 4990, Pioneer 9452, Eagle LS 4E, Hartz H 4994 and Eagle Buckshot 44.
More recent Tennessee studies evaluated conventional Group Vs. “The more resistant lines were Anand B, Delta King 5995G, Asgrow 5944, Deltapine 4748S, Pioneer 95B33 G and FFR 5700 G,” says Lentz. He notes that Roundup Ready varieties have not been evaluated but are the subject of a proposal to the state promotion board.
What about planting dates? The Tennessee studies showed some success for earlier-planted beans. Lentz' research showed that May plantings had much less damage than the June plantings. April plantings were damaged even less. “However, planting in May to avoid damage from Dectes is recommended,” says Lentz, stressing that environmental conditions could easily affect the impact of planting dates on actual stem borer damage.
Crop rotation could reduce beetle populations. “These beetles are not strong flyers,” says Sloderbeck. “If possible, avoid planting beans in fields or adjacent to fields that were infested the previous year.”
If tillage is in a grower's cropping system, he suggests a fall disking or bedding that destroys and buries stubble 2-3” deep.
“Weed management may be another way to control them because stem borers are known to use wild annual sunflower, ragweed and cocklebur as alternative hosts,” says Sloderbeck. “Good weed management in fallow fields and fence rows also may help reduce stem borer losses.”
Sloderbeck has seen some success in controlling adult stem borers by applying pyrethroid insecticides. Checkoff-funded research in 2003 is further examining chemical application rates and dates.
Until better control measures are determined, he advises growers to plan their harvest schedules to get a jump on lodging damage.
“They need to prioritize their harvesting and be ready to get into fields as soon as the crops are mature enough,” he says. “That could prevent losses if fields are showing signs of stem borer infestations.”
For further information on soybean stem borer control, visit these Web sites: web.utk.edu/˜taescomm/soyreport/croppest.html (Tennessee extension entomology) or www.oznet.ksu.edu/entomology/extension/insectinfo/sbsb/sbsb2.html (Kansas State Extension).