Farmers aren’t the only ones who like early-planted soybeans. Bugs like them, too. The relatively mild winter much of the Midwest just experienced could result in above-average populations of pests in farmers’ fields this spring—just as soybeans start to emerge.
First on Jay Johnson’s list is bean leaf beetles. “I wouldn’t be surprised to see a higher number of them than we’ve had the past couple of years,” says Johnson, owner of Prairie Crop Pro-Tech, Beaman, Iowa.
Adult bean leaf beetles burrow under plant debris along fence rows, in grassy fields or close to tree lines to ride out winter. Once spring temperatures reach 50°F to 55°F, they become active and seek host plants such as alfalfa, clover and weeds.
Timing Of Damage. As soybeans emerge, bean leaf beetles can quickly colonize and fly into the crop, where they cause damage by feeding on cotyledons, stems, unifoliate leaves and emerging trifoliates. Usually, the pest doesn’t cause extreme damage early. However, DuPont Pioneer research shows that if both leaves on a plant are lost at the VC stage (cotyledon), then yields are reduced by 8% to 9%.
One way southern growers keep the beetle and other insects at bay is by doing a burndown in fields about two weeks before planting, according to James Whitehead, Helm agronomy leader. “That gets rid of the green bridge—any weeds or vegetation that are present,” he notes. “ If you do that, the bugs die or move off to another area if they can.”
Even if insects like bean leaf beetle wreak havoc this spring, Johnson says planting early-season soybeans is still the way to go in central Iowa. “We have seen a 5- to 7-bushel-per-acre advantage of first-planted beans in mid-April over those last planted in mid-May,” he notes.
Johnson attributes the yield advantage to the soybeans having time for additional growth and development. “When the early beans start flowering, they’ll often have two additional trifoliates out there, which improves yield potential,” he explains.
Scouting for adult beetles in early beans is easy, but deciding whether to apply an insecticide is somewhat tricky, depending on crop value, beetles per plant and treatment costs. (See table). Along with that, Whitehead cautions that some populations of bean leaf beetles in the South and mid-South have developed resistance to pyrethroids, so those products could be less effective, depending on the geography.
Purdue entomologists give fairly comprehensive treatment options to control in-season soybean pests in their crops bulletin E-77-W. Their recommendations are available online at https://extension.entm.purdue.edu/publications/E-77.pdf.
Where Neonics Fit. For retailers who expect a high incidence of pest problems in farmers’ fields, a soybean seed treatment is still a good option to control pests and improve plant health and vigor. “In some areas of the country, growers see a 5- to 7-bu. yield increase from using a neonic,” Whitehead says.
Extension entomologists say to use neonicotinoids only in “targeted, high-risk situations,” such as:
· Fields recently converted from CRP or grassland to soybeans.
· Fields with animal manure, green cover crops or numerous weeds.
· Double-crop or specialty (food-grade or seed) soybean fields.
Researchers recommend an integrated pest management plan, such as rotating crops, conserving natural enemies, planting pest-resistant varieties, scouting and applying insecticides at established thresholds.
“It’s important to have a diversified [crop protection] program in place so we don’t put too much selection pressure on a single class of chemistry,” Whitehead adds.