Expect soybean aphids in large numbers this season, say experts, based on the pattern of low populations one year and high populations the next. Last year, soybean aphid numbers were down in much of the eastern and central Midwest.
Insecticidal seed treatments, such as Cruiser Maxx and Gaucho, are labeled for controlling soybean aphids and provide some early-season control, the manufacturers claim.
In four years of research at Iowa State University, insecticidal seed treatments showed good performance against soybean aphids in the early vegetative growth stages of the crop — through about 60 days from planting. In Iowa, that meant the treatment lasted until early July.
The problem is that the aphids don't show up in sizeable numbers until mid to late July, notes Ron Hammond, Ohio State University Extension entomologist. “That's too far of a time spread for the treatment to have much impact,” he says.
Another problem is that not enough aphids may show up to warrant sufficient control. “Our research showed that there was no difference in performance between treated plots and untreated plots under this situation,” Hammond says. “The [seed] treatments might prevent aphids from reaching threshold for about a week or two, but a grower will need to treat those fields sooner or later if aphid populations are building.”
The need to spray
The best way to manage the soybean aphid is through scouting and well-timed foliar insecticide applications, if the insect reaches the threshold level. “Growers should start scouting their fields in early to mid July and be ready to spray if insect populations reach 250 insects per plant,” Hammond says.
If insecticides are applied when insect levels are below threshold, “the insecticides will also wipe out natural predators that feed on the aphids, so you lose that natural control,” notes Agriliance agronomist Al Bertelsen.
The keys to the most effective control with an insecticide application are timing and proper application. “You need good coverage to control aphids,” Bertelsen notes. “High water volume and a nozzle that produces medium-size droplets at the chosen spray pressure help ensure good insecticide coverage, especially below the crop canopy.
“We also recommend using a deposition aid, such as InterLock, to get better penetration into the canopy, as well as a nonionic surfactant for better coverage and increased efficacy.”
“There is no doubt that if you have high soybean aphid populations, you can save anywhere from 5 to 18 bu./acre with such treatments,” Hammond adds. “However, we don't recommend general sprays just for plant health purposes.”
Another promising tool for aphid control is expected to hit the market in 2009. That's when Syngenta plans to begin offering aphid-resistant soybean varieties.
These varieties have been developed using a native trait that was discovered in a 25-year-old variety adapted to the U.S. Gulf Coast, explains Virgil Sparks, soybean product development head for the western region. “In 2004, we began a traditional back-crossing program that utilized marker-assisted technology to track the presence of the aphid gene,” he says. “In 2007, aphid-resistant varieties created from our development efforts will be increased by our parent seed group. There will be a limited amount of seed available in 2008.”
The first soybeans to have this trait will be Group I and II varieties, and some will likely also have the Roundup Ready trait.
“Like any kind of resistant trait, a producer will need to manage its use properly,” Sparks continues. “That will likely mean making sure there are susceptible varieties nearby. We're currently working on developing some management recommendations for how to best control the soybean aphid, while protecting this trait for long-term use. Soybean aphid resistance really needs to be considered as one more tool to use as part of a total aphid management plan. And we need as many tools as possible to help keep this pest under control.”