What’s going on with the soybean aphid?
At 700 per plant, the economic threshold for soybean aphids is fairly high.
Yet, considering the aphid’s incredibly small size and the fact that in optimal conditions a population can double in seven days, this little pest can be a crop scouter’s nightmare. Rather than trying to time an insecticide application, University of Illinois’ Carol Bonin says planting a soybean with aphid resistance may be a better idea.
Rag genes provide a genetic defense against aphids. Rag stands for resistance to Aphis glycines, the scientific name for the soybean aphid. To date, four Rag genes have been discovered. Bonin explains they are typically found in exotic soybean lines.
The first was discovered at U of I in 2004. The second, from an exotic Japanese line, was discovered in 2008. Rag3 and Rag4 were found in 2008 and 2009, respectively. Both are from Chinese lines.
• To date, four Rag genes have been found in exotic soybean lines.
• It appears the soybean aphid’s evolution has sped up considerably.
• Tank-mixing of insecticide is ratcheting up selection pressure.
Subsequent testing of Rag genes has proven their protection potential. In 2009, Matt O’Neal monitored a plot of soybeans with and without Rag1 resistance at Iowa State University. At the infestation’s height, crop scouts counted 3,500 aphids per plant on the control group.
The Rag1 plant averaged 500 aphids per plant. At harvest, Rag1 plants yielded an average of 20 bushels more per acre.Bonin notes subsequent studies have shown no yield drag in plots without an aphid infestation. Further testing continues on the other three Rag lines.
While plant breeders work to assimilate Rag genes into Midwestern varieties, the soybean aphid continues to evolve. Entomologists have identified multiple resistant biotypes. Additionally, U of I’s David Voegtlin notes that a significant evolution in where the aphid appears was first observed in 2006.
In North America, the soybean aphid’s winter host of choice has been common buckthorn. In 2006, it added glossy-leaved buckthorn to its list of winter hosts.
“The aphid-host relationships are millions of years old,” Voegtlin explains. “For this to happen in the past eight years is either a big evolutionary step or possibly additional genetic variation by new introductions of the aphid .”
While common buckthorn is readily available in Illinois, glossy-leaved buckthorn is less common. Voegtlin says glossy buckthorn grows primarily in wet areas and can be locally abundant in both Illinois and Indiana.
The addition of a winter host hasn’t had any correlation with growing-season outbreaks. However, Voegtlin says it’s becoming easier to find aphid biotypes that are capable of feeding on Rag1 plants. And the resistance appears to be expanding.
“There was a line of soybean aphids found at the University of Wisconsin that ate everything they fed it,” he adds.
The evolution appears to extend past winter host and Rag resistance. Typically, aphid populations grow exponentially when temperatures are between 75 and 80 degrees F. At 90 degrees F, reproduction is curtailed.
Throughout much of the 2011 growing season, the Midwest has experienced temperatures in the 90s quite regularly. Still, in late July, Voegtlin noted that in some parts of Iowa, soybean fields were 80% infested.
If that’s not enough evidence of something going on, Voegtlin points to the diminishing trend of a high infestation year followed by a low year. He says rampant tank-mixing of insecticide with fungicides and herbicides has negatively impacted the aphid’s natural predators. Without natural predators feeding on aphids, the ebb and flow of the aphid population has become more unpredictable.
“With the additional genetic variation mixing and recombining in the annual sexual cycle, the soybean aphid is now much less predictable,” Voegtlin concludes.
This article published in the September, 2011 edition of PRAIRIE FARMER.