Two genes that help soybeans fend off aphids worked well in 2011, both as single and stacked traits. That’s according to a six-state study of soybeans with aphid resistance genes, known as Rag1 and Rag2. Individually, the genes slowed aphid growth significantly, says Iowa State University entomologist Matt O’Neal, who led the research.
Together, the two genes were even more powerful, reducing aphid numbers well below economic injury thresholds and delivering consistent performance across a variety of environments, he says. In fact, with the Rag1-and-Rag2 combination, “It was hard to get any aphids to survive at all.”
Currently, only the Rag1 gene is available in commercial varieties.
The 2011 study included fields in Iowa, South Dakota, Minnesota, Wisconsin, Kansas and Pennsylvania. One reason for the geographic scope of the study is the genetic diversity of soybean aphids, says Mike McCarville, an Iowa State entomology doctoral student who coordinated the research. There are at least three soybean aphid biotypes, and some are immune to Rag1 or Rag2. “We don’t know how these different biotypes are distributed or where they are at different times of the year,” McCarville says.
Researchers compared an aphid-susceptible soybean line to closely related isolines that contained Rag1 resistance, Rag2 resistance and a Rag1-and-Rag2 stack. The varieties were developed at Iowa State by Walter Fehr, a distinguished agronomy professor and soybean breeder.
The Iowa, South Dakota and Minnesota research sites had heavy aphid pressure in 2011, with untreated populations on the aphid-susceptible lines well over 1,500 aphids per plant, McCarville says. In the other three locations, aphid pressure was light.
Rag1 and Rag2 provided about the same level of aphid suppression in the three states that had high aphid pressure, McCarville says. The two genes in combination “were even better than either gene alone.” In the other states, aphid numbers were too low to draw conclusions.
Brady Stueve,Wheaton, MN, has sprayed soybean aphids in three of the last four years. In 2011, he planted a maturity group 1.5 aphid-resistant soybean from Rea Hybrids on 500 acres of his toughest bean ground, where he needs cultivars with a high IDC score and SCN resistance. “We thought, if we’re going to plant a defensive bean anyway, why not get the aphid resistance, too? And it paid off.”
By the first of August, aphid populations on his susceptible soybeans reached about 200 insects per plant. Just two days later, “they skyrocketed to 800 or 1,000 per plant.” But the Rag1 beans had nary an aphid. “Most plants had zero or one or two. The most I could find was 10 or 12 aphids on a plant.”
Stueve saved about $8/acre in foliar insecticide and application expense on the Rag1 beans, which cost and yielded the same as his 1.6 RM offensive soybeans. It was a poor soybean year — too wet in May and June, too dry in August, plus an early frost before beans were mature. “One of my two best soybean fields was the aphid resistant variety, which yielded 50 bu./acre,” Stueve says. “Last year was very challenging, so we were happy with that.”
This year, Stueve again plans to plant aphid-resistant beans on about a third of his 1,500 soybean acres, moving to a 1.8 RM to capture “a little more topside potential.”