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.”

Rag1 gene availability

The Rag1 gene is available in more soybean varieties this season.

In Iowa, for example, about a dozen national and regional seed companies are offering 16 unique soybean varieties with aphid resistance, says McCarville, who surveyed 128 seed companies in January. “That’s more than we expected for 2012.”

Available maturities range from 1.2 to 3.0. About half the aphid-resistant varieties available for Iowa and similar maturity zones are Roundup Ready, McCarville says. Additional traits available in some cultivars include SCN resistance and tolerance to phytopthora and brown stem rot.

Monsanto and its regional seed companies are offering several Genuity Roundup Ready 2 Yield varieties with Rag1, in maturities from mid-Group 1 to early-Group 2, with or without Acceleron seed treatment, says Tony White, soybean product manager.

Syngenta offers aphid resistance varieties through the Aphid Management System with NK Brand soybeans. The system combines the Rag1 gene and CruiserMaxx Beans fungicide/insecticide seed treatment. Maturities range from early-Group 1 to mid-Group 2, says Quinn Showalter, soybean traits marketing manager. The company’s Aphid Management System includes a $5/acre assurance for Endigo ZC foliar insecticide if aphid populations exceed economic threshold levels.

Pioneer Hi-Bred has not yet introduced aphid resistance into its commercial lineup, says Don Schafer, senior marketing manager. However, Pioneer rates all its soybean varieties for antibiosis, the plant’s natural ability to reduce aphid growth, survival and reproduction. “Growers can use these ratings to help them determine field scouting priorities and insecticide application decisions.”

Until Rag genes are incorporated into more cultivars, O’Neal says, growers should weigh aphid resistance traits against other yield and agronomic traits. While studies show there is no yield penalty associated with Rag1, the gene “may not yet be in the best-yielding variety for your particular region” or environment, he says.

Bryan Stenzel, Wells, MN, planted 50 bags of NK 2.1 RM soybeans with Rag1 in 2011. Aphid pressure was heavy in his area. The Rag1 variety did a great job of suppressing aphids, Stenzel says, but it didn’t yield as well as his non-resistant beans, lagging by about 6 bu./acre.

If you plant a Rag1 variety this spring, keep in mind that you might still have to spray for aphids.

Although Rag1 lowers the number of aphids on plants, “it won’t always reduce them enough that growers don’t need to apply insecticide,” O’Neal says. “In certain circumstances, aphid numbers could be high enough to cause yield loss. It depends on the weather and aphid genetics.”

Host plant aphid resistance “is not a silver bullet,” agrees Monsanto’s Tony White. “The gene does an excellent job, but farmers still need to scout,” and spray if aphid numbers exceed the recommended threshold of 250 aphids per plant on 80% of plants.

If you plant an aphid-resistant variety, should you add a seed treatment, too?

“We advocate seed treatments to help with early-season aphid control,” White says. The 2011 six-state study found that insecticide seed treatments further curbed aphid numbers on resistant varieties, McCarville says. However, seed treatments did not provide an economic return on the Rag1-and-Rag2 stack.

Seed companies are working to incorporate Rag1 and Rag2 traits into their elite soybean varieties, O’Neal says, and the industry has additional sources and combinations of aphid resistance in the breeding pipeline, including Rag3 and Rag4. “The future is bright for this.”

When should you plant an aphid-resistant variety?

Soybean aphid outbreaks are not predictable at planting time, so aphid-resistant seeds are a good choice for growers in regions where infestations are common, says Matt O’Neal, Iowa State University (ISU) entomologist.

Outbreaks depend on the weather and other factors, such as natural predators, he says. However, damaging infestations tend to be more common in South Dakota, Minnesota and the northern half of Iowa, and in areas with dense stands of buckthorn, aphids’ winter habitat.

Quinn Showalter, Syngenta soybean traits marketing manager, suggests that growers try the technology on “fields that are difficult to spray,” such as those with windmills, terraces or irregular shapes, or fields near buckthorn stands.

Farmers who have sprayed for soybean aphids in two out of the last four years should consider planting aphid-resistant seed, O’Neal suggests.

The premium for soybean aphid-resistant seed may be less than the cost of a foliar insecticide application, says Mike McCarville of ISU’s entomology department. The technology fee for the Rag1 gene ranges from about $1/bag to around $10/bag, according to his January survey of Iowa seed dealers. If you are spraying for aphids half the time, it may be more cost effective to grow an aphid-resistant variety, he says.

Organic soybean producers should also consider planting Rag1 beans, O’Neal says. Aphid resistance genes are native to soybeans: they’re not genetically modified. Phil Batalden, an organic farmer from Lamberton, MN, will be planting a food-grade Rag1 soybean this spring. He’s taking a chance on a 2.3 maturity group, which is late for his area, but it’s the best food-grade choice available this season, he says. “Aphids have been devastating, and organic growers have few, if any, effective ways to manage them.”

Growers looking for a more diversified pest-management approach should consider them, too, McCarville says. These varieties can reduce the need for chemical inputs, especially later in the season.