Checkoff-funded researchers throughout the North Central region have been testing new genetic sources of aphid resistance, and the results look promising.
“We’re screening different lines from several university breeding program,” says Kelley Tilmon, South Dakota State University entomologist.
“We test them on small plots in multiple states, with varying growing conditions and varying levels of aphids, and monitor aphid population growth and yield on the different lines. That’s what we call Tier 1 screening,” she says. If a line looks good in Tier 1, it moves to Tier 2 testing, which involves larger plots and insecticides – basically mimicking how growers raise soybeans.
Aphid biotypes
“In general, there’s definitely a big difference between lines bred for resistance and the susceptible control varieties,” Tilmon says. Unfortunately, there’s a catch: aphid biotypes that can break host plant resistance.
Scientists already have identified one new aphid biotype – called biotype 1 – with some ability to overcome the source of resistance known as Rag1. “We captured biotype 1 in 2006, but there weren’t a lot of reports in 2007,” says Glen Hartman, the University of Illinois/USDA-ARS researcher who co-discovered the Rag1 and other sources of resistance with another UI researcher, Curt Hill. “What we don’t know is how predominant biotype 1 is, or how widely dispersed it is.”
Tilmon says, “In most of our trials, those lines with the Rag1 gene performed well. The wild card will be the aphid biotypes that can overcome the resistance and how quickly they can evolve or spread. So now we have to consider how that will affect management in various states.”
According to Dechun Wang, a plant breeder at Michigan State University, “We don’t know how many biotypes there are in the United States, but we know there are differences in aphids in different locations.” He doubts that aphids have already managed to overcome resistance, “since we haven’t had a resistant variety available commercially yet.”
Many sources of resistance
Nature’s ability to overcome resistance eventually, however, is one reason researchers have so many different sources in the pipeline. “The Rag1 source may be available in commercial seed in 2008, and there’s a lot more material to look at,” says Hartman.
“We have material transfer agreements with several companies for the Rag1 and another resistance source,” Hartman adds. “We have about 50 different PIs (plant introductions) that look very different from the Rag1 and other known sources. We need to do genetic tests on them, and that will take a couple years.”
Breeder Bill Schapaugh from Kansas State University also has lines in testing, including one – K1639 – that is resistant to aphids and soybean cyst nematode. “That will be a good germplasm line,” Schapaugh says, “because it also has decent agronomic traits.”
Michigan State’s Wang has new sources of resistance in the trials as well. One source, E06902, is already being crossed with commercial germplasm by Syngenta. “Our studies show that E06902 has two recessive genes controlling aphid resistance,” says Wang. Sources of resistance with more complicated genetic background will be more difficult for aphids to find a way around.
“This was the first year we looked at Dechun’s lines,” says Matt O’Neal, entomologist at Iowa State University who is leading the host plant resistance project. “Central Iowa was hit hard with aphids in 2007, and his lines looked remarkable. There was one case in Wisconsin where one of his lines didn’t do very well – and that may be due to aphid biotypes.”
One more tool in your toolbox
O’Neal adds that, even with aphid biotypes that can overcome some sources of resistance, it’s not a total loss. What this means is that resistance is going to be a good tool for soybean aphid management, but not the only tool.
“Scout and consider an insecticide if aphid populations go above the 250 threshold, even on resistant plants,” O’Neal continues. “The frequency of spraying will drop, and growers will be able to use less insecticide. But you can’t just plant aphid-resistant soybeans and walk away.
“You don’t want only one hammer for aphids, you want a lot of hammers,” he says. “That’s why we’re working on different resistance sources, and why we released Binodoxys communis. Maybe if we have several aphid-resistant soybean varieties available, we can lower the number of aphid migration events. If fewer aphids are flying in, maybe Asian lady beetles and other predators can catch up, and the resistance can hang in there.”
The above article was provided by North Central Soybean Research Program.
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