Soybean Double Trouble

LAWRENCE, Kan. (DTN) -- Soybean farmers have good reason to feel a little paranoid this spring. A new Iowa State study has found that soybean aphids are conspiring with soybean cyst nematodes (SCN) to weaken the effectiveness of some SCN-resistant soybean varieties.

"The take-home message for farmers is that when soybean aphids are feeding on soybeans that are resistant to SCN, it can lessen their resistance and therefore you could have greater than expected nematode reproduction and yield loss," said Iowa State plant pathologist Greg Tylka, one of the study's authors.

The study used a greenhouse experiment to monitor nematode reproduction on both SCN-resistant and SCN-susceptible soybean varieties. When the SCN-resistant variety was infested with low to moderate populations of aphids and nematodes, researchers found that 33% more female nematodes and eggs were produced on the plant's roots compared to aphid-less control plants.

Growers haven't reported noticing this interaction in the field yet, but Tylka said the results of the study didn't surprise the researchers. "The reason we thought to even look at this is that the nematode feeds below ground on the roots' vascular tissue, and up on the leaves, above ground, the soybean aphid feeds on the same vascular tissue," he explained. "So it's all connected from the very top of the plant to the very bottom of the roots, so we thought there might be potential for one to affect the other."

When aphid or nematode populations are very high, the increased nematode reproduction effect disappears, because the plant is simply too overwhelmed to be a good host to either pest, Tylka noted. However, growers are more likely to have fields with the low to moderate populations that allowed nematodes to flourish, he added. "The bottom line is that this trend of nematode-resistance becoming less effective is occurring in the range of numbers that are common in fields," he said. "It doesn't have to get to the point where it's a disaster -- it's with commonly found nematode numbers."

Growers can expect both short- and long-term consequences from this aphid-nematode interaction, Tylka predicted. "It has a two-prong negative effect," he said. "Number one, it's going to hurt the plant that year, in terms of yield. And then number two, it's going to allow nematode numbers to build up more than we like them to, and that creates a problem in future years."

Soybean cyst nematodes hardly need the help. Since the pest's arrival in the U.S. in the 1950s, nematodes have infested soils from southern Florida to North Dakota and from Delaware to western Kansas. They have fared especially well in the Soybean Belt and occupy every county in Illinois and all but one county in Iowa. They are widely recognized as the number-one most-yield-limiting soybean pest in the U.S, with a recent, three-year study estimating annual losses of nearly 1.3 billion soybean bushels.

The soybean aphid is also a troubling invasive pest, which arrived in Wisconsin in 2000, most likely from Japan. Despite the presence of many natural predators, farmers have increasingly turned to insecticides to control aphids. One Iowa State study found that aphid populations corresponded to a 1,423% increase in acres treated with insecticides in Iowa between 2000 and 2009.

However, in a competition for the most frustrating soybean pest, nematodes would probably win. "Nematodes are even more insidious than soybean aphids because they live in the soil and go dormant," Tylka explained. "You could literally not grow soybeans for a decade and there would still be some live, but dormant, eggs left in the soil. So it's tremendously long-lived in soil and its damage potential is as much or more than soybean aphid. We've seen 50% yield loss with soybean cyst nematode alone."

SCN-resistant soybean varieties have long been farmers' most effective and reliable means for controlling nematodes, but the pest is fighting back. For years, nematodes have shown an increased ability to reproduce on PI 88788, the most common type of SCN-resistance. As a result, aphid feeding may be chipping away at an already crumbling tool, Tylka pointed out.

"Of all the resistant varieties, 98% have exactly the same type of resistance," he said. "The nematode is building up on that common type of resistance, and now we discover that the soybean aphid makes that resistance even less effective."

Tylka said more research will be needed before it is known why aphid feeding gives nematode reproduction a boost and whether this trend affects all SCN-resistant varieties. For now, soybean growers should take the time to manage their soybean fields for both nematodes and aphids, he added.

That means scouting for and treating high aphid populations, using SCN-resistant soybean varieties, rotating types of resistance if possible, and considering the use of nematicide seed treatments on the market.

For help with scouting and treating soybean aphids, see this Purdue guide: http://goo.gl/…, and this Entomological Society of America video on aphid speed scouting: http://goo.gl/….

To see the distribution of soybean cyst nematodes in the U.S. and current research on the pest, see this Iowa State website: http://goo.gl/….

For a comprehensive soybean cyst nematode management guide, see this website from the Plant Health Institute: http://goo.gl/….

For a list of host and non-host species for the nematode, see this Pioneer guide: http://goo.gl/….

The Iowa State study, which was funded by the soybean checkoff through a grant from the Iowa Soybean Association, can be found here: http://goo.gl/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com.

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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