Soy aphid-mite issue shifting

CHANHASSEN, Minn. — University of Minnesota soybean researchers and leaders say the industry is peddling fast to help farmers stay ahead of aphids and other yield robbers, but a short-term fix doesn’t always work in the long run.

Aphids were one of the several topics among 11 regional soybean experts who spoke on production and disease panels at the annual Soybean Symposium recently at the University of Minnesota Landscape Arboretum in Chanhassen.

David Wright is staff coordinator for the North Central Soybean Research Program, based in Ankeny, Iowa, which coordinates soybean research across 12 states, including the Dakotas and Minnesota. Wright says nearly $400,000 is spent on aphid research in the 12-state region, as for related projects at a federal lab in Newark, Del.

Wright reports that improved treatment threshold recommendations have saved soybean producers nationwide $100 million to $200 million annually. The north-central area has 85 percent of the soybean crop, which is No. 2 in state of Minnesota.

“We don’t believe chemical control is the long-term solution for the soybean aphid, although it’s effective,” Wright says.

Parasites of the aphids and genetic resistance are the best long-term goals, he says.

David Ragsdale, a University of Minnesota extension entomologist, agrees. He is chairman of the university’s entomology department in St. Paul and coordinator for the north-central soybean aphid program.

Ragsdale notes that environmental consequences of some potential chemical controls are not always immediately recognized. In December, for example, a federal judge ruled that the insecticide spirotetromat must be pulled from the shelves Jan. 15 because it could be dangerously toxic to America’s honeybee population. The Bayer CropScience product was registered for cotton, fruits and vegetables — not yet soybeans.

“That material is not going to be available to us — probably ever,” Ragsdale says.

The hope is for better genetic resistance.

Aphids bull’s-eye

The primary source of genetic resistance against soybean aphids currently is the so-called “Rag” series of genes. (R stands for resistance and “aphis glycines,” which is the genus for soybean aphids.)

Studies with heavy aphid pressure indicate nonresistant varieties can lose 15 bushels of yield to the aphids, but the Rag1 gene offers about 10 bushels an acre of protection.

“They only lost 5 bushels per acre compared to an insecticide-treated control,” Ragsdale says.

“The Rag1 gene is not immunity to aphids; that’s a point I want to drive home. We’re working hard to tell you what the threshold is going to be with Rag1. (The treatment threshold) is going to be higher than 250 aphids per plant because the aphids grow slower. But we really don’t know what that number really is.”

Among other things, Ragsdale says researchers have found that Rag1 beans appear no different than normal varieties when it comes to spider mites.

“Mites attack plants from the bottom up; aphids attack from the top down,” Ragsdale says.

Both aphid and mite pests are linked by drier conditions. In 2009, aphid numbers in the region weren’t as high as in previous years. The region got droughty in July and early August, but then it was relatively cool, which helped.

“If you have a 68-degree temperature, spider mites can replace themselves in 30 days. For 10 degrees of average temperature, you go up, so at 78 degrees, you get a 10-fold increase. At 88 degrees, you get a 100-fold increase; 98 degrees, you get a 1,000-fold increase in 30 days.”

Spider mites appear to be on the increase in the state, Ragsdale says. His theory on that is the prevalence of Roundup Ready crops.

“Since we’ve gone to a Roundup-Ready system, we don’t do a good early-season weed control,” he says. “We let weeds grow and then burn down with one or two applications of Roundup. You get all of these good mite hosts out there, and you kill them and the only thing left to feed on is the soybeans.”

He acknowledges that researchers have a difficult time studying interactions between herbicides and insect pests because weather is such an important wild card.

Ragsdale warns that if farmers use a pyrethroid to control aphids, the chemicals actually may “flare” mites.

Half of the insecticides applied on soybeans are pyrethroids, Ragsdale says. Half of the chemicals used to control aphids are the organophosphates.

“EPA has the very stated goal to reducing all organophosphates to ‘not available,’ so the future is going to be other materials — not organophosphates,” Ragsdale says. “They’ve reviewed the registrations of about 3,000 compounds and have a 3,000 to go.”

If a grower decides to control aphids, he or she typically choose an organophosphate because they control both mites and aphids. But the problem is that the organophosphates are shorter in persistence than the pyrethroids. The spider mite eggs will hatch and still do fine.

“And, if you catch an aphid flight three days later, the aphids will do just fine, too,” Ragsdale says. “If you read the label on organophosphates, you must apply a second application 10 days later to catch all those eggs that hatch before they reproduce.”

Counter moves

Even as new genetics offer some resistance to aphid damage, the aphid pest is developing “biotypes” that will break it.

There already are biotypes of aphids that will break Rag1, as well as the Rag2, and what Ragsdale calls the “Rag3-plus-Rag4”combination. Ragsdale says the Rag3-plus-Rag4 technology he’s tested as parental lines is embedded in “Group Three” maturity beans. These parental lines appear are susceptible to potato leafhoppers, but commercial lines probably will be fine.

Ragsdale says he hasn’t been able to find out what companies feel their “hoped market potential is” for the Rag1 gene. Privately, he’s heard talk that it may be 20 percent of the market.

For Minnesota specifically, Ragsdale says he and other biologists want to study whether the Rag1 gene will be more stable against aphids because the so-called “biotype 2” aphid, which can break the Rag2 resistance, may not be able to over-winter as successfully in colder, northern areas.

Even with its partial resistance, the Rag1 gene probably has a use in Minnesota.

“I think that where Rag1 fits is if you have a large farm and you can’t get across your farm and spray the whole thing in a week or 10 days, the Rag1 gives you some leverage to space out some of that treatment because aphid population is s not going to rise as rapidly,” he says.

That means the “action threshold,” or the U of M’s recommendation — “a guess,” Ragsdale says — puts the time for treatment at about 500 aphids per plant on Rag1 genetics.

“You may have a little more time,” he says. “It’ll probably take seven to 10 days to go from 250 to 500 aphids per plant under ideal conditions because they grow slower,” he says.

Cruiser, from Syngenta, appears to have a complicating effect. Studies so far show that when farmers spray too early with an insurance treatment for aphids, they “flare” the spider mites.

“And with Cruiser, you flare spider mites, too,” Ragsdale says. “And guess how you’re going to get Syngenta’s Rag1 gene. With Cruiser, it won’t come untreated. And you’re going to get a $5 voucher — just in case aphids go above 250 aphids per plant — for Warrior, a pyrethroid.

The company’s aphid management system seems to be the Rag1 gene with Cruiser Max insecticide, plus a voucher for Warrior — a pyrethroid — if your aphids get above threshold.

“I’m not going to want to answer my phone in August because I won’t want to hear about all of the spider mite problems,” Ragsdale says. “And we’re going to have them.”

Tags: soybeans