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Stay on top of soybean aphids with scouting, timely treatments

Agriculture.com Staff 08/13/2009 @ 9:55am

The skies are dotted with Air Tractors as farmers start to discover soybean aphid numbers warranting treatment around the Midwest.

Earlier this week, specialists with the Iowa Soybean Association said aphid numbers are well beyond treatment threshold levels in the northern Corn Belt. But, just because you have aphids doesn't automatically mean you need to treat just yet. Scout first, and scout thoroughly. Look at every part of the plant closely, and don't assume you need to spray just because you're seeing spray planes flying, even if it's in a field right across the road.

"First of all, it’s just a waste of money," Iowa State University Extension entomologist Erin Hodgson says in an Iowa Soybean Association report. "Second, it kills all the beneficial insects that are not only reducing pests in soybeans, but pests in other systems as well. And ultimately, it could drive genetic resistance to major classes of insecticides, and we don't want that to happen to soybean aphids."

But, neglecting treatments are just as dangerous as being too quick to spray, Hodgson says. That's what makes good scouting so important, she adds.

"Under extreme outbreak conditions you could have anywhere from 40% to 70% yield loss. It's certainly something you want to take action against when they do exceed the threshold because it can cause severe economic loss," she says. "For this time of year, from now through seed set, we recommend the threshold for soybean aphid is 250 per plant, with aphid numbers increasing, and the only way you know that is through regular sampling."

Scouting's important, but so is understanding the point at which you should treat for aphids. Once the plant's passed the R 5.5 development stage, spraying's no longer needed, says Iowa State University Extension field agronomy specialist Virgil Schmitt in Muscatine County, Iowa. That's a pretty specific point in the plant's development, but with a close examination of the plant, you can get a good sense of where you are, development-wise, fairly easily.

"Once the soybeans reach growth stage R 5.5, an insecticide application is not needed," Schmitt says. "If there is a seed 1/8 inch in diameter in a pod at one of the top two nodes with a fully expanded trifoliolate leaf on the main stem of the plant, the plant is about right at R 5.5."

Though it's definitely been an "aphid year" this summer, they're inconsistent, both in numbers and location, farmers say. Many farmers and Agriculture Online Crop Talk members say they're treating aphids in their fields, where they're finding populations at or above the treatment threshold. But, not everywhere, says Crop Talk member DW11, who farms in southeastern South Dakota.

"Aphid counts from near zero to hundreds in the same field right now," he says. "The first fields were sprayed 10 days ago. Spraying the rest of our fields right now."

In some cases, farmers say they've been able to bundle insecticide applications with fungicide treatments, which has trimmed application costs. "I sprayed last week after aphid counts tripled, but were still below thresholds," says Crop Talk member idalivered. I felt I could still make it through 30-inch rows myself and was going to spray Headline anyway."

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