August 15, 2005

Aphids add insult to injury for soybean farmers

WEST LAFAYETTE, Ind. - While soybean producers are on the lookout for soybean rust from the south, a familiar foe has been riding into soybean fields on winds from the north.

Soybean aphid, a small insect that can reduce yields by 10 percent to 15 percent, has been seen in ever-increasing numbers in many Hoosier bean fields this summer.

"The highest numbers this year have occurred in the northeast corner of the state, with the northwest quarter being second highest," said Christian Krupke, a Purdue Extension entomologist.

Even farmers in southern Indiana, who normally don't feel pressure from soybean aphids, have been finding them in the fields.

At the Southern Indiana Purdue Agriculture Center in Dubois County, aphid counts blossomed to 43 per plant last week. While the economic threshold for treatment is generally 250 aphids per plant, the counts in southern Indiana were something new, Krupke said.

"Traditionally I-70 has been the border between aphid and non-aphid parts of the state," he said. "What's changed this year is aphid numbers over threshold in Vincennes in our research plots and aphids in lower numbers in the southern half of the state."

Shawn Conley, a Purdue Extension soybean specialist, said, "We're also seeing plants have been under stress for a considerable amount of the season, and this makes them more susceptible to damage and injury by the soybean aphid."

This seems to be a tough year for soybean farmers. The season started with concerns about soybean rust, then early season spider mite infestations. In addition to those pressures, the state is experiencing mild drought conditions. All this adds up to one thing: yield losses.

"We're getting into that R5 growth stage, so we're not really going to put any more pods on. What we're going to see is an impact on seed size and also the number of seeds per pod," Conley said.

Significant stress also can cause pod and seed abortion in soybeans.

"Soybean aphid can consistently cause yield losses of 10 to 15 percent," Krupke said. "It can be more than that in cases of severe infestation coupled with drought. This has not been a typical year weatherwise, especially in northeastern Indiana where the drought has been more severe, so that can only exacerbate the problem in terms of yield losses."

Soybean plants aren't the only things suffering from the drought. The aphids seem to be affected this year, as well.

"One thing that we're seeing - this may be drought related - are small, white aphids. They're not the yellowish-green of the soybean aphid," Krupke said. "In other species of aphid, stress in the parent generation causes them to produce dwarves. They don't appear to be as vigorous or do as much damage as the other aphids, but we're telling people to just count them."

Regardless of the aphids' coloration, Krupke and Conley said the best thing farmers can do is scout their fields for aphids and treat accordingly.

"We're seeing some growers that are just spraying their field simply because the aphid is in the area. That can be a waste of time and a waste of a lot of money when you're talking about large acreages," Krupke said. "It doesn't take long, and it's very easy to scout the field for soybean aphid."

Farmers should check at least 10 to 20 plants in different areas of the field and make a treatment decision based on the 250 aphid per plant threshold, he said. He also recommended that growers keep track of the insecticide residuals and preharvest intervals.

Residual refers to how long the chemical will kill the aphids, and preharvest interval is how long a farmer must wait to harvest the soybeans after spraying.

"A lot of growers are mixing them up and interchanging those numbers," he said. "It's important to note that just because your preharvest interval is 40 days you don't get 40 days of residual."

Information about scouting for, and treating, soybean aphids is available online

Writer: Kay Hagen, (765) 494-6682, kjh@purdue.edu

Sources: Christian Krupke, (765) 494-4912, ckrupke@purdue.edu

Shawn Conley, (765) 494-0895, conleysp@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
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