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Corn+Soybean Digest

The Soybean Aphid Blitz

The hottest topic in northern soybean country this past summer was the sap-sucking soybean aphid.

The super-prolific pest — with as many as 15 generations per year — clobbered some soybean fields in Minnesota, Wisconsin, Michigan and Ontario with several thousand aphids per plant. It caused generally lighter, but often still significant, damage in northern Illinois, northeastern Iowa and especially northern Ohio.

With almost rocket-like speed, it spread across Minnesota to eastern South Dakota. The pest was also found in generally lower, but sometimes yield-damaging numbers in Indiana, Missouri, Kentucky, West Virginia, Pennsylvania and New York.

Infestations in eastern Michigan and southern Ontario were so severe that a mass migration of the winged version of the aphid invaded the Toronto Blue Jays' retractable-roof baseball stadium and caused a game to be delayed. By closing the roof and reversing the stadium air system, the aphids were expelled and the game restarted.

Chris DiFonzo, Michigan State University entomologist, had a graduate student carefully count aphids on soybean plants. The low per-plant number was 1,700; the high was 13,400 aphids, she reports.

In Minnesota, “We definitely saw aphid populations top 10,000/plant, and many more populations of about 8,000/plant,” says Ken Ostlie, University of Minnesota entomologist. “That's obviously going to have a significant impact on yield.”

Tom Ferrier farms near Dover, MN, in the southeastern part of the state where aphid pressure was heavy. He sprayed about 250 of his 600 acres. “I got 53 bu/acre on one heavily infested field we sprayed, despite the drought,” he explains. “But on an unsprayed check area, the yield dropped to 38 bu.

“The aphid numbers were unreal,” Ferrier says. “They must just suck the life out of those soybeans. I had a few other fields with lower numbers, and I should have sprayed them, too.”

First identified in the U.S. in July of 2000, soybean aphids overwinter as eggs on buckthorn. Spraying with insecticides, a virtually new practice for northern soybean growers, became common in hard-hit areas last year.

Without that spraying, average statewide yields in the heaviest-impacted states would undoubtedly have taken a nosedive, say experts.

During the 2000 growing season, George Gunderson, of Boscobel, WI, saw first-hand what these aphids can do if not sprayed. His was one of the first and hardest-hit farms that year in southwestern Wisconsin. By the time aphids were discovered and identified, spraying was a waste of money.

“The aphids were so thick that they just took those beans,” Gunderson recalls. “When we found out what they were, it was too late. They affected about 40% of my beans, but where they hit very bad, beans yielded only 11 bu/acre. We had been averaging 51-53 bu on those fields.”

This past summer, Gunderson and surrounding farmers sprayed in mid-to-late July. But many, like him, suffered from severe drought, so yields were still low, even with spraying.

In any state where infestations were found in 2001, start scouting in the later vegetative stages and watch closely in the early reproductive stages, R1 (first flower) and R2. Aphids seem to peak at R3 and R4 stages, says Dave Hogg, University of Wisconsin entomologist.

Generally, begin scouting toward the end of June but more intensively in July, agree experts. Scout with a good-quality magnifying glass because these insects are tiny — less than 1/16" long.

So when should you spray? No set thresholds are yet available. But it's time to act when you see the top leaves and stems virtually covered with aphids. And when there aren't many predators like lacewings or Asian ladybeetles feeding on aphids, or fungal pathogens putting pressure on them.

Spray timing is critical, studies and field observations show. Spray too early or late and you're wasting money.

“We'll be pulling the spray trigger a little quicker this next growing season,” says Fritz Breitenbach, a University of Minnesota IPM specialist in southeastern Minnesota. “I have no doubt about that. In our area, the third week in July would probably be a good time to spray.”

To reduce chances of a needed second application, wait until the upper leaves are nearly coated with aphids, but before plants show noticeable signs of damage, suggests John Wedberg, University of Wisconsin entomologist.

Aphids move down the plant and to the undersides of leaves as the season progresses, reducing spray effectiveness.

What insecticides should be used? At least 10 are cleared for use on soybeans, but so far only four have soybean aphids on their labels. They are: Furadan 4F, Lorsban 4E, Warrior T and Penncap-M. Those four are suggested by Wedberg, but others may do well.

Hogg and DiFonzo encourage very careful monitoring of fields, then making only one spray application, if needed. Breitenbach isn't sure a single application will do it in severe situations. If two are applied, he recommends switching insecticides, using one with a different mode of action for the second treatment.

Rotating insecticides yearly may be wise even with single treatments because aphids are notorious for developing resistance to heavily used insecticides, the experts note. One other item: insecticide kills beneficial insects as well as aphids.

Yield loss from the aphids themselves is only part of the equation. The aphids, like bean leaf beetles and some other insects, can transmit diseases, including viruses like soybean mosaic. That could prove to be a double yield-cutting whammy.

And the cost of spraying? A University of Wisconsin study determined breakeven at about 3 bu/acre for $5 beans. “So roughly $15/acre would cover the material and application cost for most materials,” Hogg says. “That means that once you get past a 3-bu loss, you're losing money if you don't spray.”

The bottom line for 2002: Scout often and carefully to determine if spraying is warranted. Then, act quickly if treatment criteria are met. It could mean a big difference in your soybean profit picture.

Research Trials Show Spraying Definitely Helps

Properly timed insecticide applications can cut yield losses during heavy soybean aphid infestations, say university entomologists. Here's what three of them report from 2001 trials:

  • “Based on data we're getting from consultants and farmers, the average on strip tests in 2001 was about 12.5% yield losses on the checks,” reports Dave Hogg, University of Wisconsin entomologist. “In insecticide plots at the Arlington Research Station, yield losses were more in the 15-20% range.”

  • Minnesota's Ken Ostlie says strip trials produced results that would “blow your socks off.” He looked at results from 34 sites where people had on-farm strip trials comparing treated and untreated strips. “Results vary all the way from no increase with spraying to about 17-bu/acre yield increases,” he says, “and the average is running 7-8 bu/acre. We saw a 34% increase in pod numbers when beans were sprayed in later July.”

  • Michigan State's Chris DiFonzo reports an average 40% yield reduction between sprayed and unsprayed checks across heavily infested test plots comparing 10 insecticide treatments under drought conditions. Pod numbers were reduced about 25% per plant without spraying. Average number of aphids per plant in those plots was 7,000.

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