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What's the trigger?

FARGO, N.D. -- Both of the top North Dakota and Minnesota Extension Service insect experts in the Red River Valley say they support a 250-aphid-per-plant trigger level for spraying soybean aphids.

FARGO, N.D. -- Both of the top North Dakota and Minnesota Extension Service insect experts in the Red River Valley say they support a 250-aphid-per-plant trigger level for spraying soybean aphids.

They are Janet Knodel, North Dakota State University's extension entomologist, and her predecessor, Phil Glogoza, now a University of Minnesota area agronomist based in Moorhead, Minn.

Aphid populations in the region were lower in 2007 than in 2006, the last time the insect had a major effect. The outlook for 2008 is good, but with soybeans hitting the $14-per-bushel level, farmers are watching the horizon and making plans. There has been considerable confusion about trigger levels on aphids, mainly because of differences within the entomology community, especially in South Dakota.

Recommended levels

"The (spray) threshold we're recommending in NDSU extension is 250 aphids per plant, with increasing populations from the vegetative stage to the R-5 (growth) stage, which is when seeds are forming and filling,"

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Knodel says. "After that, when it's in full seed, or R-6, we recommend that the threshold should be increased. However, the research is under way to determine the exact number of aphids per plant."

She says the reason that isn't known is that the state hasn't had a lot of aphids infesting soybean fields that late. "We're not sure whether if you spray at R-5 that you'll see a yield benefit. We don't recommend treating R-7 or R-8, which is maturing."

Knodel says the recommendations are based on research information gathered over three years from six states -- North Dakota, Minnesota, Wisconsin, Iowa, Nebraska and Michigan. She sees the economic injury level as 660 aphids per plant, which "will cause a yield loss or economic damage." She says with the recent increase in the price of beans, she's recommending decreasing the spray interval from a seven-day window of opportunity to three or four days.

Knodel says her own research shows that plots sprayed at the 100-aphid level July 12 killed the aphids, but that the residual insect treatment didn't last long enough to kill subsequent infestations. "Part of the reason the population resurged in mid-August was because we sprayed early, killing off the beneficial insects," she says.

She says the effect of not treating and treating at the 100-aphid per plant levels showed an identical yield result -- about 50 bushels per acre.

When the spray was delayed until July 27, the residual pesticide kept the aphids in check through harvest.

"That two weeks made a big difference," she says. The yield under this scenario increased to 68 bushels per are.

"Aphids are notorious for developing insecticide resistance," she says, "especially if you use the same class of insecticide. For example, if you use Asana and Warrior, you're still exposing that insect to the same mode of action -- a pyrethroid."

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Knodel says data from Minnesota trials convinces her that "beneficial" insects have a huge impact on aphid numbers. She says where aphid predators are excluded, as in the trials of Mike Catangui, her counterpart at South Dakota State University, the aphids will multiply exponentially.

"We're dealing with farmer fields, not caged fields," she says. "We have predators moving in and out of the fields. The predators are very mobile and are keeping the population low, naturally."

Similarly, Glogoza, a former NDSU Extension Service entomologist, says the 250-aphid economic threshold was formalized in a meeting among university entomologists in Chicago in 2003, when Glogoza still was with NDSU.

"The meeting was a day, day-and-a-half, where extension entomologists working with aphid were getting together to work with trials that had been replicated for two years," Glogoza says. "One of the huge unknowns was how growth stage was influencing recommendations."

At the end of the day, the group came up with what their "best treatment guidelines would be," he says. Catangui, the South Dakota State University researcher who now recommends treatment at one to five aphids per plant, says he didn't attend that meeting because it was summertime and he was busy with field research.

By 2004 and 2005, the universities had the same protocol that was being implemented. Glogoza says infestation timing in the Red River Valley area is different than other states -- even places like southern Minnesota. It depends heavily on overwintering success, how many eggs have been laid on the buckthorn and the prevalence of buckthorn in the first place.

North Dakota and northwest Minnesota are colder and have less buckthorn he says. Quite often soybean aphids don't reach the treatment levels here. He says a level of 250 aphids has a higher predictability of breaking out and hitting the "economic injury level" of 654, than does 100 aphids per plant -- still far higher than the Catangui recommendation.

"At 100 aphids, there's no high level of confidence that you're going to get to the injury level," he says. "We've seen it time and time again, where it's at the 100-aphid level, where it stops (expanding). When you use 100 aphids, you're shooting too early."

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He says there's certainly "too much error" to suggest spraying at the one to five aphid-per-plant level. "It takes time to go from one aphid per plant to

As for 2008, Glogoza's surveys last September showed a fairly large aphid population on buckthorn but that the large storms that came through Sept. 21 to 22 contributed to a major fungal outbreak that preventned egg laying.

"That bodes well for us," he says, although he says it's possible to see migrations from other areas of the United States.

Typically, 10 percent to 15 percent of the acres in the Red River Valley and surrounding areas may need treatment.

"That would be my season outlook," he says.

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