RALEIGH, N.C. -- University Cooperative Extension entomologists across the Midwest are urging producers to scout fields carefully for soybean aphid numbers. Two questions overhanging soybean fields like summer thunderheads are: Will this be an outbreak year? Are thresholds for treatment too high?

Typically 2009 would be a year for stronger soybean aphid pressure.

"Entomologists have observed an every-other-year pattern in soybean aphid outbreaks," said Brian Ahrens, Northern Regional Manager for MANA crop protection. "Heavy pressure in 2003, 2005 and 2007 set the pattern. But then instead of being an off-year, 2008 produced quite a bit of aphid activity as well, so it's uncertain whether the usual pattern will repeat.

"Soybean aphids often start out in wooded borders containing buckthorn, and a lot of buckthorn has been seen in Minnesota, Michigan and Wisconsin. By now the aphids may have moved into the crops."

In late June Extension reports throughout the North Central Region (Iowa, South Dakota, Michigan, Minnesota, Ohio, and Wisconsin) indicated the presence of soybean aphids. Iowa State University urged growers to also watch for winged adult aphids, which often migrate to other fields. Moderate temperatures and humidity are conducive to aphid populations building rapidly.

Every soybean field should be scouted frequently, every three to four days in July. Treatment with insecticide traditionally is not recommended until populations reach an average 250 aphids per plant, with 80 percent of plants infested, according to the University of Nebraska-Lincoln Extension. The University of Minnesota reports some fields already have reached threshold.

There are some who question that threshold. Benjamin Kantack, South Dakota State University professor emeritus of entomology and plant science, has written that the 250 soybean aphid threshold "is untenable and defies logic and common sense."

His argument is that expecting growers to wait until 250 aphids per plant assumes that the economic injury level is 650 aphids per plant a week later. But, Kantack writes, in a week the aphids will be at 1,000 aphids per plant. And he notes that it's often difficult for a grower to get insecticide sprayed within a week when his neighbors also want applications, even if the South Dakota wind cooperates with applicators.

Mike Catangui, Extension entomologist at SDSU, noted that it's possible aphid populations actually double in a day under certain conditions, not the three days considered typical. Also, SDSU 2008 research plots showed that spraying for aphids at R2 increased yields by up to 28.9 bu/acre in 8-replication trials of many spray options conducted at two locations. Waiting until R5 cost 16 bu/acre. So early spraying at lower thresholds may produce a sizable return on investment.

SDSU has posted charts that relate economic injury level (EIL) thresholds to soybean yield potential, market price, and stage of soybean development. For example, for a field that produces 45 bu/acre, with a market price of $7, and an application cost of $10, the EIL would be 4.9 aphids per plant at R2, 103.5 at R-4, and 294.4 at R5. By R5 the bell curve of aphid population buildup tapers off and no further treatment is recommended thereafter, Catangui explained.

To help preserve honeybee populations, UNL suggests a pyrethroid can be a good choice of insecticide.

"MANA has a fourth-generation pyrethroid, Silencer, which contains Lamda-Cyhalothrin, the world's leading broad-spectrum insecticide active ingredient," said Ahrens. "Silencer provides extended residual control and resistance to ultraviolet light breakdown.

"MANA's Adjourn(R) contains the pyrethroid esfenvalerate, the same active as Asana(R) XL. Adjourn is an excellent choice in areas prone to periods of rain alternating with intense sunshine because it is resistant to both wash-off and to UV degradation. This kind of residual activity is important with mobile insects like aphids."

Ahrens said that another choice, MANA Alias(R) 4F, is a systemic insecticide that provides long-lasting control of aphids, as well as bean leaf beetle, Japanese beetles, leafhoppers, and whiteflies.

"Alias 4F currently is the only post-patent four-pound imidacloprid product that has a foliar soybean label. If the aphids continue late in the season, growers who need a short pre-harvest interval will find the 7-day soybeans PHI of Alias 4F a good fit," he noted.

Ahrens said many producers still opt for the very effective knockdown by Chlorpyrifos when aphids reach threshold because it can move laterally short distances under the crop canopy to get aphids not hit directly with the spray. Chlorpyrifos is not as hard on beneficial insects as some other insecticides. Chlorpyrifos also can be tank mixed with some widely used post-emergence herbicides and some fungicides.

UNL also suggested that if soybean rust is present along with aphids, a tank mix of insecticide and fungicide may be an option because both require high water pressure for adequate penetration and coverage. MANA's Bumper(R) fungicide provides preventive and curative activity against rusts, along with frogeye leaf spot, brown spot, and anthracnose.

"Once aphids reach threshold, there's no time to waste. Populations double rapidly and within a week will hit levels that cause serious yield loss," said Ahrens. "Sometimes you can tell whether a plant is infested because of the ants crawling in the sticky honeydew and sooty mold that indicate aphids. If possible, aphids need to be controlled before honeydew, mold, cupped leaves, stunted plants and other indications start appearing in a field."

Speed Sampling is the preferred scouting method these days. Extension entomologists recommend examining the upper trifoliate leaves on 11 plants about 30 paces apart. A plant is considered to be infested if it has 40 aphids or more, but you don't have to count beyond 40 on any one plant, which makes it easier to keep moving through the field. If 100 percent of the 11 plants are scored as infested, consider spraying. If there is uncertainly, scout additional sets of five plants up to 20 more plants. If 80 percent of the additional plants are infested, consider spraying.

"Timing an insecticide spray at 250 aphids per plant, or before if you use South Dakota recommendations, should reduce the amount and subsequent cost of a soybean aphid management program, while providing sufficient protection," said Ahrens. "Using MANA post-patent insecticides will further reduce the cost while providing excellent control."

An important consideration when treating fields for soybean aphid is insect resistance management.

"It's a good idea to rotate active ingredient modes of action," says Ahrens. "For example, you might use a pyrethroid like Silencer or Adjourn one time and an organophosphate like Chlorpyrifos the next time. MANA also has a special label that allows a tank mix of Silencer and Chlorpyrifos, which combines two products with different modes of action."

MANA, based in Raleigh, NC, and its parent and sister companies throughout the worl, are seventh largest among agrichemical companies, with more than 60 years of expertise.