Soybean aphids first invaded fields in part of the Midwest during the 2000 growing season.

This year, they were back and have moved into Iowa and Minnesota.

Aphids have been found across the entire eastern half of Iowa and as far west as Buena Vista County, according to Iowa State University (ISU) reports. In Minnesota, aphids have been found as far north as the Twin Cities and into many central soybean-producing counties.

"Last year, I only saw a couple of fields with aphids in southeastern Minnesota. This year, I have seen significantly more," says Chris Cook, Area agronomist for Garst Seed Co., in New Ulm, MN. "This pest is new to our region, so farmers, agronomists and university researchers are quickly trying to learn more about them."

Aphids, a native pest to Southeast Asia and China, are found on the underside of leaves in mature plants and can range from a few, to more than 1,000 per plant. The aphid is pale yellow with black-tipped tubes, called comicles, on the back of its abdomen.

These pests suck the sap from soybeans, which can stunt the growth of the plant. Symptoms include black mold on leaves, in the lower canopy, and yellowing of the edges of leaves. Soybean aphids also have the potential to transmit soybean mosaic virus throughout a field.

Fields that are stressed by drought conditions seem to be most susceptible to soybean aphids. According to information ISU researchers received from China, aphids usually peak at flowering, decline for a time, then peak again in early September, when soybeans are entering crucial stages of pod development and pod fill.

Research conducted at the University of Wisconsin says heavy infestation of soybean aphids can reduce yields by 10 to 15 bushels per acre. Aphids also cause losses of five to 10 bushels per acre from physiological plant damage. If aphids spread viruses, those can cause another five bushels per acre loss.

Cook says quick ways of scouting for aphids includes simple observations of yellow plants, lady beetles (ladybugs) or an abundance of ants.

Under severe infestations of aphids, yellowing of the plant tissue between the leaf veins and stunting of the plant can occur. However, other plant health problems, such as soybean cyst nematode (SCN), potassium deficiency, sudden death syndrome and brown stem rot, can cause similar appearances.

"The most common predator of aphids is the lady beetle," Cook says. "If walking through a field of soybeans and you discover a lady beetle, start observing the plant and often aphids will be found."

Ants also will be in abundance when aphids are present, due to the honeydew that aphids leave behind.

The big question is what to do with fields that have very large populations of aphids. ISU information says a major concern is estimating aphid population size and determining an economic threshold or treatment level. Because this is the first year of the soybean aphid infestation in Iowa and Minnesota, researchers have very little information to support any formal recommendations for farmers.

The best that can be done is to develop a nominal threshold, which is a threshold based on the subjective determinations of a person's experience.

Heavy rains and beneficial insects may reduce large populations slightly, but insecticides often are the only option, in achieving a substantial population reduction. However, there is no threshold established on when a farmer should spray based on the number of aphids per leaf.

Scott Johnson, area agronomist for Garst Seed Co., in Garwin, IA, says spraying is most effective early in the growing season. "At this time in the growing season, spraying would have varied effectiveness, because the soybeans are very tall and it would be hard to get under the soybean canopy."

Farmers probably wouldn't have a chance to spray if a second aphid peak occurs, because of harvest restrictions on insecticides. Plus, they wouldn't want to risk more yield loss by crossing the field with equipment and damaging plants.

Jerry Schoenthal, a farmer and Garst-AgriPro seed salesperson, in northeast Iowa, found significant aphid populations in mid-July. "We started to see the pests when we scouted fields and were able to treat fields with the insecticide, Warrior." Schoenthal says he treated fields at varying rates and will be eager to note yield differences at harvest.

A University of Wisconsin soybean insect and virus research project last year found that farmers spraying insecticides had significant yield gains--up to five to eight bushels per acre greater--compared to test strips where no treatment was used.

A farmer had a problem with aphids this year, it is good to be aware of the potential problem next year, but don't count on the pests making a return visit.

"Aphids have a great ability to move long distances in the wind," says Rice. "If you had a problem this year, it is not necessarily an indication that you will have a problem next year."

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