Midwest growers hit by the soybean aphid plague last summer should be on guard for recurrences this summer. Agronomy experts have learned from the past four years of aphid outbreaks that soybean fields must be carefully scouted for the pest. If a population explosion appears imminent, spraying will halt soybean damage.

The northern Corn Belt is the area most likely to be pestered by the insects, according to Alan Scott, Pioneer Hi-Bred technical information manager. This area includes the north-central and Great Lakes area where plenty of buckthorn thrives for aphid overwintering. Since last fall, the aphid eggs have hidden in these bushy areas, waiting for spring weather so they can hatch and move again into soybean fields.

Some of the overwintering eggs may have perished. Scott says aphid eggs do not survive well in cold winter temperatures. By late June and early July, growers will be able to detect how well the aphids handled the winter.

Once in a field under optimum conditions, aphid populations can double every two days because aphids have a “massive propensity to reproduce,” Scott explains. The tiny green insects are born female, pregnant, and able to produce 15 or more generations in one summer.

High temperatures can slow the swelling population. Aphids stop reproducing at 95°F and begin to suffer shortened life spans. Predators like the Asian lady beetles and parasitic wasps also keep small outbreaks under control.

Control plans

Growers can take several steps to keep aphids from ruining a soybean crop. Scott suggests planting early to try and avoid some of the heaviest aphid infestation when the plant is most vulnerable.

Otherwise, the best route is to scout the aphids throughout the summer and apply insecticide when populations grow. The trick is to consider treatments before populations reach levels that cause yield loss, which can begin at 100 to 300 aphids/plant, according to Scott.

You should start scouting in June when aphids seek nitrogen in the upper foliage of the plant. Keep weekly plant counts. Spraying now generally is not beneficial because reinfestation will occur.

In mid-season's early reproductive (R) stages, you should scout every two to four days and note aphid counts per plant. “The 50 to 100 aphids/plant number is important,” Scott says. “Predators can only hold down aphids at 50/plant. So the population is poised for an explosion. Within seven days, you can have thousands of aphids. So scouting and monitoring the population at these early R stages is important.”

Spraying choices

At this point, you need to spray unless the weather comes to your aid. Scott says a heavy rain can knock aphids off the plants. And if the higher aphid counts occur in late August, the population may become winged and move out of the field to overwinter in buckthorn.

If spraying is warranted, sooner is better than later, Scott adds. Pioneer's on-farm strip trials show delayed insecticide application will cut yields. In one 2003 test, a strip sprayed on July 31 yielded 13 bu./acre more than the strips sprayed August 8 and August 13. The fields were sprayed from the air. But Scott adds that researchers do not see a difference in yield response based on how the spraying is accomplished.

Many insecticides are on the market for treatment of soybean aphids. Scott notes, “Pyrethroids seem to work better in cooler times, and organophosphates tend to work better late in the year for a kill down.”

Field trials at Pioneer Hi-Bred showed about an 8 to 10 bu./acre yield advantage when fields were treated for aphids. With treatment costs around $8 to $14/acre, growers will find a definite economic advantage to spraying healthy soybean fields to control aphids.