The discovery of soybean aphids infesting some fields in eastern Iowa on June 8 is a signal. Now is the time to start scouting your bean fields closely and regularly for this potentially destructive pest. If an infestation increases to high enough levels, it will pay to spray an insecticide.
"There is no need to panic. But it is essential that farmers, especially in eastern Iowa, check their fields for soybean aphids," says Marlin Rice, extension entomologist at Iowa State University. "Without scouting information, it will be impossible to make an informed and responsible decision regarding how to manage this pest."
Brian Lang, ISU extension crop specialist for northeast Iowa, found one winged aphid on V1-V2 stage soybeans near Decorah on June 8. Virgil Schmitt, ISU extension crop specialist in southeast Iowa, received a report of a couple of fields being sprayed for soybean aphid between Wapello and Burlington.
Aphid activity much earlier than anticipated
Also on June 8, Brian Wischmeier, NK Seeds agronomist, received information that a field near Mediapolis in southeast Iowa had large enough populations in V4-V6 growth stage of soybeans (14-inch soybeans) that honeydew was collecting on the pant legs of the farmer as he walked the field.
Doug Tinnes, NK Seeds sales agronomist, confirmed this report. He scouted the field and noted that in a 140-acre field, the plants consistently had 100 aphids per plant and one large area of the field had 200-250 aphids per plant. In areas of the field where the aphid population was the heaviest, he could see that the plants were noticeably stunted. He observed that there "were very few" lady beetles in the field feeding on the aphids.
"This soybean aphid activity this spring in southeast Iowa is much earlier than anticipated, yet there is no need to panic and start spraying fields without accurately assessing the situation," says Rice.
How to identify soybean aphids
Soybean aphids can be found as both winged and wingless forms on soybeans. Wingless soybean aphid adults are about 1/16 inch in length, pale yellow or green, and have dark-tipped cornicles (tail pipes) near the end of the abdomen.
Aphids feed through piercing-sucking mouthparts. The winged form has a shiny black head and thorax with a dark green abdomen and black cornicles. The soybean aphid is the only aphid in North America that will reproduce on soybeans. Therefore, any small colony of aphids found on soybeans must be soybean aphids.
Scouting for aphidsâ€”what to look for
When scouting fields, check the upper two to three trifoliate leaves for aphids. Scout five locations per 20 acres. Field observations should be made weekly. Aphids are most likely to concentrate in the plant terminal. Look for ants or lady beetles on the soybean plant.
They are good indicators of the presence of aphids. Lady beetles feed on aphids while ants tend the aphids and "milk" them for honeydew.
Estimate the aphid population size per plant. The best that can be done is to count all the aphids on several leaves and plant terminals to establish what 100 or 250 aphids look like and then use this as a mental reference for gauging populations on other plants.
Use an economic threshold of 250 aphids per plant if the population is increasing and plants are in the late vegetative or early (R1-R4) reproduction stages. This economic threshold incorporates a seven-day lead-time before the aphid population would be expected to increase to 1,000 aphids per plant.
Use economic threshold to decide when to spray
That's the economic injury level and the population size that would be expected to cause economic damage (i.e., yield loss that exceeds the cost of control). This recommendation is based on research conducted by entomologists at University of Minnesota and has been adopted by other university entomologists throughout most of the Midwest.
ISU extension entomologists and field crop specialists will be monitoring the aphid populations and will issue updates on management considerations of this pest over the next several weeks.
For additional resources on aphids can be found on the Web at: www.planthealth.info/soyaphid.htm or www.ipm.iastate.edu/ipm/icm/.