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Soybean aphids
discovered in eastern Iowa
Iowa
Soybean aphids have been found in several places in eastern Iowa on June 8, according to Marlin Rice, Iowa State University Extension entomologist.
"There is no need to panic, but it is essential that farmers, especially in eastern Iowa, check their fields for soybean aphids. Without scouting information, it will be impossible to make an informed and responsible decision regarding management," Rice said.
Brian Lang, ISU Extension crop specialist for northeastern Iowa, found one winged aphid on V1 to V2 stage soybeans near Decorah. Virgil Schmitt, ISU Extension crop specialist for eastern Iowa, received a "report of a couple of fields being sprayed for soybean aphid between Wapello and Burlington."
Also on June 8, Brian Wischmeier, NK Seeds agronomist, received information that a field near Mediapolis in southeastern Iowa had large enough populations in V4 to V6 stage, 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 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 in southeastern Iowa is much earlier than anticipated, yet there is no need to panic and start spraying fields without accurately assessing the situation.
Identifying 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 aphids
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 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 to 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, which is 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 the 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 at: www.planthealth.info/soyaphid.htm or www.ipm.iastate.edu/ipm/icm.
Date: 6/22/04
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