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Researcher: Soybean aphids continue to cause problems for farmers

"By 2003, aphids had spread all across Iowa and have been known to reduce yields by as much as 50 percent," said Iowa State University graduate research assistant in entomology Kevin Johnson.

By Jennifer Bremer

APHIDS--Iowa State University graduate research assistant in entomology Kevin Johnson discusses his research on soybean aphids at the Integrated Crop Management conference held recently in Ames. (Journal photo by Jennifer Bremer.)

Soybean aphids have been a yield-reducing problem for the past several years throughout the Midwest.

Iowa State University graduate research assistant in entomology Kevin Johnson has been studying management of soybean aphids for the past five years.

Soybean farmers in Iowa and the Midwest were introduced to soybean aphids in 2000. "By 2003, aphids had spread all across Iowa and have been known to reduce yields by as much as 50 percent," said Johnson. "It can be quite devastating."

Aphids are phloem or sap-feeding pests. Once aphids start feeding on soybeans, they produce a sooty mold on the leaves which can block photosynthesis and increases the chance for viruses to attack the plants, thus leading to reduction in production and yield.

"We continue to do research on soybean aphids because it continues to be a problem for farmers," said Johnson.

Cummulative aphid days

Iowa State researchers continue to study different soybean aphid treatments and the best time to treat for the pest.

"We call it cumulative aphid days, which is a good way to visualize the treatment differences," he said. "It is analogous to degree days in corn. It's the measure of a plant's exposure to soybean aphids."

Estimating cumulative aphid days is based on the number of aphids per plant counted on each sampling date. The exposure of soybean plants to aphids between two sampling dates is calculated with the equation:

Aphid days = [mean aphids/plants at previous date + current mean aphids/plant/2] X number of days between sampling.

Summing the aphid days accumulated during the growing season provides a measure of the total aphid exposure that a soybean plant experienced, according to Johnson.

Aphid lifecycle

Soybean aphids have a complex lifecycle with sexual stages found on the primary host plant and asexual stages occurring on the secondary host plant--the soybean. Soybean aphids migrate from soybean fields, where a sexually reproductive generation produces eggs that over-winter on other plants.

In the spring, these eggs hatch and eventually produce winged adults that migrate back to soybean fields, arriving generally in early to mid-June. Soybean aphids reproduce asexually while on soybeans, increasing their numbers rapidly.

Natural enemies can help suppress aphid populations, but chemical application is needed nearly every time, according to Johnson.

Aphid activity

Johnson and his fellow researchers have been tracking the activity of the aphids for several years. In the past two years, activity has been a bit different. In 2007, the aphids arrived in Iowa on June 11, but the highest activity wasn't until August 13. In 2008, the aphids arrived on July 15 and had the highest activity on August 22.

"If we can reduce the number of aphids present, we can increase the yields," he added.

Johnson said his research has shown that seed treatment for soybean aphids isn't good enough to prevent yield loss in a bad aphid year.

"Scouting is a huge key to preventing losses in yield," he said. "The timing of chemical application is more important than the product selected, especially when the 250 aphid threshold is hit."

Johnson said even multiple applications of insecticides may not make a difference if the application is not done at the right time.

The size of the droplets of the insecticide can make a difference too. Medium to fine droplets of insecticide can mean about two bushels per acre better yields and also makes it easier to kill the aphids.

"Improved coverage means improved control, which means improved yields," said Johnson.

While aphid-resistant soybean varieties are on the horizon, Johnson warns they won't necessarily be aphid free and aphids will likely eventually become resistant to the variety.

"Overall, with proper scouting and insecticide application, yield losses from soybean aphids can be minimized tremendously," he concluded.

Jennifer Bremer can be reached by phone at 515-833-2120 or by e-mail at jbremermaj@hotmail.com.

4 Star NE\1-B

Date: 12/30/08

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