Monday, August 19, 2013

Watch for Soybean Aphids.

Doug Johnson, Extension Entomologist

Figure 1. Soybean aphid on soybean.
Soybean aphid has been found in Kentucky since its introduction into the US. Nevertheless, it has not developed into a pest species for Kentucky grown soybeans. This is attributed largely to two factors: 1.) We do not have the plant host for the overwintering stage, meaning the aphid must migrate annually into Kentucky from more northern states and 2.) Soybean aphid populations do not do well in the hot temperatures of a Kentucky July and August. So, in a typical year this pest does not pose much of a risk.

The spring and summer of 2013, however, have been anything but the typical Kentucky weather. It has been wetter, and perhaps more importantly, cooler than our average year. In addition, this has been true of states to the north of KY. For Example, my colleagues in Ohio, Iowa, Indiana, and Illinois, among others, have reported rising and, in some cases, economic populations in some soybean fields. Dr. Mike Grey at the University of Illinois is encouraging Illinois producers to scout for soybean aphid populations over the next several weeks.

I doubt that this is a major outbreak for Kentucky. Even so, with the cool weather we have experienced, the large amount of late planted soybeans, and the increased activity to our north, I believe it is prudent to pay more attention to this pest than we have in the past.

Soybean aphids are small, pale to bright yellow, soft bodied, pear shaped insects. Aphids have a pair of black cornicles (tail pipe looking structures) sticking out the rear end. You may see some small white aphids, this is just a color variation. You might see the occasional single cotton/melon aphid but soybean aphid is the only aphid that colonizes soybeans in the US.

The economic threshold for soybean aphid has not changed. It is: 250 aphids per plant, with 80% of plants infested, and an increasing population (this means at least two measures must be taken over time). Direct plant observation is the best method of detecting the soybean aphid and gives a good measure of population growth. Unfortunately, it is very time consuming. Using the “Speed Scouting” method may be better for making a control decision. This is a relatively quick and physically easier method of decision making, but does not give quite the picture of how the population is growing / declining. You may obtain a copy of the Speed Scouting form at:

On this page look toward the upper right hand side under “Resources” for the 2007 Speed Scouting Worksheet. You can download this and make copies for use in the field.

Should it become necessary, pesticides for control of soybean aphid may be found in ENT-13 Insecticide Recommendations for Soybeans. This is available at:
or you may get a copy from your local County Extension Office.

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