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Problems with Soybean Aphids South of I-80

August 11, 2003

By now, everyone probably is aware of the heavy infestations of soybean aphids that have occurred in Illinois north of I-80. As a consequence of the heavy infestations, many winged aphids have been produced, and they have been flying around quite a bit. Check out the most recent counts from the Illinois Suction Trap Network here.
Most of us would agree that a week's capture of 6,755 soybean aphids, in the trap near DeKalb, is notable. The numbers of aphids captured in some of the other traps also have increased, although not quite so dramatically.

During the week of August 4, we began to receive many reports of rather heavy infestations of soybean aphids in counties south of I-80, primarily Champaign, Ford, Kankakee, Iroquois, and Livingston counties on the eastern side of the state. However, we also have received reports of winged aphids in soybean fields in Adams, Brown, Fulton, Hancock, Henderson, Knox, McDonough, Mercer, Peoria, Pike, Schuyler, Stark, and Warren counties on the western side of the state. The numbers have been small in most fields, but they bear watching. In addition, numbers of soybean aphids in southern counties have begun to increase. Following is information about some fields surveyed by Dr. David Voegtlin's crew on August 7. I provide the percentage of plants infested (only 10 plants checked) and the average number of aphids per infested plant:

  • Bond County: Field 1--100%, 11; Field 2--100%, 26.9
  • Clay County: Field 1--40%, 1; Field 2--50%, 4.1
  • Clinton County: Field 1--90%, 15.6; Field 2--100%, 53.6
  • Effingham County: Field 1--60%, 1.1; Field 2--100%, 19.3
  • Fayette County: Field 1--40%, 2; Field 2--90%, 8.4; Field 3--90%, 45.1
  • Marion County: Field 1--100%, 16.6; Field 2--100%, 14.3
  • Richland County: Field 1--60%, 11.7; Field 2--0%

Although the numbers are not particularly alarming, they are worth noting as benchmarks for the area. It's possible that the numbers of aphids could increase very quickly in these counties, so monitoring should begin immediately. Other notes from these southern Illinois fields:

  • The plants ranged from growth stages V6 to V9.
  • Alates (winged adults) were present in most fields, indicating that the aphids had immigrated into the field recently.
  • Few natural enemies were noted.

Most of the questions I have received from several individuals recently have been similar, so I am providing the answers I have given to these questions. Please don't hesitate to contact me if you have additional questions or want to provide a report from your area.

What is the economic threshold for soybean aphids infesting soybean fields? Our monitoring guidelines and economic threshold are explained in the "Soybean Aphid" fact sheet here. Basically, we have used an average of 25 aphids per middle leaflet (remember, there are three leaflets per trifoliate) as an economic threshold, with caveats regarding the presence of alatoid nymphs and natural enemies. However, when the aphids are distributed on stems and pods, as well as leaves, a per-leaflet threshold may not be as useful. So we have checked with entomologists at the University of Minnesota, who have had more consistent experience with soybean aphids. We are recommending their threshold of an average of 250 or more soybean aphids per plant.

What insecticides can be used to control soybean aphids? The insecticides suggested for control of soybean aphids are *Asana XL at 5.9 to 9.6 oz per acre; *Furadan 4F at 1/2 pt per acre; *Lorsban 4E at 1 to 2 pt per acre; *Mustang Max at 2.8 to 4 oz per acre; *Penncap-M at 1 to 3 pt per acre; and *Warrior at 1.92 to 3.2 oz per acre. Use of products preceded by an asterisk is restricted to certified applicators. Also, please note that insecticides should not be applied at a time when bees are actively visiting flowering soybeans.

What insecticides are most effective against soybean aphids, and which ones provide the longest residual control? Data from our efficacy trials in 2001 and 2002 suggest that all of the products previously listed are effective against soybean aphids for as long as 14 days. Entomologists at the University of Minnesota have stated that the pyrethroids have provided the longest residual control. However, our data indicate that the organophosphates and carbamates have provided the same residual control as the pyrethroids.

Which is more effective, aerial or ground application? The concern seems to be coverage related to gallons per acre. However, the experience we have had thus far indicates that aerial applicators are doing a good job getting the insecticides to the aphids. We have no evidence to suggest that one type of application is better than another.

We will continue to provide updates as we learn more over the next couple of weeks. Your help with reports from the field will enhance our information. Thanks for your cooperation and support thus far.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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