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Numbers of Soybean Aphids Increasing

June 19, 2003

Although we have already issued an early warning about soybean aphids in the Bulletin ("Soybean Aphids on Seedling Soybeans," issue no. 11, June 6, 2003), it's important to keep you updated regarding the status of their populations. It seems that the numbers of soybean aphids are increasing in northern Illinois. Several people contacted us to report what they had found. Following are the highlights from those reports.

Joe Terando, a certified crop specialist with ConServ FS, found an average of 18 to 20 soybean aphids per plant on unifoliate-stage soybeans in Lake County on June 13. At that time, other fields in the area had spotty infestations, with as many as 20 aphids per plant. Joe also noted a relative absence of natural enemies (e.g., lady beetles). Steve Doench, agronomist with Pioneer Hi-Bred International, also found soybean aphids on V1-stage plants in a soybean seed field in Henderson County on June 13. The density of the aphids was only about one to two per leaflet. The field was near a wooded area that might include buckthorn, the overwintering host of soybean aphids.

Following are thumbnail reports of soybean aphids from other midwestern states:

  • Iowa--Marlin Rice, extension entomologist at Iowa State University, reported that soybean aphids were found on V1-stage soybeans in a field in northeastern Iowa on June 5.
  • Michigan--Chris DiFonzo, extension entomologist at Michigan State University, reported soybean aphids were found on V0-stage soybeans on the MSU campus on June 3. The numbers at that time were low.
  • Minnesota--University of Minnesota entomologists found a few soybean aphids on soybean plants near the campus on June 2.

The real concern for soybean aphids will not occur until later this summer, when soybeans begin to flower. However, early awareness of this potentially threatening pest is important. People involved with soybean production throughout the northern half of Illinois should begin scouting now and record the activity of soybean aphid populations in individual fields. From one week to the next, you should be able to determine whether densities are increasing or decreasing and the rate of change.

Small colony of soybean aphids (nymphs and wingless adults) on a soybean leaf. (Photo courtesy of David Voegtlin, Illinois Natural History Survey.)

Action thresholds for soybean aphids vary somewhat throughout the Midwest. The threshold of concern in Illinois is 25 soybean aphids per leaflet. However, treating too early can create more problems than it would solve. Trials conducted in other states show clearly that soybean infestations resurge in treated fields because the insecticide killed all of the natural enemies. The reproductive capacity of aphids is remarkable, so their numbers can increase quickly in the absence of natural enemies. So, while keeping track of the population dynamics of soybean aphids, don't overreact. Remember, the critical stages of soybean growth are late-V through early-R.

For more information about soybean aphids (description, life cycle, symptoms of plant damage, monitoring, management), refer to National Soybean Research Laboratory Fact Sheet #4, which can be found on the Web here. For a list of resources regarding soybean aphids, go here.--Kevin Steffey

Author: Kevin Steffey

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

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