By Kevin Steffey, University of Illinois

As has been the case throughout most of the Midwest, the high temperatures this summer have suppressed the development of soybean aphid populations. However, with projected temperatures in the 80s over the next week, small densities of soybean aphids that have been "lying in wait" could develop into potentially threatening numbers.

Consequently, we strongly encourage people to continue scouting soybean fields diligently. Soybean aphids are most likely to cause economic yield losses when infestations reach critical levels during the R1 through R5 stages of soybean development.

However, during a conversation I had recently with Dr. David Ragsdale, research entomologist at the University of Minnesota, I learned that yield losses associated with infestations of soybean aphids at the R6 stage of development have been documented in a few northern states. These situations involved enormous numbers of aphids per plant, but they confirm that soybean aphids can threaten soybean yields well into August.

Over the past three weeks, we have watched densities of soybean aphids slowly increase in 26 fields in Woodford, Marshall, Putnam, Bureau, Lee, Whiteside, Ogle, and Stephenson counties. The average numbers of soybean aphids per plant (20 plants sampled) in the 26 fields are shown in Table 1.

Average densities of aphids increased from the July 17-18 sampling period to the July 25-26 sampling period to the August 1-2 sampling period in 18 of the 26 fields sampled. From the July 25-26 sampling period to the August 1-2 sampling period, average densities of aphids increased in 22 of the 26 fields sampled. As of August 2, the largest density of soybean aphids encountered was 102.5 aphids per plant (Stephenson 5), a 10-fold increase from July 26.

Depending on temperature and some other factors, soybean aphid population densities can double in two to three days. Applying simple math to the densities of soybean aphids in Table 1 reveals that densities of approximately 20 or more aphids per plant on August 1 or 2 could build to economic threshold levels within 7 to 10 days after the last sampling date.

Keep in mind, however, that other factors (e.g., multicolored Asian lady beetles and other predators) may continue to suppress population growth of soybean aphids. Nonetheless, it is possible that numbers of soybean aphids could reach economic threshold levels in some fields in northern Illinois before the soybeans reach the R5 development stage.

As we have stated many times, the economic threshold for soybean aphids is 250 aphids per plant, with 80 percent or more of the plants infested. Keep in mind that the economic threshold is conservative, well below the economic injury level of about 1,000 aphids per plant. (The economic injury level is the density of soybean aphids [in this case] required to cause yield loss equal to the cost of control.)

Given the amount of time required for a soybean aphid population to double in size, the economic threshold of 250 aphids per plant allows for about seven days before numbers of soybean aphids reach the economic injury level.

The economic threshold for soybean aphids was developed from robust research, which is explained very well in David Ragsdale's paper published in the Proceedings of the 2006 Illinois Crop Protection Technology Conference. If you don't have a copy of the proceedings, you can access the article online (Adobe PDF).

The relationship between the economic threshold and soybean aphid population growth is explained on page 107. The full citation for the article follows: Ragsdale, D.W., E.W. Hodgson, B.P. McCornack, K.A. Koch, R.C. Venette, and B.D. Potter. 2006. Soybean aphid and the challenge of integrating recommendations within an IPM system. In Proceedings of the 2006 Illinois Crop Protection Technology Conference. University of Illinois Extension, Urbana-Champaign, pp. 103-110.

It is possible that densities of soybean aphids have already exceeded the economic threshold in some scattered fields in Illinois; we have received a couple of reports of large numbers of soybean aphids. If diligent scouting indicates that an insecticide application may be warranted (which should not be the case for most fields), pay particular attention to the growth stage of the soybean plants.

A return on investment for an insecticide application is unlikely after the soybeans reach the R6 stage of development. Products suggested for control of soybean aphids are listed in Table 2. Consider the preharvest interval when selecting an insecticide, and follow all directions and precautions.

SOURCE: The Bulletin newsletter from University of Illinois, No. 20 Article 1/Aug. 11, 2006.