No. 19 Article 3/August 3, 2007

Soybean Aphids Attract Attention in Several Areas of Illinois

Economic infestations of soybean aphids have erupted in several areas of Illinois over the past week and a half. We have received several reports of insecticide applications in northwestern, western, central, northeastern, and eastern Illinois. Memories of the outbreak in 2003 had soybean growers and agricultural professionals on the alert. However, although the outbreak in Illinois seems to be rather widespread, it's important to note that not all soybean fields harbor economic levels of soybean aphids. Our own survey data from 26 commercial soybean fields from Woodford County in north-central Illinois to Stephenson County in northwestern Illinois indicate a wide range of densities, only a few of which have near-threshold or greater-than-threshold numbers of aphids. As of August 1, only one of the 23 fields that had been sampled had a density greater than the economic threshold of 250 aphids per plant--Stephenson 6 had 324.65 (Table 1). Stephenson 3 and 10 and Woodford 6 all had near-threshold densities of soybean aphids--231.6, 178.75, and 175.65 aphids per plant, respectively. Three other fields had average densities of greater than 100 aphids per plant. However, note that the field in Bureau County had an extremely low density of 0.2 aphid per plant. But with few exceptions, the average density of soybean aphids increased noticeably from July 23 and 24 to July 30 and 31.

Soybean aphids on a soybean plant from a Stephenson County field, July 26, 2007 (photo courtesy of Jim Morrison, University of Illinois Extension).

University of Illinois Extension educators have been monitoring Asian soybean rust sentinel plots regularly for soybean aphids, and they too have reported significant increases in densities of soybean aphids over the past couple of weeks. Although the most recent sampling data from sentinel plots had not been entered into the PIPE database as of August 1, several educators have reported greater-than-threshold or near-threshold numbers of soybean aphids in sentinel plots. The most intriguing report was from Robert Bellm, who is monitoring soybean aphids in sentinel plots in Jersey and Madison counties in southern Illinois. He has found virtually no aphids all season in the Madison County plot, but he noted a remarkable increase (from about 0.5 to more 200 aphids per plant) in the Jersey County plot from July 24 to July 31. He has been sampling in different areas of the plot over time, so it's important to note that the selection of the area to be sampled can dramatically influence the results.

Most observers have found very few predators (e.g., multi-colored Asian lady beetle) in soybean aphid-infested fields, although there have been exceptions. However, regarding the assessment of soybean aphid infestations over the next couple of weeks, please take note of the activity of natural enemies, which should increase as densities of aphids increase. Also, with the projected hot temperatures, soybean aphid population growth will slow down and possibly stop. The dual regulatory action of natural enemies and high temperatures could suppress populations of soybean aphids in many fields below economic levels. If you monitor aphid populations over time, as you should, note whether the density increases from one sampling date to the next. If no population growth occurs over a week or so, an insecticide may not be necessary.

If you can find the time, it might be worthwhile to review information about the economic threshold for soybean aphids. As part of a distance education program ("Managing Soybean Aphids in 2007: How Will Biological Control Contribute?") delivered on March 6, 2007, Dr. David Ragsdale, research entomologist at the University of Minnesota, discussed economic thresholds in his presentation "Review of the Situation With Soybean Aphids in the Midwest." You can view the video for yourself on the North Central Region IPM Center web site.

In a nutshell, Dave emphasized the economic threshold of 250 aphids per plant (it's actually 273 ± 19), compared with the economic injury level (cost of control = value of yield loss) of 674 ± 47 soybean aphids per plant. These values were derived from a range of costs of control, values of soybeans per bushel, and yields of soybeans (bushels per acre). The numbers used to calculate the economic threshold and economic injury level, however, did not include the current higher value for soybeans nor the high yields that are anticipated. Because of these higher values, some people have decided to apply an insecticide to lower-than-threshold densities of soybean aphids. It's hard to argue with the logic, but keep in mind that the threshold allows for about a week before the soybean aphid density builds to the economic injury level. And it's the economic injury level that is most directly affected by higher crop values and higher yields. Consequently, the conservative threshold of 250 aphids per plant is still appropriate. This is particularly true as it relates to population growth being suppressed by natural enemies and high temperatures.

Erecting a field cage for future release of the parasitic wasp Binodoxys communis, DeKalb County, June 28, 2007 (photo courtesy of Gary Bretthauer, University of Illinois Extension).

As a side note, you may recall that we wrote a Bulletin article (issue no. 14, June 29, 2007) about releasing a parasitic wasp, Binodoxys communis, in Illinois and elsewhere this season. One of the fields we have been monitoring in preparation for this release in Illinois is in northern Mercer County. The average density of aphids in this field increased significantly over the past 3 weeks, from nearly zero on July 5 to an average of 287 aphids per plant on July 30, so we made our first release of the parasitoids on July 30. Parasitized aphids ("mummies") from Dr. David Voegtlin's lab were placed inside a field cage over about 30 soybean plants infested with soybean aphids, and the cage was closed to keep out predators and allow the aphid population to build inside the cage. The cage will be opened to encourage the spread of the parasitoids into the field. We will apprise you of the progress of this event for the rest of the season and even into the fall, when winged soybean aphids return to their primary host, buckthorn. With any luck, the parasitoid will show up in overwintering populations this winter and in field populations beyond the release site next year. We also plan releases in both DeKalb and Champaign counties over the next week or so. Stay tuned.--Kevin Steffey

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