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Concerns about Soybean Aphids Escalate

August 3, 2001
As you may recall, we first learned about soybean aphids (what we then thought were cotton aphids) at about this time last year. John Wedberg, extension entomologist at the University of Wisconsin, had reported to us that he had found some aphids in soybean plots in southern Wisconsin. We reported on their occurrence in Illinois in issue no. 20 (August 11, 2000) of the Bulletin. Last year the aphids surprised us, but this year we were prepared for their occurrence. However, being prepared hasn't made the situation any easier to evaluate this summer.

Reports of densities of soybean aphids in soybean fields in Illinois have been quite variable, with densities ranging from very few to huge numbers. In some fields, the numbers have increased dramatically over time. Les Domier, USDA-ARS plant pathologist in the Department of Crop Sciences, observed approximately 10,000 aphids per plant in a couple of fields in Winnebago County. These numbers are similar to the 13,000 aphids per plant that Chris DiFonzo, extension entomologist at Michigan State University, has observed in fields in Michigan's "thumb," where a full-scale outbreak of soybean aphids has occurred. Plants are dying as a consequence of persistent hot, dry weather and feeding by soybean aphids and twospotted spider mites. Entomologists in Minnesota report that infestations of soybean aphids are heavy in southeastern counties.

On the flip side of the coin, drastic drops in densities have occurred in fields in Illinois shortly after the appearance of a high percentage of alatoid nymphs (nymphs that will become winged adults), undoubtedly because the winged aphids left the fields. The survey team (primarily Ria Barrido and Ron Estes) coordinated by David Onstad, Department of Natural Resources and Environmental Sciences, observed a 90% drop in density in a week's time, from 260 aphids per plant to 25 aphids per plant. During that week, the proportion of alatoid nymphs in the colonies had increased from 20% to 85%. It's also entirely possible that heavy rainstorms in some areas reduced the densities of soybean aphids, a phenomenon we observed in 2000 in some fields. Finally, although densities of predators are almost as varied as densities of the aphids, in some fields predators are regulating aphid populations.

We also are aware that some soybean fields in Illinois are being treated for control of soybean aphids. In fields in which densities of aphids are very high and soil moisture is lacking, symptoms of injury have appeared--yellow, crinkled leaves and the appearance of potassium deficiency. When predators are not present or are present only in small numbers, when soybean aphid densities are increasing, and when the lack of rain is causing crop stress, an insecticide application seems justified. However, please read again how densities of aphids can decline rather suddenly, depending on the makeup of the population. If you observe a lot of aphids with "shoulder pads," you are finding alatoid nymphs, which give rise to winged adults, which probably will fly away to begin colonies elsewhere. Keep this in mind when you are trying to make a decision about whether an insecticide application is warranted.

As usual, this discussion brings us back to the question, "So when should I treat for soybean aphids?" We still hope to establish treatment guidelines soon, but it's not likely that we will have an honest-to-goodness economic threshold before the end of the season. We need research results to correlate numbers of aphids and/or amount of injury with yield losses before we can generate even preliminary economic thresholds. Without yield data, we can only guess.

And apparently that's what a lot of people are doing. I received an e-mail message from someone indicating he had heard "several different threshold recommendations. . . ." In times of uncertainty, people want some point of reference, which is understandable. I won't say that any threshold recommendation is wrong, but you have to realize that thresholds suggested right now are not necessarily correct, either. It's only guesswork.

On that note, I offer some guidelines that Chris DiFonzo has suggested. She states up front that these guidelines are based on her opinions. Nonetheless, her experience last year and this year lends more credence to her opinion than some of the ridiculously low thresholds (also based on opinions) I have heard about. Chris's guidelines for treating soybean aphids in soybean fields are

· 1,000 or more aphids per plant,

· aphids covering leaves and stems,

· honeydew and sooty mold visible on leaves,

· dry conditions (plants water-stressed), and

· pathogenic fungus not observed killing aphids.

Chris also recommends that if a field is sprayed for control of soybean aphids, an untreated check strip should be left for comparison and also as a refuge for natural enemies. We couldn't agree more. We recommend this approach for all insect control activities, just as a check on product performance, but it's rarely practiced. With a new pest, such as the soybean aphid, untreated check strips should tell us a lot.

Chris also reported early results from an insecticide efficacy trial in Michigan. R2­R3 soybeans were sprayed on July 20 when the average density of soybean aphids per plant was 6,882. By 3 days after treatment, 10% or less of the leaflets were infested in plots treated with Lorsban 4E or Penncap-M. The percentages of leaflets infested in other registered insecticide-treated plots were as follows: 38%--dimethoate, 52%--Warrior, 92%--Asana. The untreated control plot had 100% infestation 3 days after treatments were applied. To read Chris's entire article, go to http://www.msue.msu.edu/ipm/CAT01_field/FC07-26-01.htm.

We intend to conduct an insecticide efficacy trial at the University of Illinois research farm near DeKalb late this week or early next week. The trial will be designed as a split-plot, with two timings of application (early and late) as the main plots and 22 treatments as the subplots. When we have some results, we will share them with you. Likewise, when we learn from entomologists at other midwestern universities, we will share their thoughts and results.--Kevin Steffey

Author: Kevin Steffey


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

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