Soybean aphids: economic thresholds
It was much sooner than I expected, but I knew eventually someone would find soybean aphids in western Iowa. Hal Tucker (Tucker Consulting, Storm Lake) reported on June 21 that he found soybean aphids near Correctionville in Woodbury County. Brian Lang, Iowa State University Extension field specialist-crops, continues to find small populations near Decorah and also has found them at Nashua on the Northeast Research and Demonstration Farm. Now we have to begin to think seriously about scouting for this insect.
Scouting methods for the soybean aphid in Iowa have not been investigated. Therefore, procedures we use this year could have flaws. However, scouting must be conducted to determine aphid presence and abundance. In pest management, scouting information is of little use without having appropriate decision guidelines. Therefore, the following guidelines are based on the limited experience of entomologists in the Midwest regarding this insect.
If plants are covered with honeydew or sooty mold and appear stunted, an insecticide may still be of value but the optimum time for treatment would have been before these conditions became noticeable. Another question is what to do with fields that have low or moderate-sized populations of aphids. There are no clear answers but the fields should be scouted through mid-August. Heavy rains and beneficial insects may reduce low or moderate populations slightly, but insecticides may be the only option in achieving a substantial reduction if the population reaches the conditions stated above. If an insecticide is sprayed, an unsprayed test strip should be left in the field to compare and evaluate against the sprayed sections. The unsprayed test strip is needed to effectively compare the real value of the insecticide treatment and determine its performance. Data from Iowa and neighboring states show that not all insecticides provide equal levels of control. Presumably these increased aphid numbers resulted from the suppression of beneficial predators. The soybean aphid appears to rebound from some insecticides and a high level (99 percent) of control is desired. High water volume and high pressure also have been suggested as ways to improve soybean aphid control, especially in fields with a dense plant canopy. Insecticide performance data can be found in the 2001 Proceedings of the Integrated Crop Management Conference, Iowa State University. Several insecticides (Table 1) are labeled for soybean aphid (or Chinese aphid on some labels). Table 1. Insecticides and product rate per acre labeled for soybean aphids in soybean. Read and follow all label directions.
*Restricted-use insecticide. aAids in control. This article originally appeared on pages 127-128 of the IC-488(15) -- July 1, 2002 issue. Updated 06/30/2002 - 1:00pm
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