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Natural Controls Suppress Soybean Aphid Populations

July 27, 2001
We have received reports from the University of Illinois soybean aphid monitoring team of soybean aphids in fields located in Pope and Hardin counties. Earlier monitoring efforts did not find soybean aphids in Pope County, but these two counties have now been added to the growing list of counties with confirmed soybean aphid populations. Craig Kilby, Golden Harvest Seed Company, observed soybean aphids on young trifoliates and stems (10 to 12 inches aboveground) in a field located in southeastern Tazewell County. We have received similar reports from other states, so in addition to checking the newest trifoliates, check stems and lower leaves. With the recent heavy rains in some areas, soybean aphid populations on the upper portion of the plants may have been washed away.

On July 24, Ria Barrido reported more than 20,000 aphids on 50 plants in a field in LaSalle County but no noticeable signs of injury to the plants. A total of 68 predators, including Orius (aka minute pirate bugs) and lady bug adults and larvae, were also found on the 50 plants sampled. It appears that populations of natural enemies build up slowly, and their presence may not be readily apparent when aphid populations are low.

Ladybug larva feeding on soybean aphids. (Photo courtesy of Kevin Black.)

Kevin Black, Growmark, reported soybean aphid populations crashing as a result of lady bug larvae, syrphid maggots, lacewing larvae, parasitic wasps, and fungal diseases. In Ontario, half of a field was treated for soybean aphids and the other half was left untreated. Soybean aphid populations in the treated area of the field were significantly higher than in the untreated portion of the field. Ted Radcliffe, University of Minnesota, reported rapid increases in soybean aphid numbers after research plots were sprayed with lambdacyhalothrin (Warrior) and carbaryl (Sevin). He suggests that the resurgence resulted from the suppression of generalist predators. These reports suggest that natural enemies may be playing a major role in controlling soybean aphids. Treating a field for soybean aphids may not only be ineffective, it may be detrimental.

For the most recent information on distribution of soybean aphids, visit Soybean Aphid Watch 2001 ( Saphid/Aphidindex.htm) and the University of Illinois IPM Web site ( soybeans/aphids.html).--Susan Ratcliffe

Author: Susan Ratcliffe

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

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