No. 13 Article 4/June 23, 2006

Low Numbers of Soybean Aphids

The results from our weekly survey of 26 soybean fields in Woodford County (10 fields); Marshall, Putnam, Bureau, Lee, Whiteside, and Ogle counties (1 field in each county); and Stephenson County reveal very low numbers of soybean aphids in soybean fields in Illinois thus far. Although the surveyors found at least one soybean aphid in 23 of the 26 fieldssurveyed, the higheset average number was 1.85 aphids per plant (Bureau County). The most soybean aphids found on one plant was seven (Woodford County), and the greatest percentage of plants on which aphids were observed (20 plants examined in each field) was 65% (Bureau County). The soybean plants in the 26 fields surveyed were V2- to V4-stage plants. It is also worth noting that I found one soybean aphid on a V2-stage soybean plant at the Ag Engineering Farm near Urbana, and Robert Bellm, Extension crop systems educator in Edwardsville, found one aphid on a soybean plant in Madison County.

The numbers of soybean aphids in soybean fields in Illinois are similar to those being found in most other midwestern states. The hot temperatures in most areas are probably suppressing aphid development. Entomologists at the University of Minnesota determined that survivorship of soybean aphids declines as temperatures increase from 68°F to 95°F. At 95°F, soybean aphid populations suffer considerable stress and decline over time. Refer to the University of Minnesota Soybean Aphid Web site for more details. Click on "Soybean Aphid Growth Estimator" to review the relationship between temperature and soybean aphid development.

I mentioned in issue no. 11 (June 9, 2006) of the Bulletin that early-season predators may help suppress soybean aphid populations. Other entomologists in the Midwest have reported finding Orius insidiosus, the insidious flower bug, in soybean fields. These predators are present in soybean fields early in the season, feeding on thrips and other arthropods. When soybean aphids arrive in soybean fields, Orius contentedly feast on them. The combination of their early-season predation and the hot temperatures could keep soybean aphid populations in check for the time being. Let's hope for the best.--Kevin Steffey

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