No. 22 Article 1/September 5, 2008

Numbers of Soybean Aphids Have Increased Significantly in Northern Illinois

(First published on August 15)

The results from our August 13 surveys of 26 commercial soybean fields reveal that numbers of soybean aphids increased dramatically from those posted from the previous week's survey. Average densities in 10 fields in Stephenson County ranged from 504 to 313.85 aphids per plant, the latter obviously greater than the economic threshold. The soybeans in all 10 fields were in stages R4 or R5.

Although the numbers of soybean aphids found in the 10 fields in Woodford County on August 13 were considerably smaller than the numbers found in Stephenson County, the increases in densities from the previous week were significant in a few fields. The largest density in Woodford County on August 13 was 72.4 aphids per plant. The densities in Bureau, Lee, Marshall, Putnam, Ogle, and Whiteside counties also increased from August 6 and 7 to August 13.

Continued moderate temperatures and a relative lack of natural enemies currently in soybean fields will benefit population growth of soybean aphids. Keep in mind, however, that there is not widespread agreement regarding yield benefits from controlling soybean aphids with insecticides once the soybeans reach stage R6. However, for soybeans still in earlier stages of reproductive development (R1-R5), an insecticide application may be economically warranted.

If you have not begun to scout soybeans diligently for soybean aphids, you should start right away. Soybean growers throughout the northern half of the state should be alert to the possibility of rapidly increasing numbers of soybean aphids.--Kevin Steffey

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