Issue No. 19, Article 2/August 4, 2006
Weekly Survey--Soybean Aphid Numbers Continue to Increase
We continue the weekly survey for soybean aphids in Woodford County (10 fields), Stephenson County (10 fields), and in between (six fields, one each in Marshall, Putnam, Bureau, Lee, Whiteside, and Ogle counties). The average density of soybean aphids per plant increased from the July 17-18 survey to the July 25-26 survey in 23 of the 26 fields sampled. However, the average densities in all fields were well below the economic threshold of 250 aphids per plant. Nonetheless, soybean aphids were found in all 26 fields surveyed on July 25 and 26. Most of the fields surveyed July 25 and 26 were in the R2 or R3 stage of development.
The largest density of soybean aphids observed on July 25 was 32.1 aphids per plant (Marshall County). More than 200 aphids were found on two of the 20 plants sampled in that field. Ninety percent of the plants were infested with soybean aphids. However, of the 26 fields surveyed, only five had average densities greater than 10 aphids per plant.
We are quickly approaching the stages of soybean development beyond which soybean aphids should have much impact (i.e., past the R5 stage of development). However, I have received one confirmed report of a soybean field in Woodford County that was sprayed with an insecticide because the density of soybean aphids had exceeded the economic threshold. It is likely that other fields have been sprayed, although the number of fields treated in Illinois thus far in 2006 has been extremely low. It seems unlikely that soybean aphids will pose much of a threat to soybean production in Illinois in 2006, but the increases in numbers over the past couple of weeks suggest that continued scouting is appropriate. A return to cooler temperatures will encourage more rapid development of soybean aphid colonies.--Kevin Steffey