The preceding article was written as an update between issues no. 20 and 21 (August 8 and 15, 2003, respectively), but it appeared only on the Web site. As a matter of policy, we include update articles in the next printed issue. However, even more has happened with soybean aphids since the update was posted to the Web site on Monday, August 11, so following is a little more information about this troublesome pest.
Apparently economic numbers of soybean aphids are being discovered in areas where they have not been noticed up to now. Numbers of soybean aphids have increased rather significantly in Fulton, Henry, Knox, McLean, Stark, Whiteside, and Woodford counties in north-central and northwestern Illinois. We also are receiving reports of significant infestations from some more central Illinois counties such as Logan and Sangamon. If your county is not listed among these, don't assume that soybean aphids are not present. I have listed only those counties for which we have received good reports. So, if you have not begun to do so already, scout, and then scout some more. It's very important to determine whether soybean aphid densities are increasing or decreasing. At some point soon, densities of soybean aphids should begin to decline naturally as the time approaches for the generation that flies back to buckthorn.
The primary question being asked about soybean aphid management right now focuses on thresholds. It's no secret that thresholds among midwestern states vary, primarily because entomologists still have not had much time to conduct the research necessary for developing thresholds. However, the information available provides the best current information. I just finished reading an excellent article written by Eileen Cullen, Extension entomologist at the University of Wisconsin. In her article, she discusses thresholds in a very objective way. She mentions results from 16 economic threshold and spray-timing studies conducted by entomologists at the University of Wisconsin. They determined that thresholds vary with the growth stages of the soybean plants200+ aphids per plant at full bloom (R2), 1,000+ aphids per plant at beginning pod (R3), and 1,500+ aphids per plant at full pod (R4). You can read the entire article in the August 7, 2003, issue of Wisconsin Crop Manager.
With all of the research being conducted on soybean aphids this year throughout the Midwest, I have confidence that we will know more about this pest as we go into the 2004 season than we did coming into the 2003 season. We'll never have the answer to all of the questions, but we'll know more soon.--Kevin Steffey