Almost as quickly as the soybean aphid was discovered and news hit the streets, the numbers of aphids in soybean fields in northern Illinois have declined significantly. Within a week or less, fields that had been heavily infested now have many fewer aphids. Jim Morrison, Crop Systems Educator at the Rockford Extension Center, surveyed some soybean fields in Carroll, Stephenson, Whiteside, and Winnebago counties on August 22. Jim said that the numbers of aphids on soybeans increased as he traveled from southern Whiteside County through the more northern counties. However, he found very little leaf cupping or puckering, and all of the infested fields were dark green. Bill Hall, an academic hourly at the Boone County Extension Unit, visited soybean fields in Boone and McHenry counties during the weeks of August 14 and 21, and he says the numbers of aphids in infested fields have declined dramatically. Gary Bretthauer, Extension Unit Assistant in Kendall County, also observed a significant decline in numbers of aphids in soybean fields in Kendall County. |
Close-up of soybean aphids on a leaf. (Photo courtesy of Gary Bretthauser, Kendall County.)
Close-up of a soybean aphid. (Photo courtesy of Gary Bretthauer, Kendall County.)
On August 22, I spent some time with Gary Bretthauer and a video crew in a soybean field near Oswego in Kendall County. Gary indicated that the field had been heavily infested with aphids during the week of August 14, and I can attest to dramatically decreased numbers of aphids on August 22. The aphids and their cast skins were relatively easy to find, but none of the plants were heavily infested. There was plenty of evidence of the aphids' feeding injury (cupped and/or wrinkled leaves, some yellowing, some stunted plants), but in general, the field looked fine. It is doubtful that much yield will be lost as a result of the aphid infestation. I suspect that this is true for most of the fields in which aphids were present. Some growers who had their fields treated with insecticides to control the aphids have expressed regret that they reacted too quickly.
So what caused the decline in the numbers of aphids in soybeans? Heavy rainfalls in some areas washed aphids off the plants. However, I think that natural control, by predators and a pathogenic fungus, was the primary reason for the rapid decrease in densities of aphids. The field I visited seemed to be crawling with predators, especially the multicolored Asian lady beetle, Harmonia axyridis. I have never seen so many lady beetle larvae, pupae, and adults in a field of soybeans in my life. The activity of these voracious predators resembled a feeding frenzy. In addition, green lacewings (probably Chrysoperla carnea) and hover flies (the adult stage of the predatory syrphid fly maggots) were quite common. I also observed aphids that seemed to have been killed by the fungus that John Wedberg reported was causing aphid populations to crash in Wisconsin.
The video segment I referred to previously is currently being scripted and edited by our video experts in Information Technology and Communication Services in the College of ACES. We hope to have it available for viewing on our web site by the week of August 28. Look for the Update button when you visit the web site.
Now I'll add some fuel to the fire. Since our last report of the soybean aphid in Illinois, Michigan, and Wisconsin, we have learned that it has been found in Minnesota. It is logical to assume that the aphid is present in some fields in Indiana and Iowa, although no one has reported it from those states yet. But the most important news for us in Illinois is that Les Domier (research plant pathologist with the USDA here in Champaign), Gail Kampmeier (research scientist with the Illinois Natural History Survey), and David Onstad (research scientist in the Department of Natural Resources and Environmental Sciences) believe they have found the soybean aphid in a few fields in Champaign and Vermilion counties in east-central Illinois. It is important to note that David Voegtlin, our aphid specialist in the Illinois Natural History Survey, has not confirmed the species of the aphids found in Champaign and Vermilion counties; David currently is on a brief assignment in Costa Rica. However, Les, Gail, and David Onstad are reasonably certain that the aphids they found were soybean aphids, Aphis glycines.
The occurrence of the soybean aphid in east-central Illinois is an important finding. I urge people throughout Illinois to examine soybean fields for aphids as soon as possible. That having been said, however, I also urge you not to overreact. We want to be aware of the aphid's distribution, but there is no need to panic. Scientists in the states in which the aphid has been found are doing all they can to learn more about the aphid to develop strategies for its management, if necessary, in 2001 and beyond. Steve Pueppke, Associate Dean of Research in the College of ACES, and his counterparts in neighboring states are considering the formation of a Rapid Response Regional Committee. In Illinois, the members of this team will be entomologists and plant pathologists from the College of ACES, the Illinois Natural History Survey, and the USDA. Some of the team members will survey soybean fields in several counties in Illinois during the week of August 28 to determine the presence (or absence) of the soybean aphid. We will survey from south to north and east to west and report our preliminary findings in a future issue of the Bulletin. Our counterparts in neighboring states are undertaking similar surveys.
A lot of information about this aphid can be found on the Web. I have visited several informative web sites, and we'll probably find more in the near future. Check out the following web sites for reports from other states:
Iowa"Soybean Aphid Invades Midwest," by Marlin Rice: http://www.ipm.iastate.edu/ipm/icm/2000/8-21-2000/soyaphid.html
Michigan"Aphids Amuck: Soybean Aphids Discovered in U.S.," by Chris DiFonzo: http://www.msue.msu.edu/ipm/CAT00_field/FC08-17-00.htm#1
Minnesota"New Soybean Pest Appears in the Midwest: Is It in Minnesota?" by Ken Ostlie and Bill Hutchison: http://www.vegedge.umn.edu/mnvegnew/vol2/818new.htm,
Wisconsin"Important Update on Aphids in Soybean" by John Webberg: http://ipcm.wisc.edu/wcm/00-22insect1.html
For an international perspective, visit http://www.agric.nsw.gov.au/Hort/ascu/insects/aglycin.htm, "The Soybean Aphid, Aphis glycines, present in Australia," by Murray Fletcher and Peter Desborough. This site has some excellent photographs and information concerning identification, known hosts, damage, life cycle, natural enemies, distribution, and references. Most notably, one study published in China in 1996 indicated that Aphis glycines is capable of reducing soybean yield by as much as about 28%.
The bottom line regarding this aphid in Illinois right now is that an application of an insecticide to control the aphids probably will not provide an economic return. Although we do not have data regarding potential yield loss caused by this aphid in North America, most entomologists agree that most soybeans are far enough along that significant yield losses are unlikely. In addition, as I indicated previously, natural control agents are reducing densities of these aphids dramatically. Let us know what you find in your area, and we'll keep you posted with updates regarding this unique situation.--Kevin Steffey