No. 11 Article 2/June 3, 2005

The Soybean Aphid Season Has Begun

We didn't have to wait long after publishing "Waiting for Insects" in last week's issue of the Bulletin (issue no. 10, May 27, 2005). On Friday, May 27, we received two reports from individuals who had found soybean aphids in soybean fields. Dr. Bob O'Neil, entomologist at Purdue University, reported that a postdoctoral scientist (Dr. Ho Jung Yoo) working for him found five soybean aphid nymphs on a soybean plant in Miami County, Indiana, and one alate (winged) each in a field in Wabash County and a field in Carroll County on May 26. These three counties are in north-central Indiana, northeast of Purdue University (Tippecanoe County), roughly at the same latitude as Peoria and Watseka (Rt. 24).

Ben Reep, an independent crop consultant, found four soybean aphid nymphs on VE--VC stage soybeans in a field near Melvin, Illinois (Ford County), on May 27. This is the earliest report of soybean aphids found in soybean fields in Illinois since this invasive species was first discovered in 2000. The previous earliest report of soybean aphids found in soybean fields was May 29, 2003 (Jeff Wessels, Joliet Junior College, Will County). I visited the field with Ben on May 31, and after about 45 minutes of searching, we found only one more aphid. So, although aphids were present, the field certainly wasn't crawling with them. (On another note, we also found a significant infestation of twospotted spider mites in another soybean field just north across a lane. See the related article, "Twospotted Spider Mites in Soybeans Add Another Level of Concern.")

Suspected soybean aphid on soybean leaf in Ford County, May 27, 2005 (photo courtesy of Ben Reep).

As of the time I am writing this article, the identification of the aphids found in this Ford County field is unconfirmed. However, Dr. David Voegtlin, aphid research specialist with the Illinois Natural History Survey, believes the possibility is remote that an aphid in soybeans in Illinois at this time of year would be anything other than Aphis glycines (soybean aphid). David also viewed some photos that Ben Reep sent, and he believes that the aphids Ben found were soybean aphids. Nonetheless, David will be examining the aphid I brought back and will determine the species as soon as possible.

We also received a report and some photographs from Jim Donnelly, Ag View FS, who found a significant colony of soybean aphids (50 to 100) on buckthorn in Starved Rock State Park on May 28. Following is David Voegtlin's response to Jim: "The aphids certainly look like the soybean aphid; however, I have been unable to separate wingless adults and immatures of Aphis glycines and Aphis nasturtii. Both species overwinter on Rhamnus cathartica (buckthorn). Winged aphids appear to be the only way to reliably separate the two species. In my travels this spring, I have been fooled a few times, so I never am certain until I can see a winged aphid. I have found mixed colonies, colonies of only A. nasturtii and some of only A. glycines." Notable in the photographs that Jim sent is that many of the nymphs were alatoid (with "shoulder pads"). If the aphids are soybean aphids, the alatoid nymphs will molt into winged aphids that will disperse from buckthorn to soybeans.

Suspected soybean aphids on buckthorn, Starved Rock State Park, May 28, 2005 (photo courtesy of Jim Donnelly).

These early observations suggest that the aphids' "season" has begun. Consequently, it's time to step up the vigilance for soybean aphids so that colonies don't explode on us without our knowledge. Our experiences in 2003 taught us that if biological and environmental factors favor development of soybean aphids, their numbers will increase quickly. As you know, non-hot temperatures (<85°F) and low numbers of natural enemies (e.g., multicolored Asian lady beetles) both favor rapid increase in soybean aphid population densities. Projected temperatures over the next 10 days in the Melvin area are mostly low to mid-80s ( right for development of soybean aphids.

Because of the widespread concern about Asian soybean rust, Midwest soybean growers are on pins and needles as we begin this 2005 growing season. As a consequence, we anticipate the frequency of field scouting in soybeans this summer to escalate significantly. Scouting, of course, is never a bad thing. So it is likely that scouts soon will begin to encounter aphids in soybean fields. We advise against overreacting to early sightings, despite the anxiety that might arise as growers recall their experiences with aphids in 2003. Insecticides applied too early (i.e., in June, well before the densities of aphids reach the economic threshold of 250 per plant) will not prevent buildup of soybean aphids in July. If significant infestations of soybean aphids develop in Illinois, winged aphids will leave heavily infested fields to colonize other fields, which may include fields that were treated too early. Multiple insecticide applications cannot compete economically with one well-timed insecticide application later in the summer. Remember, the most critical soybean growth stages for soybean aphid infestations are late vegetative through R4. Multiple insecticide applications can also result in the onset of other pest problems if natural enemies (i.e., predators and parasitoids) are eliminated.

As you gear up to scout for soybean aphids, you might consider "Speed Scouting for Soybean Aphids," an efficient method developed by entomologists at the University of Minnesota. You can access the protocol and worksheet on the Web. Also from the University of Minnesota is the "Soybean Aphid Growth Estimator: The SAGE Model," which allows you to enter actual and forecasted temperature data to predict when the soybean aphid population density is expected to reach a threshold of 250 aphids per plant. You can access the SAGE model here. Hopefully these two sources of information will improve our decision making related to management of soybean aphids in 2005.--Kevin Steffey

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