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Observations on Soybean Aphids in Illinois in October

November 9, 2001
Most of you will recall that soybean aphids leave soybeans in late summer and fall to fly to their primary host, Rhamnus species, also known as buckthorn. In 2000, after discovering the soybean aphid for the first time in North America, entomologists spent considerable effort looking for soybean aphids on Rhamnus in the fall and winter. Unfortunately, our efforts were mostly futile--very few soybean aphids were found on Rhamnus. However, the search for soybean aphids on Rhamnus this year has been more successful. David Voegtlin, aphid research specialist at the Illinois Natural History Survey, and Phil Orwick, with the National Soybean Research Laboratory, found soybean aphids relatively easily on Rhamnus cathartica in a wooded lot next to a soybean field in southern Cook County on October 2. They observed gynoparae (fall migrants that produce the egg-laying generation on buckthorn) and nymphs that they assumed would mature into oviparae (the egg layers) on the buckthorn. They also indicated that wherever they stopped to look at soybeans that still had green leaves, they were able to find the aphids on nearby buckthorn (as well as on the soybeans). They didn't find any predators with the aphids.

During other sampling forays that same week, David and Phil found soybean aphids on buckthorn in both northern and central Illinois. They mostly found gynoparae and oviparous nymphs. (Keep in mind, however, that oviparous nymphs have to feed and mature and males have to arrive to fertilize them before they can lay eggs, the stage that overwinters.) David was struck by the fact that soybean aphids were easy to find this year (hundreds on some buckthorn), compared with the difficulty in finding them last year.

David also reported that Ben Putler, a retired entomologist in Columbia, Missouri, found soybean aphids on Rhamnus cathartica on the University of Missouri campus on October 12. Ben also had found both gynoparae and oviparae. David indicated that Columbia, Missouri, is "by far the most southern locality known to date where sexual reproduction has been observed on Rhamnus."

So, again we ask the question (similar to other questions asked about soybean aphids), "What does all of this mean?" We continue to learn as we go. Although we could find very few (only six) soybean aphids on buckthorn last fall, the aphids were quite widespread in 2001. Despite the fact that soybean aphids were not very common in Champaign County during the summer, David found several soybean aphids on a buckthorn plant on the U of I campus nearly a mile from the nearest soybean field. So, the correlation of aphids found on buckthorn in the fall and aphids found (or not found) on soybeans in the summer is not known. About the best we can do at this point is watch the buckthorn plants on which aphids have been found this fall to monitor their development next spring. Early information next spring may give us a jump on the development of populations in soybeans next summer. Stay tuned.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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