Over the past several weeks, we have been updating the aphid situation in soybean. You may remember that I indicated the available references suggested this aphid was Aphis gossypii, the cotton/melon aphid, but I had sent several specimens to an aphid specialist at the Illinois Natural History Survey (Dr. David Voegtlin) to make sure we didn't have an aphid new to our experience. Taxonomy of aphids is difficult, but it is particularly tough in the genus Aphis. Aphids that look identical to most of us can be separated into different species because of subtle differences in setae (hair) patterns and length, length of segments of appendages, minute differences in coloration etc. It is critical to make sure that you know the species with which you are dealing because such aphids can have different host ranges and biology. Dr. Voegtlin examined the aphids for us and forwarded specimens to specialists at other institutions to corroborate his finding. His findings confirmed one of our numerous concerns about this outbreak. Although the aphids we have found look "identical" to the cotton /melon aphid, Aphis gossypii, they are the soybean aphid, Aphis glycines. Prior to this discovery it has never been detected in North America, and was know only to occur in China and a few other Asian countries. It has now been confirmed from soybean fields in northern Illinois, and Dr. Chris DiFonzo at Michigan State University reports finding aphids in southern Michigan that she is currently investigating. This finding will most likely have long-term pest management concerns for soybean production in our area. The insecticide and control guidelines we have been providing over the past few weeks are unchanged, because regardless of the species, control intervention was required in some fields, and in most fields the soybean were demonstrating no outward signs of aphid-induced stressed. Accurate identification was a longer-term issue, but determining if insecticide use was warranted was an immediate concern. There is a concern for virus transmission with the soybean aphid, but the cotton/melon aphid is also known to vector certain viruses. By the time most people discovered aphids in their soybean, the aphids had been there too long to think about using an insecticide to prevent virus spread,. The concern was over direct damage to the soybeans growth and development. Dr. Craig Grau has virus studies in place and Dr. Dave Hogg and I are collaborating with him on this study. We were fortunate to have the aphids hit this plot, and we hope to have data this fall. It is important to note that we won't know at this time, the significance of this aphid as a disease vector. The good news is the aphid populations are going down hill in most fields. You can see last week's issue for information on what to look for in term of diseased and parasitized aphids. However, I would monitor soybean until early September, when winged forms should start leaving soybean fields for overwinter sites.. Nearly all of the literature on the soybean aphid is in Chinese, but we were provided with an abstract in English for a paper by Wang, C.L., Siang, N. I., Chang, G. S., and Chu, H. F., Studies on the soybean aphid, Aphis glycines Matsumura. The soybean aphid is widely distributed in the soybean growing regions of China. The host plant range is quite limited and in China it is only found on wild and cultivated soybean. Its overwintering host is Rhamnus davuricus, a species of buckthorn. We and other agencies will be examining our ornamental buckthorns and other potential hosts during late summer and fall to to locate overwintering sites. The year 2000 was a big year for corn leaf aphid, pea aphid and other species of aphids in Wisconsin. But, an infestation of the magnitude that we have seen for the soybean aphid suggests it has been present in Wisconsin prior to 2000, but at levels too low to detect. In China, they have identified three periods of damage to soybean: 1. From seedling stage to blooming stage of soybean, the aphid population reaches its highest peak. Its colonies concentrate on tender leaves and branches (e.g. new trifoliolate leaves). 2. In late July, the top growing point of soybean stops growing, the aphid colonies alter their positions from the top to the middle or the lower part of the plant and feed on the underside of soybean leaves. 3. From late August to early September the aphid colony begins to multiply rapidly again. Afterwards it migrates back to the overwintering host, where eggs are laid in preparation for winter. Points 1 and 2 above fit our observations very closely. We are watching point 3 at this time and that is why I said to spot check fields until early September. The data go on to suggest that the August populations are usually quite small and probably not a threat to soybean anymore. The nutritional quality of the soybean is not as conducive to aphid buildup. But, I would play closest attention to the double-cropped beans. They have had the most trouble already and aphid populations may behave differently in these mid-June planted fields. I wish we had better news. I was more than a little surprised when Ms. Nancy Kurtzweil, from Craig Grau's lab came back from touring the research trials a few weeks ago and produced a zip lock bag containing soybean leaves loaded with aphids. Aphids are not supposed to reach high numbers on soybean and I at first thought it was just another case to go along with an already unusual year for insect problems. In fact, some of the literature states that aphids don't colonize soybean in the U.S. We then inspected other trials and found aphids in all of them; this was followed shortly by calls from the countryside asking for advice on aphids in soybean. This chain of events led us to suspect that we might have something unusual developing and that additional identification was needed. We will keep you updated, and appreciate any observations that you may wish to contribute. Sorry, to keep you waiting this long to give the final identification, but these examinations take a long time. We thank Dr. David Voegtlin, and colleagues he contacted at other institutions (Dr. Roger Blackman, Dr. Manya Stoetzel, Dr. Susan Halbert, Dr. Bob Foottit, ) for their assistance and for giving this top priority.
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