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David Voegtlin David Voegtlin
Assistant Professor
Natural History Survey
(217) 244-2152; dvoegtlin@illinois.edu

The Soybean Aphid Roller Coaster

During the 2004 season there are lots of eyes watching soybeans for those small yellow insects that helped change the Midwest landscape in the summer of 2003 as well as the value of the crop. The soybean aphid has been in Illinois for three full growing seasons and every one of them has been very different. Is this what we can expect, what are the factors that influence each seasons population and are there suggested control action thresholds. The last point is the easiest to address, as a threshold of an average of 250 aphids/plant has been adopted across the Midwest. Other factors may influence this such as stage of crop, temperature, and even rainfall, however, incorporating these aspects will take additional research. Multiple factors can influence seasonal populations.

We are only beginning to understand the influence of the multi-colored Asian lady beetle. It appears to be unable to limit the impact of the soybean aphid in years like 2003, however, its impact at the end of the season last year may have helped to limit the size of this years populations. Over-wintering populations are critical to the survival of the aphid. We know it has Rhamnus cathartica (buckthorn) as a winter host and there are also a couple of rare native species that the aphid can successfully over-winter on. In Illinois, buckthorn is common only north of I-80, however, we have seen successful over-wintering in the southern 2/3rds of the State on buckthorns growing in urban areas. Although we do not know how far the aphids that migrate from buckthorn each spring will fly, early spring populations in an area most likely indicate the presence of buckthorn within a few miles or less of the infestation. Suction traps have shown marked differences in the fall flight of the migrants from soybean to buckthorn. In 2001 and 2003 there were very few aphids collected in September and October, while in the same time period of 2002 large numbers of aphids were caught. In the fall of 2002 the soybean aphid could be easily found on buckthorn and eggs were observed for the first time in the wild. We hope that we can use fall flight numbers as indicated in suction trap counts as well as some indication of the number of lady beetles to predict the levels of the next years population.

Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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