Western Corn Rootworm in Soybeans: Is an Adjustment in the Economic Threshold Necessary?

Com rootworm larval injury in first-year com (rotated com) was first reported in six seed production fields near Piper City, Illinois, in 1987. Initially, prolonged diapause of northern com rootworm, Diabrotica barberi, was offered as the primary explanation for this injury to rotated com. However, some of the larvae collected from affected fields were reared in the laboratory and later found to be western com rootworms, Diabrotica virgifera virgifera (Levine and Gray 1996, O\u27Neal et al. 1997). Six years later (1993), again near Piper City, new observations of com rootworm injury to first-year com seed production fields were reported. Unlike the explanation offered in the mid-1980s, a shift in the ovipositional behavior of the western com rootworm was suggested as the underlying cause of the problem. Since that first report, researchers have sought explanations for this remarkable adaptation by western com rootworm to crop rotation, including repellency by pyrethroid insecticides, prolonged diapause, and changes in feeding preferences (Steffey et al. 1992; Levine and Oloumi-Sadeghi 1996; Spencer et al. 1998, 1999): To improve immediate management options, an economic threshold based upon adult captures in soybeans and subsequent larval injury in rotated com was developed (O\u27Neal et al. 1998, 1999)