Two groups of white grubs are found in Iowa croplands: the true white grub, which can cause significant stand loss in corn and sometimes soybean; and the annual white grub, which probably does not cause stand loss in either crop. True white grubs are the larvae of June beetles (the big brown beetles that fly around yard lights during May and June evenings). They feed on the roots of seedling plants, causing them to wilt and die. True white grubs have a 3-year life cycle and can reduce plant populations during 2 of these 3 years. Predicting what corn or soybean fields might have problems from true white grubs is nearly impossible.
Iowa State University
Rice, Marlin E., "True white grub rasters" (2001). Integrated Crop Management News. 2003.
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